This is tar.info, produced by makeinfo version 6.7 from tar.texi. This manual is for GNU 'tar' (version 1.34, 4 February 2021), which creates and extracts files from archives. Copyright (C) 1992, 1994-1997, 1999-2001, 2003-2017, 2021 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License", with the Front-Cover Texts being "A GNU Manual", and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Back-Cover Text is: "You have the freedom to copy and modify this GNU manual." INFO-DIR-SECTION Archiving START-INFO-DIR-ENTRY * Tar: (tar). Making tape (or disk) archives. END-INFO-DIR-ENTRY INFO-DIR-SECTION Individual utilities START-INFO-DIR-ENTRY * tar: (tar)tar invocation. Invoking GNU 'tar'. END-INFO-DIR-ENTRY  File: tar.info, Node: one, Prev: recurse, Up: Choosing 6.10 Crossing File System Boundaries ==================================== 'tar' will normally automatically cross file system boundaries in order to archive files which are part of a directory tree. You can change this behavior by running 'tar' and specifying '--one-file-system'. This option only affects files that are archived because they are in a directory that is being archived; 'tar' will still archive files explicitly named on the command line or through '--files-from', regardless of where they reside. '--one-file-system' Prevents 'tar' from crossing file system boundaries when archiving. Use in conjunction with any write operation. The '--one-file-system' option causes 'tar' to modify its normal behavior in archiving the contents of directories. If a file in a directory is not on the same file system as the directory itself, then 'tar' will not archive that file. If the file is a directory itself, 'tar' will not archive anything beneath it; in other words, 'tar' will not cross mount points. This option is useful for making full or incremental archival backups of a file system. If this option is used in conjunction with '--verbose' ('-v'), files that are excluded are mentioned by name on the standard error. * Menu: * directory:: Changing Directory * absolute:: Absolute File Names  File: tar.info, Node: directory, Next: absolute, Up: one 6.10.1 Changing the Working Directory ------------------------------------- To change the working directory in the middle of a list of file names, either on the command line or in a file specified using '--files-from' ('-T'), use '--directory' ('-C'). This will change the working directory to the specified directory after that point in the list. '--directory=DIRECTORY' '-C DIRECTORY' Changes the working directory in the middle of a command line. For example, $ tar -c -f jams.tar grape prune -C food cherry will place the files 'grape' and 'prune' from the current directory into the archive 'jams.tar', followed by the file 'cherry' from the directory 'food'. This option is especially useful when you have several widely separated files that you want to store in the same archive. Note that the file 'cherry' is recorded in the archive under the precise name 'cherry', _not_ 'food/cherry'. Thus, the archive will contain three files that all appear to have come from the same directory; if the archive is extracted with plain 'tar --extract', all three files will be written in the current directory. Contrast this with the command, $ tar -c -f jams.tar grape prune -C food red/cherry which records the third file in the archive under the name 'red/cherry' so that, if the archive is extracted using 'tar --extract', the third file will be written in a subdirectory named 'red'. You can use the '--directory' option to make the archive independent of the original name of the directory holding the files. The following command places the files '/etc/passwd', '/etc/hosts', and '/lib/libc.a' into the archive 'foo.tar': $ tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a However, the names of the archive members will be exactly what they were on the command line: 'passwd', 'hosts', and 'libc.a'. They will not appear to be related by file name to the original directories where those files were located. Note that '--directory' options are interpreted consecutively. If '--directory' specifies a relative file name, it is interpreted relative to the then current directory, which might not be the same as the original current working directory of 'tar', due to a previous '--directory' option. When using '--files-from' (*note files::), you can put various 'tar' options (including '-C') in the file list. Notice, however, that in this case the option and its argument may not be separated by whitespace. If you use short option, its argument must either follow the option letter immediately, without any intervening whitespace, or occupy the next line. Otherwise, if you use long option, separate its argument by an equal sign. For instance, the file list for the above example will be: -C/etc passwd hosts --directory=/lib libc.a To use it, you would invoke 'tar' as follows: $ tar -c -f foo.tar --files-from list The interpretation of options in file lists is disabled by '--verbatim-files-from' and '--null' options.  File: tar.info, Node: absolute, Prev: directory, Up: one 6.10.2 Absolute File Names -------------------------- By default, GNU 'tar' drops a leading '/' on input or output, and complains about file names containing a '..' component. There is an option that turns off this behavior: '--absolute-names' '-P' Do not strip leading slashes from file names, and permit file names containing a '..' file name component. When 'tar' extracts archive members from an archive, it strips any leading slashes ('/') from the member name. This causes absolute member names in the archive to be treated as relative file names. This allows you to have such members extracted wherever you want, instead of being restricted to extracting the member in the exact directory named in the archive. For example, if the archive member has the name '/etc/passwd', 'tar' will extract it as if the name were really 'etc/passwd'. File names containing '..' can cause problems when extracting, so 'tar' normally warns you about such files when creating an archive, and rejects attempts to extracts such files. Other 'tar' programs do not do this. As a result, if you create an archive whose member names start with a slash, they will be difficult for other people with a non-GNU 'tar' program to use. Therefore, GNU 'tar' also strips leading slashes from member names when putting members into the archive. For example, if you ask 'tar' to add the file '/bin/ls' to an archive, it will do so, but the member name will be 'bin/ls'(1). Symbolic links containing '..' or leading '/' can also cause problems when extracting, so 'tar' normally extracts them last; it may create empty files as placeholders during extraction. If you use the '--absolute-names' ('-P') option, 'tar' will do none of these transformations. To archive or extract files relative to the root directory, specify the '--absolute-names' ('-P') option. Normally, 'tar' acts on files relative to the working directory--ignoring superior directory names when archiving, and ignoring leading slashes when extracting. When you specify '--absolute-names' ('-P'), 'tar' stores file names including all superior directory names, and preserves leading slashes. If you only invoked 'tar' from the root directory you would never need the '--absolute-names' option, but using this option may be more convenient than switching to root. '--absolute-names' Preserves full file names (including superior directory names) when archiving and extracting files. 'tar' prints out a message about removing the '/' from file names. This message appears once per GNU 'tar' invocation. It represents something which ought to be told; ignoring what it means can cause very serious surprises, later. Some people, nevertheless, do not want to see this message. Wanting to play really dangerously, one may of course redirect 'tar' standard error to the sink. For example, under 'sh': $ tar -c -f archive.tar /home 2> /dev/null Another solution, both nicer and simpler, would be to change to the '/' directory first, and then avoid absolute notation. For example: $ tar -c -f archive.tar -C / home *Note Integrity::, for some of the security-related implications of using this option. ---------- Footnotes ---------- (1) A side effect of this is that when '--create' is used with '--verbose' the resulting output is not, generally speaking, the same as the one you'd get running 'tar --list' command. This may be important if you use some scripts for comparing both outputs. *Note listing member and file names::, for the information on how to handle this case.  File: tar.info, Node: Date input formats, Next: Formats, Prev: Choosing, Up: Top 7 Date input formats ******************** First, a quote: Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible. Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done better than handing down our present system. It is like a set of trapezoidal building blocks, with no vertical or horizontal surfaces, like a language in which the simplest thought demands ornate constructions, useless particles and lengthy circumlocutions. Unlike the more successful patterns of language and science, which enable us to face experience boldly or at least level-headedly, our system of temporal calculation silently and persistently encourages our terror of time. ... It is as though architects had to measure length in feet, width in meters and height in ells; as though basic instruction manuals demanded a knowledge of five different languages. It is no wonder then that we often look into our own immediate past or future, last Tuesday or a week from Sunday, with feelings of helpless confusion. ... --Robert Grudin, 'Time and the Art of Living'. This section describes the textual date representations that GNU programs accept. These are the strings you, as a user, can supply as arguments to the various programs. The C interface (via the 'parse_datetime' function) is not described here. * Menu: * General date syntax:: Common rules * Calendar date items:: 21 Jul 2020 * Time of day items:: 9:20pm * Time zone items:: UTC, -0700, +0900, ... * Combined date and time of day items:: 2020-07-21T20:02:00,000000-0400 * Day of week items:: Monday and others * Relative items in date strings:: next tuesday, 2 years ago * Pure numbers in date strings:: 20200721, 1440 * Seconds since the Epoch:: @1595289600 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0" * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al.  File: tar.info, Node: General date syntax, Next: Calendar date items, Up: Date input formats 7.1 General date syntax ======================= A "date" is a string, possibly empty, containing many items separated by whitespace. The whitespace may be omitted when no ambiguity arises. The empty string means the beginning of today (i.e., midnight). Order of the items is immaterial. A date string may contain many flavors of items: * calendar date items * time of day items * time zone items * combined date and time of day items * day of the week items * relative items * pure numbers. We describe each of these item types in turn, below. A few ordinal numbers may be written out in words in some contexts. This is most useful for specifying day of the week items or relative items (see below). Among the most commonly used ordinal numbers, the word 'last' stands for -1, 'this' stands for 0, and 'first' and 'next' both stand for 1. Because the word 'second' stands for the unit of time there is no way to write the ordinal number 2, but for convenience 'third' stands for 3, 'fourth' for 4, 'fifth' for 5, 'sixth' for 6, 'seventh' for 7, 'eighth' for 8, 'ninth' for 9, 'tenth' for 10, 'eleventh' for 11 and 'twelfth' for 12. When a month is written this way, it is still considered to be written numerically, instead of being "spelled in full"; this changes the allowed strings. In the current implementation, only English is supported for words and abbreviations like 'AM', 'DST', 'EST', 'first', 'January', 'Sunday', 'tomorrow', and 'year'. The output of the 'date' command is not always acceptable as a date string, not only because of the language problem, but also because there is no standard meaning for time zone items like 'IST'. When using 'date' to generate a date string intended to be parsed later, specify a date format that is independent of language and that does not use time zone items other than 'UTC' and 'Z'. Here are some ways to do this: $ LC_ALL=C TZ=UTC0 date Tue Jul 21 23:00:37 UTC 2020 $ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ' 2020-07-21 23:00:37Z $ date --rfc-3339=ns # --rfc-3339 is a GNU extension. 2020-07-21 19:00:37.692722128-04:00 $ date --rfc-2822 # a GNU extension Tue, 21 Jul 2020 19:00:37 -0400 $ date +'%Y-%m-%d %H:%M:%S %z' # %z is a GNU extension. 2020-07-21 19:00:37 -0400 $ date +'@%s.%N' # %s and %N are GNU extensions. @1595372437.692722128 Alphabetic case is completely ignored in dates. Comments may be introduced between round parentheses, as long as included parentheses are properly nested. Hyphens not followed by a digit are currently ignored. Leading zeros on numbers are ignored. Invalid dates like '2019-02-29' or times like '24:00' are rejected. In the typical case of a host that does not support leap seconds, a time like '23:59:60' is rejected even if it corresponds to a valid leap second.  File: tar.info, Node: Calendar date items, Next: Time of day items, Prev: General date syntax, Up: Date input formats 7.2 Calendar date items ======================= A "calendar date item" specifies a day of the year. It is specified differently, depending on whether the month is specified numerically or literally. All these strings specify the same calendar date: 2020-07-20 # ISO 8601. 20-7-20 # Assume 19xx for 69 through 99, # 20xx for 00 through 68 (not recommended). 7/20/2020 # Common U.S. writing. 20 July 2020 20 Jul 2020 # Three-letter abbreviations always allowed. Jul 20, 2020 20-jul-2020 20jul2020 The year can also be omitted. In this case, the last specified year is used, or the current year if none. For example: 7/20 jul 20 Here are the rules. For numeric months, the ISO 8601 format 'YEAR-MONTH-DAY' is allowed, where YEAR is any positive number, MONTH is a number between 01 and 12, and DAY is a number between 01 and 31. A leading zero must be present if a number is less than ten. If YEAR is 68 or smaller, then 2000 is added to it; otherwise, if YEAR is less than 100, then 1900 is added to it. The construct 'MONTH/DAY/YEAR', popular in the United States, is accepted. Also 'MONTH/DAY', omitting the year. Literal months may be spelled out in full: 'January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November' or 'December'. Literal months may be abbreviated to their first three letters, possibly followed by an abbreviating dot. It is also permitted to write 'Sept' instead of 'September'. When months are written literally, the calendar date may be given as any of the following: DAY MONTH YEAR DAY MONTH MONTH DAY YEAR DAY-MONTH-YEAR Or, omitting the year: MONTH DAY  File: tar.info, Node: Time of day items, Next: Time zone items, Prev: Calendar date items, Up: Date input formats 7.3 Time of day items ===================== A "time of day item" in date strings specifies the time on a given day. Here are some examples, all of which represent the same time: 20:02:00.000000 20:02 8:02pm 20:02-0500 # In EST (U.S. Eastern Standard Time). More generally, the time of day may be given as 'HOUR:MINUTE:SECOND', where HOUR is a number between 0 and 23, MINUTE is a number between 0 and 59, and SECOND is a number between 0 and 59 possibly followed by '.' or ',' and a fraction containing one or more digits. Alternatively, ':SECOND' can be omitted, in which case it is taken to be zero. On the rare hosts that support leap seconds, SECOND may be 60. If the time is followed by 'am' or 'pm' (or 'a.m.' or 'p.m.'), HOUR is restricted to run from 1 to 12, and ':MINUTE' may be omitted (taken to be zero). 'am' indicates the first half of the day, 'pm' indicates the second half of the day. In this notation, 12 is the predecessor of 1: midnight is '12am' while noon is '12pm'. (This is the zero-oriented interpretation of '12am' and '12pm', as opposed to the old tradition derived from Latin which uses '12m' for noon and '12pm' for midnight.) The time may alternatively be followed by a time zone correction, expressed as 'SHHMM', where S is '+' or '-', HH is a number of zone hours and MM is a number of zone minutes. The zone minutes term, MM, may be omitted, in which case the one- or two-digit correction is interpreted as a number of hours. You can also separate HH from MM with a colon. When a time zone correction is given this way, it forces interpretation of the time relative to Coordinated Universal Time (UTC), overriding any previous specification for the time zone or the local time zone. For example, '+0530' and '+05:30' both stand for the time zone 5.5 hours ahead of UTC (e.g., India). This is the best way to specify a time zone correction by fractional parts of an hour. The maximum zone correction is 24 hours. Either 'am'/'pm' or a time zone correction may be specified, but not both.  File: tar.info, Node: Time zone items, Next: Combined date and time of day items, Prev: Time of day items, Up: Date input formats 7.4 Time zone items =================== A "time zone item" specifies an international time zone, indicated by a small set of letters, e.g., 'UTC' or 'Z' for Coordinated Universal Time. Any included periods are ignored. By following a non-daylight-saving time zone by the string 'DST' in a separate word (that is, separated by some white space), the corresponding daylight saving time zone may be specified. Alternatively, a non-daylight-saving time zone can be followed by a time zone correction, to add the two values. This is normally done only for 'UTC'; for example, 'UTC+05:30' is equivalent to '+05:30'. Time zone items other than 'UTC' and 'Z' are obsolescent and are not recommended, because they are ambiguous; for example, 'EST' has a different meaning in Australia than in the United States, and 'A' has different meaning as a military time zone than as an obsolescent RFC 822 time zone. Instead, it's better to use unambiguous numeric time zone corrections like '-0500', as described in the previous section. If neither a time zone item nor a time zone correction is supplied, timestamps are interpreted using the rules of the default time zone (*note Specifying time zone rules::).  File: tar.info, Node: Combined date and time of day items, Next: Day of week items, Prev: Time zone items, Up: Date input formats 7.5 Combined date and time of day items ======================================= The ISO 8601 date and time of day extended format consists of an ISO 8601 date, a 'T' character separator, and an ISO 8601 time of day. This format is also recognized if the 'T' is replaced by a space. In this format, the time of day should use 24-hour notation. Fractional seconds are allowed, with either comma or period preceding the fraction. ISO 8601 fractional minutes and hours are not supported. Typically, hosts support nanosecond timestamp resolution; excess precision is silently discarded. Here are some examples: 2012-09-24T20:02:00.052-05:00 2012-12-31T23:59:59,999999999+11:00 1970-01-01 00:00Z  File: tar.info, Node: Day of week items, Next: Relative items in date strings, Prev: Combined date and time of day items, Up: Date input formats 7.6 Day of week items ===================== The explicit mention of a day of the week will forward the date (only if necessary) to reach that day of the week in the future. Days of the week may be spelled out in full: 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday' or 'Saturday'. Days may be abbreviated to their first three letters, optionally followed by a period. The special abbreviations 'Tues' for 'Tuesday', 'Wednes' for 'Wednesday' and 'Thur' or 'Thurs' for 'Thursday' are also allowed. A number may precede a day of the week item to move forward supplementary weeks. It is best used in expression like 'third monday'. In this context, 'last DAY' or 'next DAY' is also acceptable; they move one week before or after the day that DAY by itself would represent. A comma following a day of the week item is ignored.  File: tar.info, Node: Relative items in date strings, Next: Pure numbers in date strings, Prev: Day of week items, Up: Date input formats 7.7 Relative items in date strings ================================== "Relative items" adjust a date (or the current date if none) forward or backward. The effects of relative items accumulate. Here are some examples: 1 year 1 year ago 3 years 2 days The unit of time displacement may be selected by the string 'year' or 'month' for moving by whole years or months. These are fuzzy units, as years and months are not all of equal duration. More precise units are 'fortnight' which is worth 14 days, 'week' worth 7 days, 'day' worth 24 hours, 'hour' worth 60 minutes, 'minute' or 'min' worth 60 seconds, and 'second' or 'sec' worth one second. An 's' suffix on these units is accepted and ignored. The unit of time may be preceded by a multiplier, given as an optionally signed number. Unsigned numbers are taken as positively signed. No number at all implies 1 for a multiplier. Following a relative item by the string 'ago' is equivalent to preceding the unit by a multiplier with value -1. The string 'tomorrow' is worth one day in the future (equivalent to 'day'), the string 'yesterday' is worth one day in the past (equivalent to 'day ago'). The strings 'now' or 'today' are relative items corresponding to zero-valued time displacement, these strings come from the fact a zero-valued time displacement represents the current time when not otherwise changed by previous items. They may be used to stress other items, like in '12:00 today'. The string 'this' also has the meaning of a zero-valued time displacement, but is preferred in date strings like 'this thursday'. When a relative item causes the resulting date to cross a boundary where the clocks were adjusted, typically for daylight saving time, the resulting date and time are adjusted accordingly. The fuzz in units can cause problems with relative items. For example, '2020-07-31 -1 month' might evaluate to 2020-07-01, because 2020-06-31 is an invalid date. To determine the previous month more reliably, you can ask for the month before the 15th of the current month. For example: $ date -R Thu, 31 Jul 2020 13:02:39 -0400 $ date --date='-1 month' +'Last month was %B?' Last month was July? $ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!' Last month was June! Also, take care when manipulating dates around clock changes such as daylight saving leaps. In a few cases these have added or subtracted as much as 24 hours from the clock, so it is often wise to adopt universal time by setting the 'TZ' environment variable to 'UTC0' before embarking on calendrical calculations.  File: tar.info, Node: Pure numbers in date strings, Next: Seconds since the Epoch, Prev: Relative items in date strings, Up: Date input formats 7.8 Pure numbers in date strings ================================ The precise interpretation of a pure decimal number depends on the context in the date string. If the decimal number is of the form YYYYMMDD and no other calendar date item (*note Calendar date items::) appears before it in the date string, then YYYY is read as the year, MM as the month number and DD as the day of the month, for the specified calendar date. If the decimal number is of the form HHMM and no other time of day item appears before it in the date string, then HH is read as the hour of the day and MM as the minute of the hour, for the specified time of day. MM can also be omitted. If both a calendar date and a time of day appear to the left of a number in the date string, but no relative item, then the number overrides the year.  File: tar.info, Node: Seconds since the Epoch, Next: Specifying time zone rules, Prev: Pure numbers in date strings, Up: Date input formats 7.9 Seconds since the Epoch =========================== If you precede a number with '@', it represents an internal timestamp as a count of seconds. The number can contain an internal decimal point (either '.' or ','); any excess precision not supported by the internal representation is truncated toward minus infinity. Such a number cannot be combined with any other date item, as it specifies a complete timestamp. Internally, computer times are represented as a count of seconds since an Epoch--a well-defined point of time. On GNU and POSIX systems, the Epoch is 1970-01-01 00:00:00 UTC, so '@0' represents this time, '@1' represents 1970-01-01 00:00:01 UTC, and so forth. GNU and most other POSIX-compliant systems support such times as an extension to POSIX, using negative counts, so that '@-1' represents 1969-12-31 23:59:59 UTC. Most modern systems count seconds with 64-bit two's-complement integers of seconds with nanosecond subcounts, which is a range that includes the known lifetime of the universe with nanosecond resolution. Some obsolescent systems count seconds with 32-bit two's-complement integers and can represent times from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 UTC. A few systems sport other time ranges. On most hosts, these counts ignore the presence of leap seconds. For example, on most hosts '@1483228799' represents 2016-12-31 23:59:59 UTC, '@1483228800' represents 2017-01-01 00:00:00 UTC, and there is no way to represent the intervening leap second 2016-12-31 23:59:60 UTC.  File: tar.info, Node: Specifying time zone rules, Next: Authors of parse_datetime, Prev: Seconds since the Epoch, Up: Date input formats 7.10 Specifying time zone rules =============================== Normally, dates are interpreted using the rules of the current time zone, which in turn are specified by the 'TZ' environment variable, or by a system default if 'TZ' is not set. To specify a different set of default time zone rules that apply just to one date, start the date with a string of the form 'TZ="RULE"'. The two quote characters ('"') must be present in the date, and any quotes or backslashes within RULE must be escaped by a backslash. For example, with the GNU 'date' command you can answer the question "What time is it in New York when a Paris clock shows 6:30am on October 31, 2019?" by using a date beginning with 'TZ="Europe/Paris"' as shown in the following shell transcript: $ export TZ="America/New_York" $ date --date='TZ="Europe/Paris" 2019-10-31 06:30' Sun Oct 31 01:30:00 EDT 2019 In this example, the '--date' operand begins with its own 'TZ' setting, so the rest of that operand is processed according to 'Europe/Paris' rules, treating the string '2019-10-31 06:30' as if it were in Paris. However, since the output of the 'date' command is processed according to the overall time zone rules, it uses New York time. (Paris was normally six hours ahead of New York in 2019, but this example refers to a brief Halloween period when the gap was five hours.) A 'TZ' value is a rule that typically names a location in the 'tz' database (https://www.iana.org/time-zones). A recent catalog of location names appears in the TWiki Date and Time Gateway (https://twiki.org/cgi-bin/xtra/tzdatepick.html). A few non-GNU hosts require a colon before a location name in a 'TZ' setting, e.g., 'TZ=":America/New_York"'. The 'tz' database includes a wide variety of locations ranging from 'Arctic/Longyearbyen' to 'Antarctica/South_Pole', but if you are at sea and have your own private time zone, or if you are using a non-GNU host that does not support the 'tz' database, you may need to use a POSIX rule instead. Simple POSIX rules like 'UTC0' specify a time zone without daylight saving time; other rules can specify simple daylight saving regimes. *Note Specifying the Time Zone with 'TZ': (libc)TZ Variable.  File: tar.info, Node: Authors of parse_datetime, Prev: Specifying time zone rules, Up: Date input formats 7.11 Authors of 'parse_datetime' ================================ 'parse_datetime' started life as 'getdate', as originally implemented by Steven M. Bellovin () while at the University of North Carolina at Chapel Hill. The code was later tweaked by a couple of people on Usenet, then completely overhauled by Rich $alz () and Jim Berets () in August, 1990. Various revisions for the GNU system were made by David MacKenzie, Jim Meyering, Paul Eggert and others, including renaming it to 'get_date' to avoid a conflict with the alternative Posix function 'getdate', and a later rename to 'parse_datetime'. The Posix function 'getdate' can parse more locale-specific dates using 'strptime', but relies on an environment variable and external file, and lacks the thread-safety of 'parse_datetime'. This chapter was originally produced by François Pinard () from the 'parse_datetime.y' source code, and then edited by K. Berry ().  File: tar.info, Node: Formats, Next: Media, Prev: Date input formats, Up: Top 8 Controlling the Archive Format ******************************** Due to historical reasons, there are several formats of tar archives. All of them are based on the same principles, but have some subtle differences that often make them incompatible with each other. GNU tar is able to create and handle archives in a variety of formats. The most frequently used formats are (in alphabetical order): gnu Format used by GNU 'tar' versions up to 1.13.25. This format derived from an early POSIX standard, adding some improvements such as sparse file handling and incremental archives. Unfortunately these features were implemented in a way incompatible with other archive formats. Archives in 'gnu' format are able to hold file names of unlimited length. oldgnu Format used by GNU 'tar' of versions prior to 1.12. v7 Archive format, compatible with the V7 implementation of tar. This format imposes a number of limitations. The most important of them are: 1. The maximum length of a file name is limited to 99 characters. 2. The maximum length of a symbolic link is limited to 99 characters. 3. It is impossible to store special files (block and character devices, fifos etc.) 4. Maximum value of user or group ID is limited to 2097151 (7777777 octal) 5. V7 archives do not contain symbolic ownership information (user and group name of the file owner). This format has traditionally been used by Automake when producing Makefiles. This practice will change in the future, in the meantime, however this means that projects containing file names more than 99 characters long will not be able to use GNU 'tar' 1.34 and Automake prior to 1.9. ustar Archive format defined by POSIX.1-1988 specification. It stores symbolic ownership information. It is also able to store special files. However, it imposes several restrictions as well: 1. The maximum length of a file name is limited to 256 characters, provided that the file name can be split at a directory separator in two parts, first of them being at most 155 bytes long. So, in most cases the maximum file name length will be shorter than 256 characters. 2. The maximum length of a symbolic link name is limited to 100 characters. 3. Maximum size of a file the archive is able to accommodate is 8GB 4. Maximum value of UID/GID is 2097151. 5. Maximum number of bits in device major and minor numbers is 21. star Format used by Jörg Schilling 'star' implementation. GNU 'tar' is able to read 'star' archives but currently does not produce them. posix Archive format defined by POSIX.1-2001 specification. This is the most flexible and feature-rich format. It does not impose any restrictions on file sizes or file name lengths. This format is quite recent, so not all tar implementations are able to handle it properly. However, this format is designed in such a way that any tar implementation able to read 'ustar' archives will be able to read most 'posix' archives as well, with the only exception that any additional information (such as long file names etc.) will in such case be extracted as plain text files along with the files it refers to. This archive format will be the default format for future versions of GNU 'tar'. The following table summarizes the limitations of each of these formats: Format UID File Size File Name Devn -------------------------------------------------------------------- gnu 1.8e19 Unlimited Unlimited 63 oldgnu 1.8e19 Unlimited Unlimited 63 v7 2097151 8GB 99 n/a ustar 2097151 8GB 256 21 posix Unlimited Unlimited Unlimited Unlimited The default format for GNU 'tar' is defined at compilation time. You may check it by running 'tar --help', and examining the last lines of its output. Usually, GNU 'tar' is configured to create archives in 'gnu' format, however, future version will switch to 'posix'. * Menu: * Compression:: Using Less Space through Compression * Attributes:: Handling File Attributes * Portability:: Making 'tar' Archives More Portable * cpio:: Comparison of 'tar' and 'cpio'  File: tar.info, Node: Compression, Next: Attributes, Up: Formats 8.1 Using Less Space through Compression ======================================== * Menu: * gzip:: Creating and Reading Compressed Archives * sparse:: Archiving Sparse Files  File: tar.info, Node: gzip, Next: sparse, Up: Compression 8.1.1 Creating and Reading Compressed Archives ---------------------------------------------- GNU 'tar' is able to create and read compressed archives. It supports a wide variety of compression programs, namely: 'gzip', 'bzip2', 'lzip', 'lzma', 'lzop', 'zstd', 'xz' and traditional 'compress'. The latter is supported mostly for backward compatibility, and we recommend against using it, because it is by far less effective than the other compression programs(1). Creating a compressed archive is simple: you just specify a "compression option" along with the usual archive creation commands. Available compression options are summarized in the table below: Long Short Archive format --------------------------------------------------------------------------- '--gzip' '-z' 'gzip' '--bzip2' '-j' 'bzip2' '--xz' '-J' 'xz' '--lzip' 'lzip' '--lzma' 'lzma' '--lzop' 'lzop' '--zstd' 'zstd' '--compress' '-Z' 'compress' For example: $ tar czf archive.tar.gz . You can also let GNU 'tar' select the compression program based on the suffix of the archive file name. This is done using '--auto-compress' ('-a') command line option. For example, the following invocation will use 'bzip2' for compression: $ tar caf archive.tar.bz2 . whereas the following one will use 'lzma': $ tar caf archive.tar.lzma . For a complete list of file name suffixes recognized by GNU 'tar', see *note auto-compress::. Reading compressed archive is even simpler: you don't need to specify any additional options as GNU 'tar' recognizes its format automatically. Thus, the following commands will list and extract the archive created in previous example: # List the compressed archive $ tar tf archive.tar.gz # Extract the compressed archive $ tar xf archive.tar.gz The format recognition algorithm is based on "signatures", a special byte sequences in the beginning of file, that are specific for certain compression formats. If this approach fails, 'tar' falls back to using archive name suffix to determine its format (*note auto-compress::, for a list of recognized suffixes). Some compression programs are able to handle different compression formats. GNU 'tar' uses this, if the principal decompressor for the given format is not available. For example, if 'compress' is not installed, 'tar' will try to use 'gzip'. As of version 1.34 the following alternatives are tried(2): Format Main decompressor Alternatives --------------------------------------------------------------------- compress compress gzip lzma lzma xz bzip2 bzip2 lbzip2 The only case when you have to specify a decompression option while reading the archive is when reading from a pipe or from a tape drive that does not support random access. However, in this case GNU 'tar' will indicate which option you should use. For example: $ cat archive.tar.gz | tar tf - tar: Archive is compressed. Use -z option tar: Error is not recoverable: exiting now If you see such diagnostics, just add the suggested option to the invocation of GNU 'tar': $ cat archive.tar.gz | tar tzf - Notice also, that there are several restrictions on operations on compressed archives. First of all, compressed archives cannot be modified, i.e., you cannot update ('--update', alias '-u') them or delete ('--delete') members from them or add ('--append', alias '-r') members to them. Likewise, you cannot append another 'tar' archive to a compressed archive using '--concatenate' ('-A'). Secondly, multi-volume archives cannot be compressed. The following options allow to select a particular compressor program: '-z' '--gzip' '--ungzip' Filter the archive through 'gzip'. '-J' '--xz' Filter the archive through 'xz'. '-j' '--bzip2' Filter the archive through 'bzip2'. '--lzip' Filter the archive through 'lzip'. '--lzma' Filter the archive through 'lzma'. '--lzop' Filter the archive through 'lzop'. '--zstd' Filter the archive through 'zstd'. '-Z' '--compress' '--uncompress' Filter the archive through 'compress'. When any of these options is given, GNU 'tar' searches the compressor binary in the current path and invokes it. The name of the compressor program is specified at compilation time using a corresponding '--with-COMPNAME' option to 'configure', e.g. '--with-bzip2' to select a specific 'bzip2' binary. *Note lbzip2::, for a detailed discussion. The output produced by 'tar --help' shows the actual compressor names along with each of these options. You can use any of these options on physical devices (tape drives, etc.) and remote files as well as on normal files; data to or from such devices or remote files is reblocked by another copy of the 'tar' program to enforce the specified (or default) record size. The default compression parameters are used. You can override them by using the '-I' option (see below), e.g.: $ tar -cf archive.tar.gz -I 'gzip -9 -n' subdir A more traditional way to do this is to use a pipe: $ tar cf - subdir | gzip -9 -n > archive.tar.gz Compressed archives are easily corrupted, because compressed files have little redundancy. The adaptive nature of the compression scheme means that the compression tables are implicitly spread all over the archive. If you lose a few blocks, the dynamic construction of the compression tables becomes unsynchronized, and there is little chance that you could recover later in the archive. Other compression options provide better control over creating compressed archives. These are: '--auto-compress' '-a' Select a compression program to use by the archive file name suffix. The following suffixes are recognized: Suffix Compression program ------------------------------------------------------------------- '.gz' 'gzip' '.tgz' 'gzip' '.taz' 'gzip' '.Z' 'compress' '.taZ' 'compress' '.bz2' 'bzip2' '.tz2' 'bzip2' '.tbz2' 'bzip2' '.tbz' 'bzip2' '.lz' 'lzip' '.lzma' 'lzma' '.tlz' 'lzma' '.lzo' 'lzop' '.xz' 'xz' '.zst' 'zstd' '.tzst' 'zstd' '--use-compress-program=COMMAND' '-I=COMMAND' Use external compression program COMMAND. Use this option if you want to specify options for the compression program, or if you are not happy with the compression program associated with the suffix at compile time, or if you have a compression program that GNU 'tar' does not support. The COMMAND argument is a valid command invocation, as you would type it at the command line prompt, with any additional options as needed. Enclose it in quotes if it contains white space (*note Running External Commands: external.). The COMMAND should follow two conventions: First, when invoked without additional options, it should read data from standard input, compress it and output it on standard output. Secondly, if invoked with the additional '-d' option, it should do exactly the opposite, i.e., read the compressed data from the standard input and produce uncompressed data on the standard output. The latter requirement means that you must not use the '-d' option as a part of the COMMAND itself. The '--use-compress-program' option, in particular, lets you implement your own filters, not necessarily dealing with compression/decompression. For example, suppose you wish to implement PGP encryption on top of compression, using 'gpg' (*note gpg: (gpg)Top.). The following script does that: #! /bin/sh case $1 in -d) gpg --decrypt - | gzip -d -c;; '') gzip -c | gpg -s;; *) echo "Unknown option $1">&2; exit 1;; esac Suppose you name it 'gpgz' and save it somewhere in your 'PATH'. Then the following command will create a compressed archive signed with your private key: $ tar -cf foo.tar.gpgz -Igpgz . Likewise, the command below will list its contents: $ tar -tf foo.tar.gpgz -Igpgz . * Menu: * lbzip2:: Using lbzip2 with GNU 'tar'. ---------- Footnotes ---------- (1) It also had patent problems in the past. (2) To verbosely trace the decompressor selection, use the '--warning=decompress-program' option (*note decompress-program: warnings.).  File: tar.info, Node: lbzip2, Up: gzip 8.1.1.1 Using lbzip2 with GNU 'tar'. .................................... 'Lbzip2' is a multithreaded utility for handling 'bzip2' compression, written by Laszlo Ersek. It makes use of multiple processors to speed up its operation and in general works considerably faster than 'bzip2'. For a detailed description of 'lbzip2' see and lbzip2: parallel bzip2 utility (http://www.linuxinsight.com/lbzip2-parallel-bzip2-utility.html). Recent versions of 'lbzip2' are mostly command line compatible with 'bzip2', which makes it possible to automatically invoke it via the '--bzip2' GNU 'tar' command line option. To do so, GNU 'tar' must be configured with the '--with-bzip2' command line option, like this: $ ./configure --with-bzip2=lbzip2 [OTHER-OPTIONS] Once configured and compiled this way, 'tar --help' will show the following: $ tar --help | grep -- --bzip2 -j, --bzip2 filter the archive through lbzip2 which means that running 'tar --bzip2' will invoke 'lbzip2'.  File: tar.info, Node: sparse, Prev: gzip, Up: Compression 8.1.2 Archiving Sparse Files ---------------------------- Files in the file system occasionally have "holes". A "hole" in a file is a section of the file's contents which was never written. The contents of a hole reads as all zeros. On many operating systems, actual disk storage is not allocated for holes, but they are counted in the length of the file. If you archive such a file, 'tar' could create an archive longer than the original. To have 'tar' attempt to recognize the holes in a file, use '--sparse' ('-S'). When you use this option, then, for any file using less disk space than would be expected from its length, 'tar' searches the file for holes. It then records in the archive for the file where the holes (consecutive stretches of zeros) are, and only archives the "real contents" of the file. On extraction (using '--sparse' is not needed on extraction) any such files have also holes created wherever the holes were found. Thus, if you use '--sparse', 'tar' archives won't take more space than the original. GNU 'tar' uses two methods for detecting holes in sparse files. These methods are described later in this subsection. '-S' '--sparse' This option instructs 'tar' to test each file for sparseness before attempting to archive it. If the file is found to be sparse it is treated specially, thus allowing to decrease the amount of space used by its image in the archive. This option is meaningful only when creating or updating archives. It has no effect on extraction. Consider using '--sparse' when performing file system backups, to avoid archiving the expanded forms of files stored sparsely in the system. Even if your system has no sparse files currently, some may be created in the future. If you use '--sparse' while making file system backups as a matter of course, you can be assured the archive will never take more space on the media than the files take on disk (otherwise, archiving a disk filled with sparse files might take hundreds of tapes). *Note Incremental Dumps::. However, be aware that '--sparse' option may present a serious drawback. Namely, in order to determine the positions of holes in a file 'tar' may have to read it before trying to archive it, so in total the file may be read *twice*. This may happen when your OS or your FS does not support "SEEK_HOLE/SEEK_DATA" feature in "lseek" (See '--hole-detection', below). When using 'POSIX' archive format, GNU 'tar' is able to store sparse files using in three distinct ways, called "sparse formats". A sparse format is identified by its "number", consisting, as usual of two decimal numbers, delimited by a dot. By default, format '1.0' is used. If, for some reason, you wish to use an earlier format, you can select it using '--sparse-version' option. '--sparse-version=VERSION' Select the format to store sparse files in. Valid VERSION values are: '0.0', '0.1' and '1.0'. *Note Sparse Formats::, for a detailed description of each format. Using '--sparse-format' option implies '--sparse'. '--hole-detection=METHOD' Enforce concrete hole detection method. Before the real contents of sparse file are stored, 'tar' needs to gather knowledge about file sparseness. This is because it needs to have the file's map of holes stored into tar header before it starts archiving the file contents. Currently, two methods of hole detection are implemented: * '--hole-detection=seek' Seeking the file for data and holes. It uses enhancement of the 'lseek' system call ('SEEK_HOLE' and 'SEEK_DATA') which is able to reuse file system knowledge about sparse file contents - so the detection is usually very fast. To use this feature, your file system and operating system must support it. At the time of this writing (2015) this feature, in spite of not being accepted by POSIX, is fairly widely supported by different operating systems. * '--hole-detection=raw' Reading byte-by-byte the whole sparse file before the archiving. This method detects holes like consecutive stretches of zeroes. Comparing to the previous method, it is usually much slower, although more portable. When no '--hole-detection' option is given, 'tar' uses the 'seek', if supported by the operating system. Using '--hole-detection' option implies '--sparse'.  File: tar.info, Node: Attributes, Next: Portability, Prev: Compression, Up: Formats 8.2 Handling File Attributes ============================ When 'tar' reads files, it updates their access times. To avoid this, use the '--atime-preserve[=METHOD]' option, which can either reset the access time retroactively or avoid changing it in the first place. '--atime-preserve' '--atime-preserve=replace' '--atime-preserve=system' Preserve the access times of files that are read. This works only for files that you own, unless you have superuser privileges. '--atime-preserve=replace' works on most systems, but it also restores the data modification time and updates the status change time. Hence it doesn't interact with incremental dumps nicely (*note Incremental Dumps::), and it can set access or data modification times incorrectly if other programs access the file while 'tar' is running. '--atime-preserve=system' avoids changing the access time in the first place, if the operating system supports this. Unfortunately, this may or may not work on any given operating system or file system. If 'tar' knows for sure it won't work, it complains right away. Currently '--atime-preserve' with no operand defaults to '--atime-preserve=replace', but this is intended to change to '--atime-preserve=system' when the latter is better-supported. '-m' '--touch' Do not extract data modification time. When this option is used, 'tar' leaves the data modification times of the files it extracts as the times when the files were extracted, instead of setting it to the times recorded in the archive. This option is meaningless with '--list' ('-t'). '--same-owner' Create extracted files with the same ownership they have in the archive. This is the default behavior for the superuser, so this option is meaningful only for non-root users, when 'tar' is executed on those systems able to give files away. This is considered as a security flaw by many people, at least because it makes quite difficult to correctly account users for the disk space they occupy. Also, the 'suid' or 'sgid' attributes of files are easily and silently lost when files are given away. When writing an archive, 'tar' writes the user ID and user name separately. If it can't find a user name (because the user ID is not in '/etc/passwd'), then it does not write one. When restoring, it tries to look the name (if one was written) up in '/etc/passwd'. If it fails, then it uses the user ID stored in the archive instead. '--no-same-owner' '-o' Do not attempt to restore ownership when extracting. This is the default behavior for ordinary users, so this option has an effect only for the superuser. '--numeric-owner' The '--numeric-owner' option allows (ANSI) archives to be written without user/group name information or such information to be ignored when extracting. It effectively disables the generation and/or use of user/group name information. This option forces extraction using the numeric ids from the archive, ignoring the names. This is useful in certain circumstances, when restoring a backup from an emergency floppy with different passwd/group files for example. It is otherwise impossible to extract files with the right ownerships if the password file in use during the extraction does not match the one belonging to the file system(s) being extracted. This occurs, for example, if you are restoring your files after a major crash and had booted from an emergency floppy with no password file or put your disk into another machine to do the restore. The numeric ids are _always_ saved into 'tar' archives. The identifying names are added at create time when provided by the system, unless '--format=oldgnu' is used. Numeric ids could be used when moving archives between a collection of machines using a centralized management for attribution of numeric ids to users and groups. This is often made through using the NIS capabilities. When making a 'tar' file for distribution to other sites, it is sometimes cleaner to use a single owner for all files in the distribution, and nicer to specify the write permission bits of the files as stored in the archive independently of their actual value on the file system. The way to prepare a clean distribution is usually to have some Makefile rule creating a directory, copying all needed files in that directory, then setting ownership and permissions as wanted (there are a lot of possible schemes), and only then making a 'tar' archive out of this directory, before cleaning everything out. Of course, we could add a lot of options to GNU 'tar' for fine tuning permissions and ownership. This is not the good way, I think. GNU 'tar' is already crowded with options and moreover, the approach just explained gives you a great deal of control already. '-p' '--same-permissions' '--preserve-permissions' Extract all protection information. This option causes 'tar' to set the modes (access permissions) of extracted files exactly as recorded in the archive. If this option is not used, the current 'umask' setting limits the permissions on extracted files. This option is by default enabled when 'tar' is executed by a superuser. This option is meaningless with '--list' ('-t').  File: tar.info, Node: Portability, Next: cpio, Prev: Attributes, Up: Formats 8.3 Making 'tar' Archives More Portable ======================================= Creating a 'tar' archive on a particular system that is meant to be useful later on many other machines and with other versions of 'tar' is more challenging than you might think. 'tar' archive formats have been evolving since the first versions of Unix. Many such formats are around, and are not always compatible with each other. This section discusses a few problems, and gives some advice about making 'tar' archives more portable. One golden rule is simplicity. For example, limit your 'tar' archives to contain only regular files and directories, avoiding other kind of special files. Do not attempt to save sparse files or contiguous files as such. Let's discuss a few more problems, in turn. * Menu: * Portable Names:: Portable Names * dereference:: Symbolic Links * hard links:: Hard Links * old:: Old V7 Archives * ustar:: Ustar Archives * gnu:: GNU and old GNU format archives. * posix:: POSIX archives * Checksumming:: Checksumming Problems * Large or Negative Values:: Large files, negative time stamps, etc. * Other Tars:: How to Extract GNU-Specific Data Using Other 'tar' Implementations  File: tar.info, Node: Portable Names, Next: dereference, Up: Portability 8.3.1 Portable Names -------------------- Use portable file and member names. A name is portable if it contains only ASCII letters and digits, '/', '.', '_', and '-'; it cannot be empty, start with '-' or '//', or contain '/-'. Avoid deep directory nesting. For portability to old Unix hosts, limit your file name components to 14 characters or less. If you intend to have your 'tar' archives to be read under MSDOS, you should not rely on case distinction for file names, and you might use the GNU 'doschk' program for helping you further diagnosing illegal MSDOS names, which are even more limited than System V's.  File: tar.info, Node: dereference, Next: hard links, Prev: Portable Names, Up: Portability 8.3.2 Symbolic Links -------------------- Normally, when 'tar' archives a symbolic link, it writes a block to the archive naming the target of the link. In that way, the 'tar' archive is a faithful record of the file system contents. When '--dereference' ('-h') is used with '--create' ('-c'), 'tar' archives the files symbolic links point to, instead of the links themselves. When creating portable archives, use '--dereference' ('-h'): some systems do not support symbolic links, and moreover, your distribution might be unusable if it contains unresolved symbolic links. When reading from an archive, the '--dereference' ('-h') option causes 'tar' to follow an already-existing symbolic link when 'tar' writes or reads a file named in the archive. Ordinarily, 'tar' does not follow such a link, though it may remove the link before writing a new file. *Note Dealing with Old Files::. The '--dereference' option is unsafe if an untrusted user can modify directories while 'tar' is running. *Note Security::.  File: tar.info, Node: hard links, Next: old, Prev: dereference, Up: Portability 8.3.3 Hard Links ---------------- Normally, when 'tar' archives a hard link, it writes a block to the archive naming the target of the link (a '1' type block). In that way, the actual file contents is stored in file only once. For example, consider the following two files: $ ls -l -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden Here, 'jeden' is a link to 'one'. When archiving this directory with a verbose level 2, you will get an output similar to the following: $ tar cvvf ../archive.tar . drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./ -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden The last line shows that, instead of storing two copies of the file, 'tar' stored it only once, under the name 'jeden', and stored file 'one' as a hard link to this file. It may be important to know that all hard links to the given file are stored in the archive. For example, this may be necessary for exact reproduction of the file system. The following option does that: '--check-links' '-l' Check the number of links dumped for each processed file. If this number does not match the total number of hard links for the file, print a warning message. For example, trying to archive only file 'jeden' with this option produces the following diagnostics: $ tar -c -f ../archive.tar -l jeden tar: Missing links to 'jeden'. Although creating special records for hard links helps keep a faithful record of the file system contents and makes archives more compact, it may present some difficulties when extracting individual members from the archive. For example, trying to extract file 'one' from the archive created in previous examples produces, in the absence of file 'jeden': $ tar xf archive.tar ./one tar: ./one: Cannot hard link to './jeden': No such file or directory tar: Error exit delayed from previous errors The reason for this behavior is that 'tar' cannot seek back in the archive to the previous member (in this case, 'one'), to extract it(1). If you wish to avoid such problems at the cost of a bigger archive, use the following option: '--hard-dereference' Dereference hard links and store the files they refer to. For example, trying this option on our two sample files, we get two copies in the archive, each of which can then be extracted independently of the other: $ tar -c -vv -f ../archive.tar --hard-dereference . drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./ -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one ---------- Footnotes ---------- (1) There are plans to fix this in future releases.  File: tar.info, Node: old, Next: ustar, Prev: hard links, Up: Portability 8.3.4 Old V7 Archives --------------------- Certain old versions of 'tar' cannot handle additional information recorded by newer 'tar' programs. To create an archive in V7 format (not ANSI), which can be read by these old versions, specify the '--format=v7' option in conjunction with the '--create' ('-c') ('tar' also accepts '--portability' or '--old-archive' for this option). When you specify it, 'tar' leaves out information about directories, pipes, fifos, contiguous files, and device files, and specifies file ownership by group and user IDs instead of group and user names. When updating an archive, do not use '--format=v7' unless the archive was created using this option. In most cases, a _new_ format archive can be read by an _old_ 'tar' program without serious trouble, so this option should seldom be needed. On the other hand, most modern 'tar's are able to read old format archives, so it might be safer for you to always use '--format=v7' for your distributions. Notice, however, that 'ustar' format is a better alternative, as it is free from many of 'v7''s drawbacks.  File: tar.info, Node: ustar, Next: gnu, Prev: old, Up: Portability 8.3.5 Ustar Archive Format -------------------------- The archive format defined by the POSIX.1-1988 specification is called 'ustar'. Although it is more flexible than the V7 format, it still has many restrictions (*note ustar: Formats, for the detailed description of 'ustar' format). Along with V7 format, 'ustar' format is a good choice for archives intended to be read with other implementations of 'tar'. To create an archive in 'ustar' format, use the '--format=ustar' option in conjunction with '--create' ('-c').  File: tar.info, Node: gnu, Next: posix, Prev: ustar, Up: Portability 8.3.6 GNU and old GNU 'tar' format ---------------------------------- GNU 'tar' was based on an early draft of the POSIX 1003.1 'ustar' standard. GNU extensions to 'tar', such as the support for file names longer than 100 characters, use portions of the 'tar' header record which were specified in that POSIX draft as unused. Subsequent changes in POSIX have allocated the same parts of the header record for other purposes. As a result, GNU 'tar' format is incompatible with the current POSIX specification, and with 'tar' programs that follow it. In the majority of cases, 'tar' will be configured to create this format by default. This will change in future releases, since we plan to make 'POSIX' format the default. To force creation a GNU 'tar' archive, use option '--format=gnu'.  File: tar.info, Node: posix, Next: Checksumming, Prev: gnu, Up: Portability 8.3.7 GNU 'tar' and POSIX 'tar' ------------------------------- Starting from version 1.14 GNU 'tar' features full support for POSIX.1-2001 archives. A POSIX conformant archive will be created if 'tar' was given '--format=posix' ('--format=pax') option. No special option is required to read and extract from a POSIX archive. * Menu: * PAX keywords:: Controlling Extended Header Keywords.  File: tar.info, Node: PAX keywords, Up: posix 8.3.7.1 Controlling Extended Header Keywords ............................................ '--pax-option=KEYWORD-LIST' Handle keywords in PAX extended headers. This option is equivalent to '-o' option of the 'pax' utility. KEYWORD-LIST is a comma-separated list of keyword options, each keyword option taking one of the following forms: 'delete=PATTERN' When used with one of archive-creation commands, this option instructs 'tar' to omit from extended header records that it produces any keywords matching the string PATTERN. When used in extract or list mode, this option instructs tar to ignore any keywords matching the given PATTERN in the extended header records. In both cases, matching is performed using the pattern matching notation described in POSIX 1003.2, 3.13 (*note wildcards::). For example: --pax-option delete=security.* would suppress security-related information. 'exthdr.name=STRING' This keyword allows user control over the name that is written into the ustar header blocks for the extended headers. The name is obtained from STRING after making the following substitutions: Meta-character Replaced By ------------------------------------------------------------ %d The directory name of the file, equivalent to the result of the 'dirname' utility on the translated file name. %f The name of the file with the directory information stripped, equivalent to the result of the 'basename' utility on the translated file name. %p The process ID of the 'tar' process. %% A '%' character. Any other '%' characters in STRING produce undefined results. If no option 'exthdr.name=string' is specified, 'tar' will use the following default value: %d/PaxHeaders/%f This default is selected to ensure the reproducibility of the archive. POSIX standard recommends to use '%d/PaxHeaders.%p/%f' instead, which means the two archives created with the same set of options and containing the same set of files will be byte-to-byte different. This default will be used if the environment variable 'POSIXLY_CORRECT' is set. 'exthdr.mtime=VALUE' This keyword defines the value of the 'mtime' field that is written into the ustar header blocks for the extended headers. By default, the 'mtime' field is set to the modification time of the archive member described by that extended header (or to the value of the '--mtime' option, if supplied). 'globexthdr.name=STRING' This keyword allows user control over the name that is written into the ustar header blocks for global extended header records. The name is obtained from the contents of STRING, after making the following substitutions: Meta-character Replaced By ------------------------------------------------------------ %n An integer that represents the sequence number of the global extended header record in the archive, starting at 1. %p The process ID of the 'tar' process. %% A '%' character. Any other '%' characters in STRING produce undefined results. If no option 'globexthdr.name=string' is specified, 'tar' will use the following default value: $TMPDIR/GlobalHead.%n If the environment variable 'POSIXLY_CORRECT' is set, the following value is used instead: $TMPDIR/GlobalHead.%p.%n In both cases, '$TMPDIR' stands for the value of the TMPDIR environment variable. If TMPDIR is not set, 'tar' uses '/tmp'. 'globexthdr.mtime=VALUE' This keyword defines the value of the 'mtime' field that is written into the ustar header blocks for the global extended headers. By default, the 'mtime' field is set to the time when 'tar' was invoked. 'KEYWORD=VALUE' When used with one of archive-creation commands, these keyword/value pairs will be included at the beginning of the archive in a global extended header record. When used with one of archive-reading commands, 'tar' will behave as if it has encountered these keyword/value pairs at the beginning of the archive in a global extended header record. 'KEYWORD:=VALUE' When used with one of archive-creation commands, these keyword/value pairs will be included as records at the beginning of an extended header for each file. This is effectively equivalent to KEYWORD=VALUE form except that it creates no global extended header records. When used with one of archive-reading commands, 'tar' will behave as if these keyword/value pairs were included as records at the end of each extended header; thus, they will override any global or file-specific extended header record keywords of the same names. For example, in the command: tar --format=posix --create \ --file archive --pax-option gname:=user . the group name will be forced to a new value for all files stored in the archive. In any of the forms described above, the VALUE may be a string enclosed in curly braces. In that case, the string between the braces is understood either as a textual time representation, as described in *note Date input formats::, or a name of the existing file, starting with '/' or '.'. In the latter case, the modification time of that file is used. For example, to set all modification times to the current date, you use the following option: --pax-option='mtime:={now}' Note quoting of the option's argument. As another example, here is the option that ensures that any two archives created using it, will be binary equivalent if they have the same contents: --pax-option=atime:=0 If you extract files from such an archive and recreate the archive from them, you will also need to eliminate changes due to ctime, as shown in examples below: --pax-option=atime:=0,ctime:=0 or --pax-option=atime:=0,delete=ctime Notice, that if you create an archive in POSIX format (*note posix::) and the environment variable 'POSIXLY_CORRECT' is set, then the two archives created using the same options on the same set of files will not be byte-to-byte equivalent even with the above option. This is because the posix default for extended header names includes the PID of the tar process, which is different at each run. To produce byte-to-byte equivalent archives in this case, either unset 'POSIXLY_CORRECT', or use the following option: ---pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0,ctime:=0  File: tar.info, Node: Checksumming, Next: Large or Negative Values, Prev: posix, Up: Portability 8.3.8 Checksumming Problems --------------------------- SunOS and HP-UX 'tar' fail to accept archives created using GNU 'tar' and containing non-ASCII file names, that is, file names having characters with the eighth bit set, because they use signed checksums, while GNU 'tar' uses unsigned checksums while creating archives, as per POSIX standards. On reading, GNU 'tar' computes both checksums and accepts either of them. It is somewhat worrying that a lot of people may go around doing backup of their files using faulty (or at least non-standard) software, not learning about it until it's time to restore their missing files with an incompatible file extractor, or vice versa. GNU 'tar' computes checksums both ways, and accepts either of them on read, so GNU tar can read Sun tapes even with their wrong checksums. GNU 'tar' produces the standard checksum, however, raising incompatibilities with Sun. That is to say, GNU 'tar' has not been modified to _produce_ incorrect archives to be read by buggy 'tar''s. I've been told that more recent Sun 'tar' now read standard archives, so maybe Sun did a similar patch, after all? The story seems to be that when Sun first imported 'tar' sources on their system, they recompiled it without realizing that the checksums were computed differently, because of a change in the default signing of 'char''s in their compiler. So they started computing checksums wrongly. When they later realized their mistake, they merely decided to stay compatible with it, and with themselves afterwards. Presumably, but I do not really know, HP-UX has chosen their 'tar' archives to be compatible with Sun's. The current standards do not favor Sun 'tar' format. In any case, it now falls on the shoulders of SunOS and HP-UX users to get a 'tar' able to read the good archives they receive.  File: tar.info, Node: Large or Negative Values, Next: Other Tars, Prev: Checksumming, Up: Portability 8.3.9 Large or Negative Values ------------------------------ _(This message will disappear, once this node revised.)_ The above sections suggest to use 'oldest possible' archive format if in doubt. However, sometimes it is not possible. If you attempt to archive a file whose metadata cannot be represented using required format, GNU 'tar' will print error message and ignore such a file. You will than have to switch to a format that is able to handle such values. The format summary table (*note Formats::) will help you to do so. In particular, when trying to archive files larger than 8GB or with timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16 12:56:31 UTC, you will have to chose between GNU and POSIX archive formats. When considering which format to choose, bear in mind that the GNU format uses two's-complement base-256 notation to store values that do not fit into standard ustar range. Such archives can generally be read only by a GNU 'tar' implementation. Moreover, they sometimes cannot be correctly restored on another hosts even by GNU 'tar'. For example, using two's complement representation for negative time stamps that assumes a signed 32-bit 'time_t' generates archives that are not portable to hosts with differing 'time_t' representations. On the other hand, POSIX archives, generally speaking, can be extracted by any tar implementation that understands older ustar format. The only exception are files larger than 8GB.  File: tar.info, Node: Other Tars, Prev: Large or Negative Values, Up: Portability 8.3.10 How to Extract GNU-Specific Data Using Other 'tar' Implementations ------------------------------------------------------------------------- In previous sections you became acquainted with various quirks necessary to make your archives portable. Sometimes you may need to extract archives containing GNU-specific members using some third-party 'tar' implementation or an older version of GNU 'tar'. Of course your best bet is to have GNU 'tar' installed, but if it is for some reason impossible, this section will explain how to cope without it. When we speak about "GNU-specific" members we mean two classes of them: members split between the volumes of a multi-volume archive and sparse members. You will be able to always recover such members if the archive is in PAX format. In addition split members can be recovered from archives in old GNU format. The following subsections describe the required procedures in detail. * Menu: * Split Recovery:: Members Split Between Volumes * Sparse Recovery:: Sparse Members  File: tar.info, Node: Split Recovery, Next: Sparse Recovery, Up: Other Tars 8.3.10.1 Extracting Members Split Between Volumes ................................................. If a member is split between several volumes of an old GNU format archive most third party 'tar' implementation will fail to extract it. To extract it, use 'tarcat' program (*note Tarcat::). This program is available from GNU 'tar' home page (http://www.gnu.org/software/tar/utils/tarcat.html). It concatenates several archive volumes into a single valid archive. For example, if you have three volumes named from 'vol-1.tar' to 'vol-3.tar', you can do the following to extract them using a third-party 'tar': $ tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf - You could use this approach for most (although not all) PAX format archives as well. However, extracting split members from a PAX archive is a much easier task, because PAX volumes are constructed in such a way that each part of a split member is extracted to a different file by 'tar' implementations that are not aware of GNU extensions. More specifically, the very first part retains its original name, and all subsequent parts are named using the pattern: %d/GNUFileParts/%f.%n where symbols preceded by '%' are "macro characters" that have the following meaning: Meta-character Replaced By ------------------------------------------------------------ %d The directory name of the file, equivalent to the result of the 'dirname' utility on its full name. %f The file name of the file, equivalent to the result of the 'basename' utility on its full name. %p The process ID of the 'tar' process that created the archive. %n Ordinal number of this particular part. For example, if the file 'var/longfile' was split during archive creation between three volumes, then the member names will be: var/longfile var/GNUFileParts/longfile.1 var/GNUFileParts/longfile.2 When you extract your archive using a third-party 'tar', these files will be created on your disk, and the only thing you will need to do to restore your file in its original form is concatenate them in the proper order, for example: $ cd var $ cat GNUFileParts/longfile.1 \ GNUFileParts/longfile.2 >> longfile $ rm -f GNUFileParts Notice, that if the 'tar' implementation you use supports PAX format archives, it will probably emit warnings about unknown keywords during extraction. They will look like this: Tar file too small Unknown extended header keyword 'GNU.volume.filename' ignored. Unknown extended header keyword 'GNU.volume.size' ignored. Unknown extended header keyword 'GNU.volume.offset' ignored. You can safely ignore these warnings. If your 'tar' implementation is not PAX-aware, you will get more warnings and more files generated on your disk, e.g.: $ tar xf vol-1.tar var/PaxHeaders/longfile: Unknown file type 'x', extracted as normal file Unexpected EOF in archive $ tar xf vol-2.tar tmp/GlobalHead.1: Unknown file type 'g', extracted as normal file GNUFileParts/PaxHeaders/sparsefile.1: Unknown file type 'x', extracted as normal file Ignore these warnings. The 'PaxHeaders.*' directories created will contain files with "extended header keywords" describing the extracted files. You can delete them, unless they describe sparse members. Read further to learn more about them.  File: tar.info, Node: Sparse Recovery, Prev: Split Recovery, Up: Other Tars 8.3.10.2 Extracting Sparse Members .................................. Any 'tar' implementation will be able to extract sparse members from a PAX archive. However, the extracted files will be "condensed", i.e., any zero blocks will be removed from them. When we restore such a condensed file to its original form, by adding zero blocks (or "holes") back to their original locations, we call this process "expanding" a compressed sparse file. To expand a file, you will need a simple auxiliary program called 'xsparse'. It is available in source form from GNU 'tar' home page (http://www.gnu.org/software/tar/utils/xsparse.html). Let's begin with archive members in "sparse format version 1.0"(1), which are the easiest to expand. The condensed file will contain both file map and file data, so no additional data will be needed to restore it. If the original file name was 'DIR/NAME', then the condensed file will be named 'DIR/GNUSparseFile.N/NAME', where N is a decimal number(2). To expand a version 1.0 file, run 'xsparse' as follows: $ xsparse cond-file where 'cond-file' is the name of the condensed file. The utility will deduce the name for the resulting expanded file using the following algorithm: 1. If 'cond-file' does not contain any directories, '../cond-file' will be used; 2. If 'cond-file' has the form 'DIR/T/NAME', where both T and NAME are simple names, with no '/' characters in them, the output file name will be 'DIR/NAME'. 3. Otherwise, if 'cond-file' has the form 'DIR/NAME', the output file name will be 'NAME'. In the unlikely case when this algorithm does not suit your needs, you can explicitly specify output file name as a second argument to the command: $ xsparse cond-file out-file It is often a good idea to run 'xsparse' in "dry run" mode first. In this mode, the command does not actually expand the file, but verbosely lists all actions it would be taking to do so. The dry run mode is enabled by '-n' command line argument: $ xsparse -n /home/gray/GNUSparseFile.6058/sparsefile Reading v.1.0 sparse map Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to '/home/gray/sparsefile' Finished dry run To actually expand the file, you would run: $ xsparse /home/gray/GNUSparseFile.6058/sparsefile The program behaves the same way all UNIX utilities do: it will keep quiet unless it has something important to tell you (e.g. an error condition or something). If you wish it to produce verbose output, similar to that from the dry run mode, use '-v' option: $ xsparse -v /home/gray/GNUSparseFile.6058/sparsefile Reading v.1.0 sparse map Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to '/home/gray/sparsefile' Done Additionally, if your 'tar' implementation has extracted the "extended headers" for this file, you can instruct 'xstar' to use them in order to verify the integrity of the expanded file. The option '-x' sets the name of the extended header file to use. Continuing our example: $ xsparse -v -x /home/gray/PaxHeaders/sparsefile \ /home/gray/GNUSparseFile/sparsefile Reading extended header file Found variable GNU.sparse.major = 1 Found variable GNU.sparse.minor = 0 Found variable GNU.sparse.name = sparsefile Found variable GNU.sparse.realsize = 217481216 Reading v.1.0 sparse map Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to '/home/gray/sparsefile' Done An "extended header" is a special 'tar' archive header that precedes an archive member and contains a set of "variables", describing the member properties that cannot be stored in the standard 'ustar' header. While optional for expanding sparse version 1.0 members, the use of extended headers is mandatory when expanding sparse members in older sparse formats: v.0.0 and v.0.1 (The sparse formats are described in detail in *note Sparse Formats::.) So, for these formats, the question is: how to obtain extended headers from the archive? If you use a 'tar' implementation that does not support PAX format, extended headers for each member will be extracted as a separate file. If we represent the member name as 'DIR/NAME', then the extended header file will be named 'DIR/PaxHeaders/NAME'. Things become more difficult if your 'tar' implementation does support PAX headers, because in this case you will have to manually extract the headers. We recommend the following algorithm: 1. Consult the documentation of your 'tar' implementation for an option that prints "block numbers" along with the archive listing (analogous to GNU 'tar''s '-R' option). For example, 'star' has '-block-number'. 2. Obtain verbose listing using the 'block number' option, and find block numbers of the sparse member in question and the member immediately following it. For example, running 'star' on our archive we obtain: $ star -t -v -block-number -f arc.tar ... star: Unknown extended header keyword 'GNU.sparse.size' ignored. star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored. star: Unknown extended header keyword 'GNU.sparse.name' ignored. star: Unknown extended header keyword 'GNU.sparse.map' ignored. block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README ... (as usual, ignore the warnings about unknown keywords.) 3. Let SIZE be the size of the sparse member, BS be its block number and BN be the block number of the next member. Compute: N = BS - BN - SIZE/512 - 2 This number gives the size of the extended header part in tar "blocks". In our example, this formula gives: '897 - 56 - 425984 / 512 - 2 = 7'. 4. Use 'dd' to extract the headers: dd if=ARCHIVE of=HNAME bs=512 skip=BS count=N where ARCHIVE is the archive name, HNAME is a name of the file to store the extended header in, BS and N are computed in previous steps. In our example, this command will be $ dd if=arc.tar of=xhdr bs=512 skip=56 count=7 Finally, you can expand the condensed file, using the obtained header: $ xsparse -v -x xhdr GNUSparseFile.6058/sparsefile Reading extended header file Found variable GNU.sparse.size = 217481216 Found variable GNU.sparse.numblocks = 208 Found variable GNU.sparse.name = sparsefile Found variable GNU.sparse.map = 0,2048,1050624,2048,... Expanding file 'GNUSparseFile.28124/sparsefile' to 'sparsefile' Done ---------- Footnotes ---------- (1) *Note PAX 1::. (2) Technically speaking, N is a "process ID" of the 'tar' process which created the archive (*note PAX keywords::).  File: tar.info, Node: cpio, Prev: Portability, Up: Formats 8.4 Comparison of 'tar' and 'cpio' ================================== _(This message will disappear, once this node revised.)_ The 'cpio' archive formats, like 'tar', do have maximum file name lengths. The binary and old ASCII formats have a maximum file length of 256, and the new ASCII and CRC ASCII formats have a max file length of 1024. GNU 'cpio' can read and write archives with arbitrary file name lengths, but other 'cpio' implementations may crash unexplainedly trying to read them. 'tar' handles symbolic links in the form in which it comes in BSD; 'cpio' doesn't handle symbolic links in the form in which it comes in System V prior to SVR4, and some vendors may have added symlinks to their system without enhancing 'cpio' to know about them. Others may have enhanced it in a way other than the way I did it at Sun, and which was adopted by AT&T (and which is, I think, also present in the 'cpio' that Berkeley picked up from AT&T and put into a later BSD release--I think I gave them my changes). (SVR4 does some funny stuff with 'tar'; basically, its 'cpio' can handle 'tar' format input, and write it on output, and it probably handles symbolic links. They may not have bothered doing anything to enhance 'tar' as a result.) 'cpio' handles special files; traditional 'tar' doesn't. 'tar' comes with V7, System III, System V, and BSD source; 'cpio' comes only with System III, System V, and later BSD (4.3-tahoe and later). 'tar''s way of handling multiple hard links to a file can handle file systems that support 32-bit i-numbers (e.g., the BSD file system); 'cpio's way requires you to play some games (in its "binary" format, i-numbers are only 16 bits, and in its "portable ASCII" format, they're 18 bits--it would have to play games with the "file system ID" field of the header to make sure that the file system ID/i-number pairs of different files were always different), and I don't know which 'cpio's, if any, play those games. Those that don't might get confused and think two files are the same file when they're not, and make hard links between them. 'tar's way of handling multiple hard links to a file places only one copy of the link on the tape, but the name attached to that copy is the _only_ one you can use to retrieve the file; 'cpio's way puts one copy for every link, but you can retrieve it using any of the names. What type of check sum (if any) is used, and how is this calculated. See the attached manual pages for 'tar' and 'cpio' format. 'tar' uses a checksum which is the sum of all the bytes in the 'tar' header for a file; 'cpio' uses no checksum. If anyone knows why 'cpio' was made when 'tar' was present at the unix scene, It wasn't. 'cpio' first showed up in PWB/UNIX 1.0; no generally-available version of UNIX had 'tar' at the time. I don't know whether any version that was generally available _within AT&T_ had 'tar', or, if so, whether the people within AT&T who did 'cpio' knew about it. On restore, if there is a corruption on a tape 'tar' will stop at that point, while 'cpio' will skip over it and try to restore the rest of the files. The main difference is just in the command syntax and header format. 'tar' is a little more tape-oriented in that everything is blocked to start on a record boundary. Is there any differences between the ability to recover crashed archives between the two of them. (Is there any chance of recovering crashed archives at all.) Theoretically it should be easier under 'tar' since the blocking lets you find a header with some variation of 'dd skip=NN'. However, modern 'cpio''s and variations have an option to just search for the next file header after an error with a reasonable chance of resyncing. However, lots of tape driver software won't allow you to continue past a media error which should be the only reason for getting out of sync unless a file changed sizes while you were writing the archive. If anyone knows why 'cpio' was made when 'tar' was present at the unix scene, please tell me about this too. Probably because it is more media efficient (by not blocking everything and using only the space needed for the headers where 'tar' always uses 512 bytes per file header) and it knows how to archive special files. You might want to look at the freely available alternatives. The major ones are 'afio', GNU 'tar', and 'pax', each of which have their own extensions with some backwards compatibility. Sparse files were 'tar'red as sparse files (which you can easily test, because the resulting archive gets smaller, and GNU 'cpio' can no longer read it).  File: tar.info, Node: Media, Next: Reliability and security, Prev: Formats, Up: Top 9 Tapes and Other Archive Media ******************************* _(This message will disappear, once this node revised.)_ A few special cases about tape handling warrant more detailed description. These special cases are discussed below. Many complexities surround the use of 'tar' on tape drives. Since the creation and manipulation of archives located on magnetic tape was the original purpose of 'tar', it contains many features making such manipulation easier. Archives are usually written on dismountable media--tape cartridges, mag tapes, or floppy disks. The amount of data a tape or disk holds depends not only on its size, but also on how it is formatted. A 2400 foot long reel of mag tape holds 40 megabytes of data when formatted at 1600 bits per inch. The physically smaller EXABYTE tape cartridge holds 2.3 gigabytes. Magnetic media are re-usable--once the archive on a tape is no longer needed, the archive can be erased and the tape or disk used over. Media quality does deteriorate with use, however. Most tapes or disks should be discarded when they begin to produce data errors. EXABYTE tape cartridges should be discarded when they generate an "error count" (number of non-usable bits) of more than 10k. Magnetic media are written and erased using magnetic fields, and should be protected from such fields to avoid damage to stored data. Sticking a floppy disk to a filing cabinet using a magnet is probably not a good idea. * Menu: * Device:: Device selection and switching * Remote Tape Server:: * Common Problems and Solutions:: * Blocking:: Blocking * Many:: Many archives on one tape * Using Multiple Tapes:: Using Multiple Tapes * label:: Including a Label in the Archive * verify:: * Write Protection::  File: tar.info, Node: Device, Next: Remote Tape Server, Up: Media 9.1 Device Selection and Switching ================================== _(This message will disappear, once this node revised.)_ '-f [HOSTNAME:]FILE' '--file=[HOSTNAME:]FILE' Use archive file or device FILE on HOSTNAME. This option is used to specify the file name of the archive 'tar' works on. If the file name is '-', 'tar' reads the archive from standard input (when listing or extracting), or writes it to standard output (when creating). If the '-' file name is given when updating an archive, 'tar' will read the original archive from its standard input, and will write the entire new archive to its standard output. If the file name contains a ':', it is interpreted as 'hostname:file name'. If the HOSTNAME contains an "at" sign ('@'), it is treated as 'user@hostname:file name'. In either case, 'tar' will invoke the command 'rsh' (or 'remsh') to start up an '/usr/libexec/rmt' on the remote machine. If you give an alternate login name, it will be given to the 'rsh'. Naturally, the remote machine must have an executable '/usr/libexec/rmt'. This program is free software from the University of California, and a copy of the source code can be found with the sources for 'tar'; it's compiled and installed by default. The exact path to this utility is determined when configuring the package. It is 'PREFIX/libexec/rmt', where PREFIX stands for your installation prefix. This location may also be overridden at runtime by using the '--rmt-command=COMMAND' option (*Note --rmt-command: Option Summary, for detailed description of this option. *Note Remote Tape Server::, for the description of 'rmt' command). If this option is not given, but the environment variable 'TAPE' is set, its value is used; otherwise, old versions of 'tar' used a default archive name (which was picked when 'tar' was compiled). The default is normally set up to be the "first" tape drive or other transportable I/O medium on the system. Starting with version 1.11.5, GNU 'tar' uses standard input and standard output as the default device, and I will not try anymore supporting automatic device detection at installation time. This was failing really in too many cases, it was hopeless. This is now completely left to the installer to override standard input and standard output for default device, if this seems preferable. Further, I think _most_ actual usages of 'tar' are done with pipes or disks, not really tapes, cartridges or diskettes. Some users think that using standard input and output is running after trouble. This could lead to a nasty surprise on your screen if you forget to specify an output file name--especially if you are going through a network or terminal server capable of buffering large amounts of output. We had so many bug reports in that area of configuring default tapes automatically, and so many contradicting requests, that we finally consider the problem to be portably intractable. We could of course use something like '/dev/tape' as a default, but this is _also_ running after various kind of trouble, going from hung processes to accidental destruction of real tapes. After having seen all this mess, using standard input and output as a default really sounds like the only clean choice left, and a very useful one too. GNU 'tar' reads and writes archive in records, I suspect this is the main reason why block devices are preferred over character devices. Most probably, block devices are more efficient too. The installer could also check for 'DEFTAPE' in ''. '--force-local' Archive file is local even if it contains a colon. '--rsh-command=COMMAND' Use remote COMMAND instead of 'rsh'. This option exists so that people who use something other than the standard 'rsh' (e.g., a Kerberized 'rsh') can access a remote device. When this command is not used, the shell command found when the 'tar' program was installed is used instead. This is the first found of '/usr/ucb/rsh', '/usr/bin/remsh', '/usr/bin/rsh', '/usr/bsd/rsh' or '/usr/bin/nsh'. The installer may have overridden this by defining the environment variable 'RSH' _at installation time_. '-[0-7][lmh]' Specify drive and density. '-M' '--multi-volume' Create/list/extract multi-volume archive. This option causes 'tar' to write a "multi-volume" archive--one that may be larger than will fit on the medium used to hold it. *Note Multi-Volume Archives::. '-L NUM' '--tape-length=SIZE[SUF]' Change tape after writing SIZE units of data. Unless SUF is given, SIZE is treated as kilobytes, i.e. 'SIZE x 1024' bytes. The following suffixes alter this behavior: Suffix Units Byte Equivalent ------------------------------------------------------------- b Blocks SIZE x 512 B Kilobytes SIZE x 1024 c Bytes SIZE G Gigabytes SIZE x 1024^3 K Kilobytes SIZE x 1024 k Kilobytes SIZE x 1024 M Megabytes SIZE x 1024^2 P Petabytes SIZE x 1024^5 T Terabytes SIZE x 1024^4 w Words SIZE x 2 Table 9.1: Size Suffixes This option might be useful when your tape drivers do not properly detect end of physical tapes. By being slightly conservative on the maximum tape length, you might avoid the problem entirely. '-F COMMAND' '--info-script=COMMAND' '--new-volume-script=COMMAND' Execute COMMAND at end of each tape. This implies '--multi-volume' ('-M'). *Note info-script::, for a detailed description of this option.  File: tar.info, Node: Remote Tape Server, Next: Common Problems and Solutions, Prev: Device, Up: Media 9.2 Remote Tape Server ====================== In order to access the tape drive on a remote machine, 'tar' uses the remote tape server written at the University of California at Berkeley. The remote tape server must be installed as 'PREFIX/libexec/rmt' on any machine whose tape drive you want to use. 'tar' calls 'rmt' by running an 'rsh' or 'remsh' to the remote machine, optionally using a different login name if one is supplied. A copy of the source for the remote tape server is provided. Its source code can be freely distributed. It is compiled and installed by default. Unless you use the '--absolute-names' ('-P') option, GNU 'tar' will not allow you to create an archive that contains absolute file names (a file name beginning with '/'). If you try, 'tar' will automatically remove the leading '/' from the file names it stores in the archive. It will also type a warning message telling you what it is doing. When reading an archive that was created with a different 'tar' program, GNU 'tar' automatically extracts entries in the archive which have absolute file names as if the file names were not absolute. This is an important feature. A visitor here once gave a 'tar' tape to an operator to restore; the operator used Sun 'tar' instead of GNU 'tar', and the result was that it replaced large portions of our '/bin' and friends with versions from the tape; needless to say, we were unhappy about having to recover the file system from backup tapes. For example, if the archive contained a file '/usr/bin/computoy', GNU 'tar' would extract the file to 'usr/bin/computoy', relative to the current directory. If you want to extract the files in an archive to the same absolute names that they had when the archive was created, you should do a 'cd /' before extracting the files from the archive, or you should either use the '--absolute-names' option, or use the command 'tar -C / ...'. Some versions of Unix (Ultrix 3.1 is known to have this problem), can claim that a short write near the end of a tape succeeded, when it actually failed. This will result in the -M option not working correctly. The best workaround at the moment is to use a significantly larger blocking factor than the default 20. In order to update an archive, 'tar' must be able to backspace the archive in order to reread or rewrite a record that was just read (or written). This is currently possible only on two kinds of files: normal disk files (or any other file that can be backspaced with 'lseek'), and industry-standard 9-track magnetic tape (or any other kind of tape that can be backspaced with the 'MTIOCTOP' 'ioctl'). This means that the '--append', '--concatenate', and '--delete' commands will not work on any other kind of file. Some media simply cannot be backspaced, which means these commands and options will never be able to work on them. These non-backspacing media include pipes and cartridge tape drives. Some other media can be backspaced, and 'tar' will work on them once 'tar' is modified to do so. Archives created with the '--multi-volume', '--label', and '--incremental' ('-G') options may not be readable by other version of 'tar'. In particular, restoring a file that was split over a volume boundary will require some careful work with 'dd', if it can be done at all. Other versions of 'tar' may also create an empty file whose name is that of the volume header. Some versions of 'tar' may create normal files instead of directories archived with the '--incremental' ('-G') option.  File: tar.info, Node: Common Problems and Solutions, Next: Blocking, Prev: Remote Tape Server, Up: Media 9.3 Some Common Problems and their Solutions ============================================ errors from system: permission denied no such file or directory not owner errors from 'tar': directory checksum error header format error errors from media/system: i/o error device busy  File: tar.info, Node: Blocking, Next: Many, Prev: Common Problems and Solutions, Up: Media 9.4 Blocking ============ "Block" and "record" terminology is rather confused, and it is also confusing to the expert reader. On the other hand, readers who are new to the field have a fresh mind, and they may safely skip the next two paragraphs, as the remainder of this manual uses those two terms in a quite consistent way. John Gilmore, the writer of the public domain 'tar' from which GNU 'tar' was originally derived, wrote (June 1995): The nomenclature of tape drives comes from IBM, where I believe they were invented for the IBM 650 or so. On IBM mainframes, what is recorded on tape are tape blocks. The logical organization of data is into records. There are various ways of putting records into blocks, including 'F' (fixed sized records), 'V' (variable sized records), 'FB' (fixed blocked: fixed size records, N to a block), 'VB' (variable size records, N to a block), 'VSB' (variable spanned blocked: variable sized records that can occupy more than one block), etc. The 'JCL' 'DD RECFORM=' parameter specified this to the operating system. The Unix man page on 'tar' was totally confused about this. When I wrote 'PD TAR', I used the historically correct terminology ('tar' writes data records, which are grouped into blocks). It appears that the bogus terminology made it into POSIX (no surprise here), and now François has migrated that terminology back into the source code too. The term "physical block" means the basic transfer chunk from or to a device, after which reading or writing may stop without anything being lost. In this manual, the term "block" usually refers to a disk physical block, _assuming_ that each disk block is 512 bytes in length. It is true that some disk devices have different physical blocks, but 'tar' ignore these differences in its own format, which is meant to be portable, so a 'tar' block is always 512 bytes in length, and "block" always mean a 'tar' block. The term "logical block" often represents the basic chunk of allocation of many disk blocks as a single entity, which the operating system treats somewhat atomically; this concept is only barely used in GNU 'tar'. The term "physical record" is another way to speak of a physical block, those two terms are somewhat interchangeable. In this manual, the term "record" usually refers to a tape physical block, _assuming_ that the 'tar' archive is kept on magnetic tape. It is true that archives may be put on disk or used with pipes, but nevertheless, 'tar' tries to read and write the archive one "record" at a time, whatever the medium in use. One record is made up of an integral number of blocks, and this operation of putting many disk blocks into a single tape block is called "reblocking", or more simply, "blocking". The term "logical record" refers to the logical organization of many characters into something meaningful to the application. The term "unit record" describes a small set of characters which are transmitted whole to or by the application, and often refers to a line of text. Those two last terms are unrelated to what we call a "record" in GNU 'tar'. When writing to tapes, 'tar' writes the contents of the archive in chunks known as "records". To change the default blocking factor, use the '--blocking-factor=512-SIZE' ('-b 512-SIZE') option. Each record will then be composed of 512-SIZE blocks. (Each 'tar' block is 512 bytes. *Note Standard::.) Each file written to the archive uses at least one full record. As a result, using a larger record size can result in more wasted space for small files. On the other hand, a larger record size can often be read and written much more efficiently. Further complicating the problem is that some tape drives ignore the blocking entirely. For these, a larger record size can still improve performance (because the software layers above the tape drive still honor the blocking), but not as dramatically as on tape drives that honor blocking. When reading an archive, 'tar' can usually figure out the record size on itself. When this is the case, and a non-standard record size was used when the archive was created, 'tar' will print a message about a non-standard blocking factor, and then operate normally(1). On some tape devices, however, 'tar' cannot figure out the record size itself. On most of those, you can specify a blocking factor (with '--blocking-factor') larger than the actual blocking factor, and then use the '--read-full-records' ('-B') option. (If you specify a blocking factor with '--blocking-factor' and don't use the '--read-full-records' option, then 'tar' will not attempt to figure out the recording size itself.) On some devices, you must always specify the record size exactly with '--blocking-factor' when reading, because 'tar' cannot figure it out. In any case, use '--list' ('-t') before doing any extractions to see whether 'tar' is reading the archive correctly. 'tar' blocks are all fixed size (512 bytes), and its scheme for putting them into records is to put a whole number of them (one or more) into each record. 'tar' records are all the same size; at the end of the file there's a block containing all zeros, which is how you tell that the remainder of the last record(s) are garbage. In a standard 'tar' file (no options), the block size is 512 and the record size is 10240, for a blocking factor of 20. What the '--blocking-factor' option does is sets the blocking factor, changing the record size while leaving the block size at 512 bytes. 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives; most tape drives these days prefer much bigger records in order to stream and not waste tape. When writing tapes for myself, some tend to use a factor of the order of 2048, say, giving a record size of around one megabyte. If you use a blocking factor larger than 20, older 'tar' programs might not be able to read the archive, so we recommend this as a limit to use in practice. GNU 'tar', however, will support arbitrarily large record sizes, limited only by the amount of virtual memory or the physical characteristics of the tape device. * Menu: * Format Variations:: Format Variations * Blocking Factor:: The Blocking Factor of an Archive ---------- Footnotes ---------- (1) If this message is not needed, you can turn it off using the '--warning=no-record-size' option.  File: tar.info, Node: Format Variations, Next: Blocking Factor, Up: Blocking 9.4.1 Format Variations ----------------------- _(This message will disappear, once this node revised.)_ Format parameters specify how an archive is written on the archive media. The best choice of format parameters will vary depending on the type and number of files being archived, and on the media used to store the archive. To specify format parameters when accessing or creating an archive, you can use the options described in the following sections. If you do not specify any format parameters, 'tar' uses default parameters. You cannot modify a compressed archive. If you create an archive with the '--blocking-factor' option specified (*note Blocking Factor::), you must specify that blocking-factor when operating on the archive. *Note Formats::, for other examples of format parameter considerations.  File: tar.info, Node: Blocking Factor, Prev: Format Variations, Up: Blocking 9.4.2 The Blocking Factor of an Archive --------------------------------------- _(This message will disappear, once this node revised.)_ The data in an archive is grouped into blocks, which are 512 bytes. Blocks are read and written in whole number multiples called "records". The number of blocks in a record (i.e., the size of a record in units of 512 bytes) is called the "blocking factor". The '--blocking-factor=512-SIZE' ('-b 512-SIZE') option specifies the blocking factor of an archive. The default blocking factor is typically 20 (i.e., 10240 bytes), but can be specified at installation. To find out the blocking factor of an existing archive, use 'tar --list --file=ARCHIVE-NAME'. This may not work on some devices. Records are separated by gaps, which waste space on the archive media. If you are archiving on magnetic tape, using a larger blocking factor (and therefore larger records) provides faster throughput and allows you to fit more data on a tape (because there are fewer gaps). If you are archiving on cartridge, a very large blocking factor (say 126 or more) greatly increases performance. A smaller blocking factor, on the other hand, may be useful when archiving small files, to avoid archiving lots of nulls as 'tar' fills out the archive to the end of the record. In general, the ideal record size depends on the size of the inter-record gaps on the tape you are using, and the average size of the files you are archiving. *Note create::, for information on writing archives. Archives with blocking factors larger than 20 cannot be read by very old versions of 'tar', or by some newer versions of 'tar' running on old machines with small address spaces. With GNU 'tar', the blocking factor of an archive is limited only by the maximum record size of the device containing the archive, or by the amount of available virtual memory. Also, on some systems, not using adequate blocking factors, as sometimes imposed by the device drivers, may yield unexpected diagnostics. For example, this has been reported: Cannot write to /dev/dlt: Invalid argument In such cases, it sometimes happen that the 'tar' bundled by the system is aware of block size idiosyncrasies, while GNU 'tar' requires an explicit specification for the block size, which it cannot guess. This yields some people to consider GNU 'tar' is misbehaving, because by comparison, 'the bundle 'tar' works OK'. Adding '-b 256', for example, might resolve the problem. If you use a non-default blocking factor when you create an archive, you must specify the same blocking factor when you modify that archive. Some archive devices will also require you to specify the blocking factor when reading that archive, however this is not typically the case. Usually, you can use '--list' ('-t') without specifying a blocking factor--'tar' reports a non-default record size and then lists the archive members as it would normally. To extract files from an archive with a non-standard blocking factor (particularly if you're not sure what the blocking factor is), you can usually use the '--read-full-records' ('-B') option while specifying a blocking factor larger then the blocking factor of the archive (i.e., 'tar --extract --read-full-records --blocking-factor=300'). *Note list::, for more information on the '--list' ('-t') operation. *Note Reading::, for a more detailed explanation of that option. '--blocking-factor=NUMBER' '-b NUMBER' Specifies the blocking factor of an archive. Can be used with any operation, but is usually not necessary with '--list' ('-t'). Device blocking '-b BLOCKS' '--blocking-factor=BLOCKS' Set record size to BLOCKS*512 bytes. This option is used to specify a "blocking factor" for the archive. When reading or writing the archive, 'tar', will do reads and writes of the archive in records of BLOCK*512 bytes. This is true even when the archive is compressed. Some devices requires that all write operations be a multiple of a certain size, and so, 'tar' pads the archive out to the next record boundary. The default blocking factor is set when 'tar' is compiled, and is typically 20. Blocking factors larger than 20 cannot be read by very old versions of 'tar', or by some newer versions of 'tar' running on old machines with small address spaces. With a magnetic tape, larger records give faster throughput and fit more data on a tape (because there are fewer inter-record gaps). If the archive is in a disk file or a pipe, you may want to specify a smaller blocking factor, since a large one will result in a large number of null bytes at the end of the archive. When writing cartridge or other streaming tapes, a much larger blocking factor (say 126 or more) will greatly increase performance. However, you must specify the same blocking factor when reading or updating the archive. Apparently, Exabyte drives have a physical block size of 8K bytes. If we choose our blocksize as a multiple of 8k bytes, then the problem seems to disappear. Id est, we are using block size of 112 right now, and we haven't had the problem since we switched... With GNU 'tar' the blocking factor is limited only by the maximum record size of the device containing the archive, or by the amount of available virtual memory. However, deblocking or reblocking is virtually avoided in a special case which often occurs in practice, but which requires all the following conditions to be simultaneously true: * the archive is subject to a compression option, * the archive is not handled through standard input or output, nor redirected nor piped, * the archive is directly handled to a local disk, instead of any special device, * '--blocking-factor' is not explicitly specified on the 'tar' invocation. If the output goes directly to a local disk, and not through stdout, then the last write is not extended to a full record size. Otherwise, reblocking occurs. Here are a few other remarks on this topic: * 'gzip' will complain about trailing garbage if asked to uncompress a compressed archive on tape, there is an option to turn the message off, but it breaks the regularity of simply having to use 'PROG -d' for decompression. It would be nice if gzip was silently ignoring any number of trailing zeros. I'll ask Jean-loup Gailly, by sending a copy of this message to him. * 'compress' does not show this problem, but as Jean-loup pointed out to Michael, 'compress -d' silently adds garbage after the result of decompression, which tar ignores because it already recognized its end-of-file indicator. So this bug may be safely ignored. * 'gzip -d -q' will be silent about the trailing zeros indeed, but will still return an exit status of 2 which tar reports in turn. 'tar' might ignore the exit status returned, but I hate doing that, as it weakens the protection 'tar' offers users against other possible problems at decompression time. If 'gzip' was silently skipping trailing zeros _and_ also avoiding setting the exit status in this innocuous case, that would solve this situation. * 'tar' should become more solid at not stopping to read a pipe at the first null block encountered. This inelegantly breaks the pipe. 'tar' should rather drain the pipe out before exiting itself. '-i' '--ignore-zeros' Ignore blocks of zeros in archive (means EOF). The '--ignore-zeros' ('-i') option causes 'tar' to ignore blocks of zeros in the archive. Normally a block of zeros indicates the end of the archive, but when reading a damaged archive, or one which was created by concatenating several archives together, this option allows 'tar' to read the entire archive. This option is not on by default because many versions of 'tar' write garbage after the zeroed blocks. Note that this option causes 'tar' to read to the end of the archive file, which may sometimes avoid problems when multiple files are stored on a single physical tape. '-B' '--read-full-records' Reblock as we read (for reading 4.2BSD pipes). If '--read-full-records' is used, 'tar' will not panic if an attempt to read a record from the archive does not return a full record. Instead, 'tar' will keep reading until it has obtained a full record. This option is turned on by default when 'tar' is reading an archive from standard input, or from a remote machine. This is because on BSD Unix systems, a read of a pipe will return however much happens to be in the pipe, even if it is less than 'tar' requested. If this option was not used, 'tar' would fail as soon as it read an incomplete record from the pipe. This option is also useful with the commands for updating an archive. Tape blocking When handling various tapes or cartridges, you have to take care of selecting a proper blocking, that is, the number of disk blocks you put together as a single tape block on the tape, without intervening tape gaps. A "tape gap" is a small landing area on the tape with no information on it, used for decelerating the tape to a full stop, and for later regaining the reading or writing speed. When the tape driver starts reading a record, the record has to be read whole without stopping, as a tape gap is needed to stop the tape motion without losing information. Using higher blocking (putting more disk blocks per tape block) will use the tape more efficiently as there will be less tape gaps. But reading such tapes may be more difficult for the system, as more memory will be required to receive at once the whole record. Further, if there is a reading error on a huge record, this is less likely that the system will succeed in recovering the information. So, blocking should not be too low, nor it should be too high. 'tar' uses by default a blocking of 20 for historical reasons, and it does not really matter when reading or writing to disk. Current tape technology would easily accommodate higher blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs. We were told that for some DLT drives, the blocking should be a multiple of 4Kb, preferably 64Kb ('-b 128') or 256 for decent performance. Other manufacturers may use different recommendations for the same tapes. This might also depends of the buffering techniques used inside modern tape controllers. Some imposes a minimum blocking, or a maximum blocking. Others request blocking to be some exponent of two. So, there is no fixed rule for blocking. But blocking at read time should ideally be the same as blocking used at write time. At one place I know, with a wide variety of equipment, they found it best to use a blocking of 32 to guarantee that their tapes are fully interchangeable. I was also told that, for recycled tapes, prior erasure (by the same drive unit that will be used to create the archives) sometimes lowers the error rates observed at rewriting time. I might also use '--number-blocks' instead of '--block-number', so '--block' will then expand to '--blocking-factor' unambiguously.  File: tar.info, Node: Many, Next: Using Multiple Tapes, Prev: Blocking, Up: Media 9.5 Many Archives on One Tape ============================= Most tape devices have two entries in the '/dev' directory, or entries that come in pairs, which differ only in the minor number for this device. Let's take for example '/dev/tape', which often points to the only or usual tape device of a given system. There might be a corresponding '/dev/nrtape' or '/dev/ntape'. The simpler name is the _rewinding_ version of the device, while the name having 'nr' in it is the _no rewinding_ version of the same device. A rewinding tape device will bring back the tape to its beginning point automatically when this device is opened or closed. Since 'tar' opens the archive file before using it and closes it afterwards, this means that a simple: $ tar cf /dev/tape DIRECTORY will reposition the tape to its beginning both prior and after saving DIRECTORY contents to it, thus erasing prior tape contents and making it so that any subsequent write operation will destroy what has just been saved. So, a rewinding device is normally meant to hold one and only one file. If you want to put more than one 'tar' archive on a given tape, you will need to avoid using the rewinding version of the tape device. You will also have to pay special attention to tape positioning. Errors in positioning may overwrite the valuable data already on your tape. Many people, burnt by past experiences, will only use rewinding devices and limit themselves to one file per tape, precisely to avoid the risk of such errors. Be fully aware that writing at the wrong position on a tape loses all information past this point and most probably until the end of the tape, and this destroyed information _cannot_ be recovered. To save DIRECTORY-1 as a first archive at the beginning of a tape, and leave that tape ready for a second archive, you should use: $ mt -f /dev/nrtape rewind $ tar cf /dev/nrtape DIRECTORY-1 "Tape marks" are special magnetic patterns written on the tape media, which are later recognizable by the reading hardware. These marks are used after each file, when there are many on a single tape. An empty file (that is to say, two tape marks in a row) signal the logical end of the tape, after which no file exist. Usually, non-rewinding tape device drivers will react to the close request issued by 'tar' by first writing two tape marks after your archive, and by backspacing over one of these. So, if you remove the tape at that time from the tape drive, it is properly terminated. But if you write another file at the current position, the second tape mark will be erased by the new information, leaving only one tape mark between files. So, you may now save DIRECTORY-2 as a second archive after the first on the same tape by issuing the command: $ tar cf /dev/nrtape DIRECTORY-2 and so on for all the archives you want to put on the same tape. Another usual case is that you do not write all the archives the same day, and you need to remove and store the tape between two archive sessions. In general, you must remember how many files are already saved on your tape. Suppose your tape already has 16 files on it, and that you are ready to write the 17th. You have to take care of skipping the first 16 tape marks before saving DIRECTORY-17, say, by using these commands: $ mt -f /dev/nrtape rewind $ mt -f /dev/nrtape fsf 16 $ tar cf /dev/nrtape DIRECTORY-17 In all the previous examples, we put aside blocking considerations, but you should do the proper things for that as well. *Note Blocking::. * Menu: * Tape Positioning:: Tape Positions and Tape Marks * mt:: The 'mt' Utility  File: tar.info, Node: Tape Positioning, Next: mt, Up: Many 9.5.1 Tape Positions and Tape Marks ----------------------------------- _(This message will disappear, once this node revised.)_ Just as archives can store more than one file from the file system, tapes can store more than one archive file. To keep track of where archive files (or any other type of file stored on tape) begin and end, tape archive devices write magnetic "tape marks" on the archive media. Tape drives write one tape mark between files, two at the end of all the file entries. If you think of data as a series of records "rrrr"'s, and tape marks as "*"'s, a tape might look like the following: rrrr*rrrrrr*rrrrr*rr*rrrrr**------------------------- Tape devices read and write tapes using a read/write "tape head"--a physical part of the device which can only access one point on the tape at a time. When you use 'tar' to read or write archive data from a tape device, the device will begin reading or writing from wherever on the tape the tape head happens to be, regardless of which archive or what part of the archive the tape head is on. Before writing an archive, you should make sure that no data on the tape will be overwritten (unless it is no longer needed). Before reading an archive, you should make sure the tape head is at the beginning of the archive you want to read. You can do it manually via 'mt' utility (*note mt::). The 'restore' script does that automatically (*note Scripted Restoration::). If you want to add new archive file entries to a tape, you should advance the tape to the end of the existing file entries, backspace over the last tape mark, and write the new archive file. If you were to add two archives to the example above, the tape might look like the following: rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------  File: tar.info, Node: mt, Prev: Tape Positioning, Up: Many 9.5.2 The 'mt' Utility ---------------------- _(This message will disappear, once this node revised.)_ *Note Blocking Factor::. You can use the 'mt' utility to advance or rewind a tape past a specified number of archive files on the tape. This will allow you to move to the beginning of an archive before extracting or reading it, or to the end of all the archives before writing a new one. The syntax of the 'mt' command is: mt [-f TAPENAME] OPERATION [NUMBER] where TAPENAME is the name of the tape device, NUMBER is the number of times an operation is performed (with a default of one), and OPERATION is one of the following: 'eof' 'weof' Writes NUMBER tape marks at the current position on the tape. 'fsf' Moves tape position forward NUMBER files. 'bsf' Moves tape position back NUMBER files. 'rewind' Rewinds the tape. (Ignores NUMBER.) 'offline' 'rewoff1' Rewinds the tape and takes the tape device off-line. (Ignores NUMBER.) 'status' Prints status information about the tape unit. If you don't specify a TAPENAME, 'mt' uses the environment variable 'TAPE'; if 'TAPE' is not set, 'mt' will use the default device specified in your 'sys/mtio.h' file ('DEFTAPE' variable). If this is not defined, the program will display a descriptive error message and exit with code 1. 'mt' returns a 0 exit status when the operation(s) were successful, 1 if the command was unrecognized, and 2 if an operation failed.  File: tar.info, Node: Using Multiple Tapes, Next: label, Prev: Many, Up: Media 9.6 Using Multiple Tapes ======================== Often you might want to write a large archive, one larger than will fit on the actual tape you are using. In such a case, you can run multiple 'tar' commands, but this can be inconvenient, particularly if you are using options like '--exclude=PATTERN' or dumping entire file systems. Therefore, 'tar' provides a special mode for creating multi-volume archives. "Multi-volume" archive is a single 'tar' archive, stored on several media volumes of fixed size. Although in this section we will often call 'volume' a "tape", there is absolutely no requirement for multi-volume archives to be stored on tapes. Instead, they can use whatever media type the user finds convenient, they can even be located on files. When creating a multi-volume archive, GNU 'tar' continues to fill current volume until it runs out of space, then it switches to next volume (usually the operator is queried to replace the tape on this point), and continues working on the new volume. This operation continues until all requested files are dumped. If GNU 'tar' detects end of media while dumping a file, such a file is archived in split form. Some very big files can even be split across several volumes. Each volume is itself a valid GNU 'tar' archive, so it can be read without any special options. Consequently any file member residing entirely on one volume can be extracted or otherwise operated upon without needing the other volume. Sure enough, to extract a split member you would need all volumes its parts reside on. Multi-volume archives suffer from several limitations. In particular, they cannot be compressed. GNU 'tar' is able to create multi-volume archives of two formats (*note Formats::): 'GNU' and 'POSIX'. * Menu: * Multi-Volume Archives:: Archives Longer than One Tape or Disk * Tape Files:: Tape Files * Tarcat:: Concatenate Volumes into a Single Archive  File: tar.info, Node: Multi-Volume Archives, Next: Tape Files, Up: Using Multiple Tapes 9.6.1 Archives Longer than One Tape or Disk ------------------------------------------- To create an archive that is larger than will fit on a single unit of the media, use the '--multi-volume' ('-M') option in conjunction with the '--create' option (*note create::). A "multi-volume" archive can be manipulated like any other archive (provided the '--multi-volume' option is specified), but is stored on more than one tape or file. When you specify '--multi-volume', 'tar' does not report an error when it comes to the end of an archive volume (when reading), or the end of the media (when writing). Instead, it prompts you to load a new storage volume. If the archive is on a magnetic tape, you should change tapes when you see the prompt; if the archive is on a floppy disk, you should change disks; etc. '--multi-volume' '-M' Creates a multi-volume archive, when used in conjunction with '--create' ('-c'). To perform any other operation on a multi-volume archive, specify '--multi-volume' in conjunction with that operation. For example: $ tar --create --multi-volume --file=/dev/tape FILES The method 'tar' uses to detect end of tape is not perfect, and fails on some operating systems or on some devices. If 'tar' cannot detect the end of the tape itself, you can use '--tape-length' option to inform it about the capacity of the tape: '--tape-length=SIZE[SUF]' '-L SIZE[SUF]' Set maximum length of a volume. The SUF, if given, specifies units in which SIZE is expressed, e.g. '2M' mean 2 megabytes (*note Table 9.1: size-suffixes, for a list of allowed size suffixes). Without SUF, units of 1024 bytes (kilobyte) are assumed. This option selects '--multi-volume' automatically. For example: $ tar --create --tape-length=41943040 --file=/dev/tape FILES or, which is equivalent: $ tar --create --tape-length=4G --file=/dev/tape FILES When GNU 'tar' comes to the end of a storage media, it asks you to change the volume. The built-in prompt for POSIX locale is(1): Prepare volume #N for 'ARCHIVE' and hit return: where N is the ordinal number of the volume to be created and ARCHIVE is archive file or device name. When prompting for a new tape, 'tar' accepts any of the following responses: '?' Request 'tar' to explain possible responses. 'q' Request 'tar' to exit immediately. 'n FILE-NAME' Request 'tar' to write the next volume on the file FILE-NAME. '!' Request 'tar' to run a subshell. This option can be disabled by giving '--restrict' command line option to 'tar'(2). 'y' Request 'tar' to begin writing the next volume. (You should only type 'y' after you have changed the tape; otherwise 'tar' will write over the volume it just finished.) The volume number used by 'tar' in its tape-changing prompt can be changed; if you give the '--volno-file=FILE-OF-NUMBER' option, then FILE-OF-NUMBER should be an non-existing file to be created, or else, a file already containing a decimal number. That number will be used as the volume number of the first volume written. When 'tar' is finished, it will rewrite the file with the now-current volume number. (This does not change the volume number written on a tape label, as per *note label::, it _only_ affects the number used in the prompt.) If you want more elaborate behavior than this, you can write a special "new volume script", that will be responsible for changing the volume, and instruct 'tar' to use it instead of its normal prompting procedure: '--info-script=COMMAND' '--new-volume-script=COMMAND' '-F COMMAND' Specify the command to invoke when switching volumes. The COMMAND can be used to eject cassettes, or to broadcast messages such as 'Someone please come change my tape' when performing unattended backups. The COMMAND can contain additional options, if such are needed. *Note Running External Commands: external, for a detailed discussion of the way GNU 'tar' runs external commands. It inherits 'tar''s shell environment. Additional data is passed to it via the following environment variables: 'TAR_VERSION' GNU 'tar' version number. 'TAR_ARCHIVE' The name of the archive 'tar' is processing. 'TAR_BLOCKING_FACTOR' Current blocking factor (*note Blocking::). 'TAR_VOLUME' Ordinal number of the volume 'tar' is about to start. 'TAR_SUBCOMMAND' A short option describing the operation 'tar' is executing. *Note Operations::, for a complete list of subcommand options. 'TAR_FORMAT' Format of the archive being processed. *Note Formats::, for a complete list of archive format names. 'TAR_FD' File descriptor which can be used to communicate the new volume name to 'tar'. These variables can be used in the COMMAND itself, provided that they are properly quoted to prevent them from being expanded by the shell that invokes 'tar'. The volume script can instruct 'tar' to use new archive name, by writing in to file descriptor '$TAR_FD' (see below for an example). If the info script fails, 'tar' exits; otherwise, it begins writing the next volume. If you want 'tar' to cycle through a series of files or tape drives, there are three approaches to choose from. First of all, you can give 'tar' multiple '--file' options. In this case the specified files will be used, in sequence, as the successive volumes of the archive. Only when the first one in the sequence needs to be used again will 'tar' prompt for a tape change (or run the info script). For example, suppose someone has two tape drives on a system named '/dev/tape0' and '/dev/tape1'. For having GNU 'tar' to switch to the second drive when it needs to write the second tape, and then back to the first tape, etc., just do either of: $ tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 FILES $ tar -cM -f /dev/tape0 -f /dev/tape1 FILES The second method is to use the 'n' response to the tape-change prompt. Finally, the most flexible approach is to use a volume script, that writes new archive name to the file descriptor '$TAR_FD'. For example, the following volume script will create a series of archive files, named 'ARCHIVE-VOL', where ARCHIVE is the name of the archive being created (as given by '--file' option) and VOL is the ordinal number of the archive being created: #! /bin/bash # For this script it's advisable to use a shell, such as Bash, # that supports a TAR_FD value greater than 9. echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE. name=`expr $TAR_ARCHIVE : '\(.*\)-.*'` case $TAR_SUBCOMMAND in -c) ;; -d|-x|-t) test -r ${name:-$TAR_ARCHIVE}-$TAR_VOLUME || exit 1 ;; *) exit 1 esac echo ${name:-$TAR_ARCHIVE}-$TAR_VOLUME >&$TAR_FD The same script can be used while listing, comparing or extracting from the created archive. For example: # Create a multi-volume archive: $ tar -c -L1024 -f archive.tar -F new-volume . # Extract from the created archive: $ tar -x -f archive.tar -F new-volume . Notice, that the first command had to use '-L' option, since otherwise GNU 'tar' will end up writing everything to file 'archive.tar'. You can read each individual volume of a multi-volume archive as if it were an archive by itself. For example, to list the contents of one volume, use '--list', without '--multi-volume' specified. To extract an archive member from one volume (assuming it is described that volume), use '--extract', again without '--multi-volume'. If an archive member is split across volumes (i.e., its entry begins on one volume of the media and ends on another), you need to specify '--multi-volume' to extract it successfully. In this case, you should load the volume where the archive member starts, and use 'tar --extract --multi-volume'--'tar' will prompt for later volumes as it needs them. *Note extracting archives::, for more information about extracting archives. Multi-volume archives can be modified like any other archive. To add files to a multi-volume archive, you need to only mount the last volume of the archive media (and new volumes, if needed). For all other operations, you need to use the entire archive. If a multi-volume archive was labeled using '--label=ARCHIVE-LABEL' (*note label::) when it was created, 'tar' will not automatically label volumes which are added later. To label subsequent volumes, specify '--label=ARCHIVE-LABEL' again in conjunction with the '--append', '--update' or '--concatenate' operation. Notice that multi-volume support is a GNU extension and the archives created in this mode should be read only using GNU 'tar'. If you absolutely have to process such archives using a third-party 'tar' implementation, read *note Split Recovery::. ---------- Footnotes ---------- (1) If you run GNU 'tar' under a different locale, the translation to the locale's language will be used. (2) *Note --restrict::, for more information about this option.  File: tar.info, Node: Tape Files, Next: Tarcat, Prev: Multi-Volume Archives, Up: Using Multiple Tapes 9.6.2 Tape Files ---------------- _(This message will disappear, once this node revised.)_ To give the archive a name which will be recorded in it, use the '--label=VOLUME-LABEL' ('-V VOLUME-LABEL') option. This will write a special block identifying VOLUME-LABEL as the name of the archive to the front of the archive which will be displayed when the archive is listed with '--list'. If you are creating a multi-volume archive with '--multi-volume' (*note Using Multiple Tapes::), then the volume label will have 'Volume NNN' appended to the name you give, where NNN is the number of the volume of the archive. If you use the '--label=VOLUME-LABEL' option when reading an archive, it checks to make sure the label on the tape matches the one you gave. *Note label::. When 'tar' writes an archive to tape, it creates a single tape file. If multiple archives are written to the same tape, one after the other, they each get written as separate tape files. When extracting, it is necessary to position the tape at the right place before running 'tar'. To do this, use the 'mt' command. For more information on the 'mt' command and on the organization of tapes into a sequence of tape files, see *note mt::. People seem to often do: --label="SOME-PREFIX `date +SOME-FORMAT`" or such, for pushing a common date in all volumes or an archive set.  File: tar.info, Node: Tarcat, Prev: Tape Files, Up: Using Multiple Tapes 9.6.3 Concatenate Volumes into a Single Archive ----------------------------------------------- Sometimes it is necessary to convert existing GNU 'tar' multi-volume archive to a single 'tar' archive. Simply concatenating all volumes into one will not work, since each volume carries an additional information at the beginning. GNU 'tar' is shipped with the shell script 'tarcat' designed for this purpose. The script takes a list of files comprising a multi-volume archive and creates the resulting archive at the standard output. For example: tarcat vol.1 vol.2 vol.3 | tar tf - The script implements a simple heuristics to determine the format of the first volume file and to decide how to process the rest of the files. However, it makes no attempt to verify whether the files are given in order or even if they are valid 'tar' archives. It uses 'dd' and does not filter its standard error, so you will usually see lots of spurious messages.  File: tar.info, Node: label, Next: verify, Prev: Using Multiple Tapes, Up: Media 9.7 Including a Label in the Archive ==================================== To avoid problems caused by misplaced paper labels on the archive media, you can include a "label" entry -- an archive member which contains the name of the archive -- in the archive itself. Use the '--label=ARCHIVE-LABEL' ('-V ARCHIVE-LABEL') option(1) in conjunction with the '--create' operation to include a label entry in the archive as it is being created. '--label=ARCHIVE-LABEL' '-V ARCHIVE-LABEL' Includes an "archive-label" at the beginning of the archive when the archive is being created, when used in conjunction with the '--create' operation. Checks to make sure the archive label matches the one specified (when used in conjunction with any other operation). If you create an archive using both '--label=ARCHIVE-LABEL' ('-V ARCHIVE-LABEL') and '--multi-volume' ('-M'), each volume of the archive will have an archive label of the form 'ARCHIVE-LABEL Volume N', where N is 1 for the first volume, 2 for the next, and so on. *Note Using Multiple Tapes::, for information on creating multiple volume archives. The volume label will be displayed by '--list' along with the file contents. If verbose display is requested, it will also be explicitly marked as in the example below: $ tar --verbose --list --file=iamanarchive V--------- 0/0 0 1992-03-07 12:01 iamalabel--Volume Header-- -rw-r--r-- ringo/user 40 1990-05-21 13:30 iamafilename However, '--list' option will cause listing entire contents of the archive, which may be undesirable (for example, if the archive is stored on a tape). You can request checking only the volume label by specifying '--test-label' option. This option reads only the first block of an archive, so it can be used with slow storage devices. For example: $ tar --test-label --file=iamanarchive iamalabel If '--test-label' is used with one or more command line arguments, 'tar' compares the volume label with each argument. It exits with code 0 if a match is found, and with code 1 otherwise(2). No output is displayed, unless you also used the '--verbose' option. For example: $ tar --test-label --file=iamanarchive 'iamalabel' => 0 $ tar --test-label --file=iamanarchive 'alabel' => 1 When used with the '--verbose' option, 'tar' prints the actual volume label (if any), and a verbose diagnostics in case of a mismatch: $ tar --test-label --verbose --file=iamanarchive 'iamalabel' iamalabel => 0 $ tar --test-label --verbose --file=iamanarchive 'alabel' iamalabel tar: Archive label mismatch => 1 If you request any operation, other than '--create', along with using '--label' option, 'tar' will first check if the archive label matches the one specified and will refuse to proceed if it does not. Use this as a safety precaution to avoid accidentally overwriting existing archives. For example, if you wish to add files to 'archive', presumably labeled with string 'My volume', you will get: $ tar -rf archive --label 'My volume' . tar: Archive not labeled to match 'My volume' in case its label does not match. This will work even if 'archive' is not labeled at all. Similarly, 'tar' will refuse to list or extract the archive if its label doesn't match the ARCHIVE-LABEL specified. In those cases, ARCHIVE-LABEL argument is interpreted as a globbing-style pattern which must match the actual magnetic volume label. *Note exclude::, for a precise description of how match is attempted(3). If the switch '--multi-volume' ('-M') is being used, the volume label matcher will also suffix ARCHIVE-LABEL by ' Volume [1-9]*' if the initial match fails, before giving up. Since the volume numbering is automatically added in labels at creation time, it sounded logical to equally help the user taking care of it when the archive is being read. You can also use '--label' to get a common information on all tapes of a series. For having this information different in each series created through a single script used on a regular basis, just manage to get some date string as part of the label. For example: $ tar -cM -f /dev/tape -V "Daily backup for `date +%Y-%m-%d`" $ tar --create --file=/dev/tape --multi-volume \ --label="Daily backup for `date +%Y-%m-%d`" Some more notes about volume labels: * Each label has its own date and time, which corresponds to the time when GNU 'tar' initially attempted to write it, often soon after the operator launches 'tar' or types the carriage return telling that the next tape is ready. * Comparing date labels to get an idea of tape throughput is unreliable. It gives correct results only if the delays for rewinding tapes and the operator switching them were negligible, which is usually not the case. ---------- Footnotes ---------- (1) Until version 1.10, that option was called '--volume', but is not available under that name anymore. (2) Note that GNU 'tar' versions up to 1.23 indicated mismatch with an exit code 2 and printed a spurious diagnostics on stderr. (3) Previous versions of 'tar' used full regular expression matching, or before that, only exact string matching, instead of wildcard matchers. We decided for the sake of simplicity to use a uniform matching device through 'tar'.  File: tar.info, Node: verify, Next: Write Protection, Prev: label, Up: Media 9.8 Verifying Data as It is Stored ================================== '-W' '--verify' Attempt to verify the archive after writing. This option causes 'tar' to verify the archive after writing it. Each volume is checked after it is written, and any discrepancies are recorded on the standard error output. Verification requires that the archive be on a back-space-able medium. This means pipes, some cartridge tape drives, and some other devices cannot be verified. You can insure the accuracy of an archive by comparing files in the system with archive members. 'tar' can compare an archive to the file system as the archive is being written, to verify a write operation, or can compare a previously written archive, to insure that it is up to date. To check for discrepancies in an archive immediately after it is written, use the '--verify' ('-W') option in conjunction with the '--create' operation. When this option is specified, 'tar' checks archive members against their counterparts in the file system, and reports discrepancies on the standard error. To verify an archive, you must be able to read it from before the end of the last written entry. This option is useful for detecting data errors on some tapes. Archives written to pipes, some cartridge tape drives, and some other devices cannot be verified. One can explicitly compare an already made archive with the file system by using the '--compare' ('--diff', '-d') option, instead of using the more automatic '--verify' option. *Note compare::. Note that these two options have a slightly different intent. The '--compare' option checks how identical are the logical contents of some archive with what is on your disks, while the '--verify' option is really for checking if the physical contents agree and if the recording media itself is of dependable quality. So, for the '--verify' operation, 'tar' tries to defeat all in-memory cache pertaining to the archive, while it lets the speed optimization undisturbed for the '--compare' option. If you nevertheless use '--compare' for media verification, you may have to defeat the in-memory cache yourself, maybe by opening and reclosing the door latch of your recording unit, forcing some doubt in your operating system about the fact this is really the same volume as the one just written or read. The '--verify' option would not be necessary if drivers were indeed able to detect dependably all write failures. This sometimes require many magnetic heads, some able to read after the writes occurred. One would not say that drivers unable to detect all cases are necessarily flawed, as long as programming is concerned. The '--verify' ('-W') option will not work in conjunction with the '--multi-volume' ('-M') option or the '--append' ('-r'), '--update' ('-u') and '--delete' operations. *Note Operations::, for more information on these operations. Also, since 'tar' normally strips leading '/' from file names (*note absolute::), a command like 'tar --verify -cf /tmp/foo.tar /etc' will work as desired only if the working directory is '/', as 'tar' uses the archive's relative member names (e.g., 'etc/motd') when verifying the archive.  File: tar.info, Node: Write Protection, Prev: verify, Up: Media 9.9 Write Protection ==================== Almost all tapes and diskettes, and in a few rare cases, even disks can be "write protected", to protect data on them from being changed. Once an archive is written, you should write protect the media to prevent the archive from being accidentally overwritten or deleted. (This will protect the archive from being changed with a tape or floppy drive--it will not protect it from magnet fields or other physical hazards.) The write protection device itself is usually an integral part of the physical media, and can be a two position (write enabled/write disabled) switch, a notch which can be popped out or covered, a ring which can be removed from the center of a tape reel, or some other changeable feature.  File: tar.info, Node: Reliability and security, Next: Changes, Prev: Media, Up: Top 10 Reliability and Security *************************** The 'tar' command reads and writes files as any other application does, and is subject to the usual caveats about reliability and security. This section contains some commonsense advice on the topic. * Menu: * Reliability:: * Security::  File: tar.info, Node: Reliability, Next: Security, Up: Reliability and security 10.1 Reliability ================ Ideally, when 'tar' is creating an archive, it reads from a file system that is not being modified, and encounters no errors or inconsistencies while reading and writing. If this is the case, the archive should faithfully reflect what was read. Similarly, when extracting from an archive, ideally 'tar' ideally encounters no errors and the extracted files faithfully reflect what was in the archive. However, when reading or writing real-world file systems, several things can go wrong; these include permissions problems, corruption of data, and race conditions. * Menu: * Permissions problems:: * Data corruption and repair:: * Race conditions::  File: tar.info, Node: Permissions problems, Next: Data corruption and repair, Up: Reliability 10.1.1 Permissions Problems --------------------------- If 'tar' encounters errors while reading or writing files, it normally reports an error and exits with nonzero status. The work it does may therefore be incomplete. For example, when creating an archive, if 'tar' cannot read a file then it cannot copy the file into the archive.  File: tar.info, Node: Data corruption and repair, Next: Race conditions, Prev: Permissions problems, Up: Reliability 10.1.2 Data Corruption and Repair --------------------------------- If an archive becomes corrupted by an I/O error, this may corrupt the data in an extracted file. Worse, it may corrupt the file's metadata, which may cause later parts of the archive to become misinterpreted. An tar-format archive contains a checksum that most likely will detect errors in the metadata, but it will not detect errors in the data. If data corruption is a concern, you can compute and check your own checksums of an archive by using other programs, such as 'cksum'. When attempting to recover from a read error or data corruption in an archive, you may need to skip past the questionable data and read the rest of the archive. This requires some expertise in the archive format and in other software tools.  File: tar.info, Node: Race conditions, Prev: Data corruption and repair, Up: Reliability 10.1.3 Race conditions ---------------------- If some other process is modifying the file system while 'tar' is reading or writing files, the result may well be inconsistent due to race conditions. For example, if another process creates some files in a directory while 'tar' is creating an archive containing the directory's files, 'tar' may see some of the files but not others, or it may see a file that is in the process of being created. The resulting archive may not be a snapshot of the file system at any point in time. If an application such as a database system depends on an accurate snapshot, restoring from the 'tar' archive of a live file system may therefore break that consistency and may break the application. The simplest way to avoid the consistency issues is to avoid making other changes to the file system while tar is reading it or writing it. When creating an archive, several options are available to avoid race conditions. Some hosts have a way of snapshotting a file system, or of temporarily suspending all changes to a file system, by (say) suspending the only virtual machine that can modify a file system; if you use these facilities and have 'tar -c' read from a snapshot when creating an archive, you can avoid inconsistency problems. More drastically, before starting 'tar' you could suspend or shut down all processes other than 'tar' that have access to the file system, or you could unmount the file system and then mount it read-only. When extracting from an archive, one approach to avoid race conditions is to create a directory that no other process can write to, and extract into that.  File: tar.info, Node: Security, Prev: Reliability, Up: Reliability and security 10.2 Security ============= In some cases 'tar' may be used in an adversarial situation, where an untrusted user is attempting to gain information about or modify otherwise-inaccessible files. Dealing with untrusted data (that is, data generated by an untrusted user) typically requires extra care, because even the smallest mistake in the use of 'tar' is more likely to be exploited by an adversary than by a race condition. * Menu: * Privacy:: * Integrity:: * Live untrusted data:: * Security rules of thumb::  File: tar.info, Node: Privacy, Next: Integrity, Up: Security 10.2.1 Privacy -------------- Standard privacy concerns apply when using 'tar'. For example, suppose you are archiving your home directory into a file '/archive/myhome.tar'. Any secret information in your home directory, such as your SSH secret keys, are copied faithfully into the archive. Therefore, if your home directory contains any file that should not be read by some other user, the archive itself should be not be readable by that user. And even if the archive's data are inaccessible to untrusted users, its metadata (such as size or last-modified date) may reveal some information about your home directory; if the metadata are intended to be private, the archive's parent directory should also be inaccessible to untrusted users. One precaution is to create '/archive' so that it is not accessible to any user, unless that user also has permission to access all the files in your home directory. Similarly, when extracting from an archive, take care that the permissions of the extracted files are not more generous than what you want. Even if the archive itself is readable only to you, files extracted from it have their own permissions that may differ.  File: tar.info, Node: Integrity, Next: Live untrusted data, Prev: Privacy, Up: Security 10.2.2 Integrity ---------------- When creating archives, take care that they are not writable by a untrusted user; otherwise, that user could modify the archive, and when you later extract from the archive you will get incorrect data. When 'tar' extracts from an archive, by default it writes into files relative to the working directory. If the archive was generated by an untrusted user, that user therefore can write into any file under the working directory. If the working directory contains a symbolic link to another directory, the untrusted user can also write into any file under the referenced directory. When extracting from an untrusted archive, it is therefore good practice to create an empty directory and run 'tar' in that directory. When extracting from two or more untrusted archives, each one should be extracted independently, into different empty directories. Otherwise, the first archive could create a symbolic link into an area outside the working directory, and the second one could follow the link and overwrite data that is not under the working directory. For example, when restoring from a series of incremental dumps, the archives should have been created by a trusted process, as otherwise the incremental restores might alter data outside the working directory. If you use the '--absolute-names' ('-P') option when extracting, 'tar' respects any file names in the archive, even file names that begin with '/' or contain '..'. As this lets the archive overwrite any file in your system that you can write, the '--absolute-names' ('-P') option should be used only for trusted archives. Conversely, with the '--keep-old-files' ('-k') and '--skip-old-files' options, 'tar' refuses to replace existing files when extracting. The difference between the two options is that the former treats existing files as errors whereas the latter just silently ignores them. Finally, with the '--no-overwrite-dir' option, 'tar' refuses to replace the permissions or ownership of already-existing directories. These options may help when extracting from untrusted archives.  File: tar.info, Node: Live untrusted data, Next: Security rules of thumb, Prev: Integrity, Up: Security 10.2.3 Dealing with Live Untrusted Data --------------------------------------- Extra care is required when creating from or extracting into a file system that is accessible to untrusted users. For example, superusers who invoke 'tar' must be wary about its actions being hijacked by an adversary who is reading or writing the file system at the same time that 'tar' is operating. When creating an archive from a live file system, 'tar' is vulnerable to denial-of-service attacks. For example, an adversarial user could create the illusion of an indefinitely-deep directory hierarchy 'd/e/f/g/...' by creating directories one step ahead of 'tar', or the illusion of an indefinitely-long file by creating a sparse file but arranging for blocks to be allocated just before 'tar' reads them. There is no easy way for 'tar' to distinguish these scenarios from legitimate uses, so you may need to monitor 'tar', just as you'd need to monitor any other system service, to detect such attacks. While a superuser is extracting from an archive into a live file system, an untrusted user might replace a directory with a symbolic link, in hopes that 'tar' will follow the symbolic link and extract data into files that the untrusted user does not have access to. Even if the archive was generated by the superuser, it may contain a file such as 'd/etc/passwd' that the untrusted user earlier created in order to break in; if the untrusted user replaces the directory 'd/etc' with a symbolic link to '/etc' while 'tar' is running, 'tar' will overwrite '/etc/passwd'. This attack can be prevented by extracting into a directory that is inaccessible to untrusted users. Similar attacks via symbolic links are also possible when creating an archive, if the untrusted user can modify an ancestor of a top-level argument of 'tar'. For example, an untrusted user that can modify '/home/eve' can hijack a running instance of 'tar -cf - /home/eve/Documents/yesterday' by replacing '/home/eve/Documents' with a symbolic link to some other location. Attacks like these can be prevented by making sure that untrusted users cannot modify any files that are top-level arguments to 'tar', or any ancestor directories of these files.  File: tar.info, Node: Security rules of thumb, Prev: Live untrusted data, Up: Security 10.2.4 Security Rules of Thumb ------------------------------ This section briefly summarizes rules of thumb for avoiding security pitfalls. * Protect archives at least as much as you protect any of the files being archived. * Extract from an untrusted archive only into an otherwise-empty directory. This directory and its parent should be accessible only to trusted users. For example: $ chmod go-rwx . $ mkdir -m go-rwx dir $ cd dir $ tar -xvf /archives/got-it-off-the-net.tar.gz As a corollary, do not do an incremental restore from an untrusted archive. * Do not let untrusted users access files extracted from untrusted archives without checking first for problems such as setuid programs. * Do not let untrusted users modify directories that are ancestors of top-level arguments of 'tar'. For example, while you are executing 'tar -cf /archive/u-home.tar /u/home', do not let an untrusted user modify '/', '/archive', or '/u'. * Pay attention to the diagnostics and exit status of 'tar'. * When archiving live file systems, monitor running instances of 'tar' to detect denial-of-service attacks. * Avoid unusual options such as '--absolute-names' ('-P'), '--dereference' ('-h'), '--overwrite', '--recursive-unlink', and '--remove-files' unless you understand their security implications.  File: tar.info, Node: Changes, Next: Recipes, Prev: Reliability and security, Up: Top Appendix A Changes ****************** This appendix lists some important user-visible changes between various versions of GNU 'tar'. An up-to-date version of this document is available at the GNU 'tar' documentation page (http://www.gnu.org/software/tar/manual/changes.html). Use of globbing patterns when listing and extracting. Previous versions of GNU tar assumed shell-style globbing when extracting from or listing an archive. For example: $ tar xf foo.tar '*.c' would extract all files whose names end in '.c'. This behavior was not documented and was incompatible with traditional tar implementations. Therefore, starting from version 1.15.91, GNU tar no longer uses globbing by default. For example, the above invocation is now interpreted as a request to extract from the archive the file named '*.c'. To facilitate transition to the new behavior for those users who got used to the previous incorrect one, 'tar' will print a warning if it finds out that a requested member was not found in the archive and its name looks like a globbing pattern. For example: $ tar xf foo.tar '*.c' tar: Pattern matching characters used in file names. Please, tar: use --wildcards to enable pattern matching, or --no-wildcards to tar: suppress this warning. tar: *.c: Not found in archive tar: Error exit delayed from previous errors To treat member names as globbing patterns, use the '--wildcards' option. If you want to tar to mimic the behavior of versions prior to 1.15.91, add this option to your 'TAR_OPTIONS' variable. *Note wildcards::, for the detailed discussion of the use of globbing patterns by GNU 'tar'. Use of short option '-o'. Earlier versions of GNU 'tar' understood '-o' command line option as a synonym for '--old-archive'. GNU 'tar' starting from version 1.13.90 understands this option as a synonym for '--no-same-owner'. This is compatible with UNIX98 'tar' implementations. However, to facilitate transition, '-o' option retains its old semantics when it is used with one of archive-creation commands. Users are encouraged to use '--format=oldgnu' instead. It is especially important, since versions of GNU Automake up to and including 1.8.4 invoke tar with this option to produce distribution tarballs. *Note v7: Formats, for the detailed discussion of this issue and its implications. *Note tar-formats: (automake)Options, for a description on how to use various archive formats with 'automake'. Future versions of GNU 'tar' will understand '-o' only as a synonym for '--no-same-owner'. Use of short option '-l' Earlier versions of GNU 'tar' understood '-l' option as a synonym for '--one-file-system'. Since such usage contradicted to UNIX98 specification and harmed compatibility with other implementations, it was declared deprecated in version 1.14. However, to facilitate transition to its new semantics, it was supported by versions 1.15 and 1.15.90. The present use of '-l' as a short variant of '--check-links' was introduced in version 1.15.91. Use of options '--portability' and '--old-archive' These options are deprecated. Please use '--format=v7' instead. Use of option '--posix' This option is deprecated. Please use '--format=posix' instead.  File: tar.info, Node: Recipes, Next: Configuring Help Summary, Prev: Changes, Up: Top Appendix B Recipes ****************** This appendix provides several recipes for performing common tasks using GNU 'tar'. * Menu: * copy directory hierarchy:: * intermediate directories::  File: tar.info, Node: copy directory hierarchy, Next: intermediate directories, Up: Recipes B.1 Copying directory hierarchies ================================= This is a traditional way to copy a directory hierarchy preserving the dates, modes, owners and link-structure of all the files therein. It was used back when the 'cp' command lacked the '-a' option: $ (cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -) You can avoid subshells by using '-C' option: $ tar -C sourcedir -cf - . | tar -C targetdir -xf - The same command using long option forms: $ (cd sourcedir; tar --create --file=- . ) \ | (cd targetdir; tar --extract --file=-) or $ tar --directory sourcedir --create --file=- . \ | tar --directory targetdir --extract --file=-  File: tar.info, Node: intermediate directories, Prev: copy directory hierarchy, Up: Recipes B.2 Restoring Intermediate Directories ====================================== A common concern is how to extract permissions and ownerships of intermediate directories when extracting only selected members from the archive. To illustrate this, consider the following archive: # tar tvf A.tar drwxr-xr-x root/root 0 2017-11-16 14:39 foo/ dr-xr-x--- gray/user 0 2017-11-16 14:39 foo/bar/ -rw-r--r-- gray/user 10 2017-11-16 14:40 foo/bar/file Suppose you extract only the file 'foo/bar/file', while being 'root': # tar xvf A.tar foo/bar/file foo/bar/file Now, let's inspect the content of the created directories: # find foo -ls 427257 0 drwxr-xr-x 3 root root 16 Nov 17 16:10 foo 427258 0 drwxr-xr-x 2 root root 17 Nov 17 16:10 foo/bar 427259 0 -rw-r--r-- 1 gray user 10 Nov 6 14:40 foo/bar/file The requested file is restored, including its ownership and permissions. The intermediate directories, however, are created with the default permissions, current timestamp and owned by the current user. This is because by the time 'tar' has reached the requested file, it had already skipped the entries for its parent directories, so it has no iformation about their ownership and modes. To restore meta information about the intermediate directories, you'll need to specify them explicitly in the command line and use the '--no-recursive' option (*note recurse::) to avoid extracting their content. To automate this process, 'Neal P. Murphy' proposed the following shell script(1): #! /bin/sh (while read path do path=`dirname $path` while [ -n "$path" -a "$path" != "." ] do echo $path path=`dirname $path` done done < $2 | sort | uniq) | tar -x --no-recursion -v -f $1 -T - -T $2 The script takes two arguments: the name of the archive file, and the name of the file list file. To complete our example, the file list will contain single line: foo/bar/file Supposing its name is 'file.list' and the script is named 'restore.sh', you can invoke it as follows: # sh restore.sh A.tar file.list ---------- Footnotes ---------- (1) The original version of the script can be seen at  File: tar.info, Node: Configuring Help Summary, Next: Fixing Snapshot Files, Prev: Recipes, Up: Top Appendix C Configuring Help Summary *********************************** Running 'tar --help' displays the short 'tar' option summary (*note help::). This summary is organized by "groups" of semantically close options. The options within each group are printed in the following order: a short option, eventually followed by a list of corresponding long option names, followed by a short description of the option. For example, here is an excerpt from the actual 'tar --help' output: Main operation mode: -A, --catenate, --concatenate append tar files to an archive -c, --create create a new archive -d, --diff, --compare find differences between archive and file system --delete delete from the archive The exact visual representation of the help output is configurable via 'ARGP_HELP_FMT' environment variable. The value of this variable is a comma-separated list of "format variable" assignments. There are two kinds of format variables. An "offset variable" keeps the offset of some part of help output text from the leftmost column on the screen. A "boolean" variable is a flag that toggles some output feature on or off. Depending on the type of the corresponding variable, there are two kinds of assignments: Offset assignment The assignment to an offset variable has the following syntax: VARIABLE=VALUE where VARIABLE is the variable name, and VALUE is a numeric value to be assigned to the variable. Boolean assignment To assign 'true' value to a variable, simply put this variable name. To assign 'false' value, prefix the variable name with 'no-'. For example: # Assign true value: dup-args # Assign false value: no-dup-args Following variables are declared: -- Help Output: boolean dup-args If true, arguments for an option are shown with both short and long options, even when a given option has both forms, for example: -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE If false, then if an option has both short and long forms, the argument is only shown with the long one, for example: -f, --file=ARCHIVE use archive file or device ARCHIVE and a message indicating that the argument is applicable to both forms is printed below the options. This message can be disabled using 'dup-args-note' (see below). The default is false. -- Help Output: boolean dup-args-note If this variable is true, which is the default, the following notice is displayed at the end of the help output: Mandatory or optional arguments to long options are also mandatory or optional for any corresponding short options. Setting 'no-dup-args-note' inhibits this message. Normally, only one of variables 'dup-args' or 'dup-args-note' should be set. -- Help Output: offset short-opt-col Column in which short options start. Default is 2. $ tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE -- Help Output: offset long-opt-col Column in which long options start. Default is 6. For example: $ tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE -- Help Output: offset doc-opt-col Column in which "doc options" start. A doc option isn't actually an option, but rather an arbitrary piece of documentation that is displayed in much the same manner as the options. For example, in the description of '--format' option: -H, --format=FORMAT create archive of the given format. FORMAT is one of the following: gnu GNU tar 1.13.x format oldgnu GNU format as per tar <= 1.12 pax POSIX 1003.1-2001 (pax) format posix same as pax ustar POSIX 1003.1-1988 (ustar) format v7 old V7 tar format the format names are doc options. Thus, if you set 'ARGP_HELP_FMT=doc-opt-col=6' the above part of the help output will look as follows: -H, --format=FORMAT create archive of the given format. FORMAT is one of the following: gnu GNU tar 1.13.x format oldgnu GNU format as per tar <= 1.12 pax POSIX 1003.1-2001 (pax) format posix same as pax ustar POSIX 1003.1-1988 (ustar) format v7 old V7 tar format -- Help Output: offset opt-doc-col Column in which option description starts. Default is 29. $ tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE Notice, that the description starts on a separate line if 'opt-doc-col' value is too small. -- Help Output: offset header-col Column in which "group headers" are printed. A group header is a descriptive text preceding an option group. For example, in the following text: Main operation mode: -A, --catenate, --concatenate append tar files to an archive -c, --create create a new archive 'Main operation mode:' is the group header. The default value is 1. -- Help Output: offset usage-indent Indentation of wrapped usage lines. Affects '--usage' output. Default is 12. -- Help Output: offset rmargin Right margin of the text output. Used for wrapping.  File: tar.info, Node: Fixing Snapshot Files, Next: Tar Internals, Prev: Configuring Help Summary, Up: Top Appendix D Fixing Snapshot Files ******************************** Various situations can cause device numbers to change: upgrading your kernel version, reconfiguring your hardware, loading kernel modules in a different order, using virtual volumes that are assembled dynamically (such as with LVM or RAID), hot-plugging drives (e.g. external USB or Firewire drives), etc. In the majority of cases this change is unnoticed by the users. However, it influences 'tar' incremental backups: the device number is stored in tar snapshot files (*note Snapshot Files::) and is used to determine whether the file has changed since the last backup. If the device numbers change for some reason, by default the next backup you run will be a full backup. To minimize the impact in these cases, GNU 'tar' comes with the 'tar-snapshot-edit' utility for inspecting and updating device numbers in snapshot files. (The utility, written by Dustin J. Mitchell, is also available from the GNU 'tar' home page (http://www.gnu.org/software/tar/utils/tar-snapshot-edit.html).) To obtain a summary of the device numbers found in the snapshot file, run $ tar-snapshot-edit SNAPFILE where SNAPFILE is the name of the snapshot file (you can supply as many files as you wish in a single command line). You can then compare the numbers across snapshot files, or against those currently in use on the live filesystem (using 'ls -l' or 'stat'). Assuming the device numbers have indeed changed, it's often possible to simply tell GNU 'tar' to ignore the device number when processing the incremental snapshot files for these backups, using the '--no-check-device' option (*note device numbers::). Alternatively, you can use the 'tar-edit-snapshot' script's '-r' option to update all occurrences of the given device number in the snapshot file(s). It takes a single argument of the form 'OLDDEV-NEWDEV', where OLDDEV is the device number used in the snapshot file, and NEWDEV is the corresponding new device number. Both numbers may be specified in hex (e.g., '0xfe01'), decimal (e.g., '65025'), or as a major:minor number pair (e.g., '254:1'). To change several device numbers at once, specify them in a single comma-separated list, as in '-r 0x3060-0x4500,0x307-0x4600'. Before updating the snapshot file, it is a good idea to create a backup copy of it. This is accomplished by '-b' option. The name of the backup file is obtained by appending '~' to the original file name. An example session: $ tar-snapshot-edit root_snap.0 boot_snap.0 File: root_snap.0 Detected snapshot file version: 2 Device 0x0000 occurs 1 times. Device 0x0003 occurs 1 times. Device 0x0005 occurs 1 times. Device 0x0013 occurs 1 times. Device 0x6801 occurs 1 times. Device 0x6803 occurs 6626 times. Device 0xfb00 occurs 1 times. File: boot_snap.0 Detected snapshot file version: 2 Device 0x6801 occurs 3 times. $ tar-snapshot-edit -b -r 0x6801-0x6901,0x6803-0x6903 root_snap.0 boot_snap.0 File: root_snap.0 Detected snapshot file version: 2 Updated 6627 records. File: boot_snap.0 Detected snapshot file version: 2 Updated 3 records.  File: tar.info, Node: Tar Internals, Next: Genfile, Prev: Fixing Snapshot Files, Up: Top Appendix E Tar Internals ************************ * Menu: * Standard:: Basic Tar Format * Extensions:: GNU Extensions to the Archive Format * Sparse Formats:: Storing Sparse Files * Snapshot Files:: * Dumpdir::  File: tar.info, Node: Standard, Next: Extensions, Up: Tar Internals Basic Tar Format ================ _(This message will disappear, once this node revised.)_ While an archive may contain many files, the archive itself is a single ordinary file. Like any other file, an archive file can be written to a storage device such as a tape or disk, sent through a pipe or over a network, saved on the active file system, or even stored in another archive. An archive file is not easy to read or manipulate without using the 'tar' utility or Tar mode in GNU Emacs. Physically, an archive consists of a series of file entries terminated by an end-of-archive entry, which consists of two 512 blocks of zero bytes. A file entry usually describes one of the files in the archive (an "archive member"), and consists of a file header and the contents of the file. File headers contain file names and statistics, checksum information which 'tar' uses to detect file corruption, and information about file types. Archives are permitted to have more than one member with the same member name. One way this situation can occur is if more than one version of a file has been stored in the archive. For information about adding new versions of a file to an archive, see *note update::. In addition to entries describing archive members, an archive may contain entries which 'tar' itself uses to store information. *Note label::, for an example of such an archive entry. A 'tar' archive file contains a series of blocks. Each block contains 'BLOCKSIZE' bytes. Although this format may be thought of as being on magnetic tape, other media are often used. Each file archived is represented by a header block which describes the file, followed by zero or more blocks which give the contents of the file. At the end of the archive file there are two 512-byte blocks filled with binary zeros as an end-of-file marker. A reasonable system should write such end-of-file marker at the end of an archive, but must not assume that such a block exists when reading an archive. In particular GNU 'tar' always issues a warning if it does not encounter it. The blocks may be "blocked" for physical I/O operations. Each record of N blocks (where N is set by the '--blocking-factor=512-SIZE' ('-b 512-SIZE') option to 'tar') is written with a single 'write ()' operation. On magnetic tapes, the result of such a write is a single record. When writing an archive, the last record of blocks should be written at the full size, with blocks after the zero block containing all zeros. When reading an archive, a reasonable system should properly handle an archive whose last record is shorter than the rest, or which contains garbage records after a zero block. The header block is defined in C as follows. In the GNU 'tar' distribution, this is part of file 'src/tar.h': /* tar Header Block, from POSIX 1003.1-1990. */ /* POSIX header. */ struct posix_header { /* byte offset */ char name[100]; /* 0 */ char mode[8]; /* 100 */ char uid[8]; /* 108 */ char gid[8]; /* 116 */ char size[12]; /* 124 */ char mtime[12]; /* 136 */ char chksum[8]; /* 148 */ char typeflag; /* 156 */ char linkname[100]; /* 157 */ char magic[6]; /* 257 */ char version[2]; /* 263 */ char uname[32]; /* 265 */ char gname[32]; /* 297 */ char devmajor[8]; /* 329 */ char devminor[8]; /* 337 */ char prefix[155]; /* 345 */ /* 500 */ }; #define TMAGIC "ustar" /* ustar and a null */ #define TMAGLEN 6 #define TVERSION "00" /* 00 and no null */ #define TVERSLEN 2 /* Values used in typeflag field. */ #define REGTYPE '0' /* regular file */ #define AREGTYPE '\0' /* regular file */ #define LNKTYPE '1' /* link */ #define SYMTYPE '2' /* reserved */ #define CHRTYPE '3' /* character special */ #define BLKTYPE '4' /* block special */ #define DIRTYPE '5' /* directory */ #define FIFOTYPE '6' /* FIFO special */ #define CONTTYPE '7' /* reserved */ #define XHDTYPE 'x' /* Extended header referring to the next file in the archive */ #define XGLTYPE 'g' /* Global extended header */ /* Bits used in the mode field, values in octal. */ #define TSUID 04000 /* set UID on execution */ #define TSGID 02000 /* set GID on execution */ #define TSVTX 01000 /* reserved */ /* file permissions */ #define TUREAD 00400 /* read by owner */ #define TUWRITE 00200 /* write by owner */ #define TUEXEC 00100 /* execute/search by owner */ #define TGREAD 00040 /* read by group */ #define TGWRITE 00020 /* write by group */ #define TGEXEC 00010 /* execute/search by group */ #define TOREAD 00004 /* read by other */ #define TOWRITE 00002 /* write by other */ #define TOEXEC 00001 /* execute/search by other */ /* tar Header Block, GNU extensions. */ /* In GNU tar, SYMTYPE is for to symbolic links, and CONTTYPE is for contiguous files, so maybe disobeying the "reserved" comment in POSIX header description. I suspect these were meant to be used this way, and should not have really been "reserved" in the published standards. */ /* *BEWARE* *BEWARE* *BEWARE* that the following information is still boiling, and may change. Even if the OLDGNU format description should be accurate, the so-called GNU format is not yet fully decided. It is surely meant to use only extensions allowed by POSIX, but the sketch below repeats some ugliness from the OLDGNU format, which should rather go away. Sparse files should be saved in such a way that they do *not* require two passes at archive creation time. Huge files get some POSIX fields to overflow, alternate solutions have to be sought for this. */ /* Descriptor for a single file hole. */ struct sparse { /* byte offset */ char offset[12]; /* 0 */ char numbytes[12]; /* 12 */ /* 24 */ }; /* Sparse files are not supported in POSIX ustar format. For sparse files with a POSIX header, a GNU extra header is provided which holds overall sparse information and a few sparse descriptors. When an old GNU header replaces both the POSIX header and the GNU extra header, it holds some sparse descriptors too. Whether POSIX or not, if more sparse descriptors are still needed, they are put into as many successive sparse headers as necessary. The following constants tell how many sparse descriptors fit in each kind of header able to hold them. */ #define SPARSES_IN_EXTRA_HEADER 16 #define SPARSES_IN_OLDGNU_HEADER 4 #define SPARSES_IN_SPARSE_HEADER 21 /* Extension header for sparse files, used immediately after the GNU extra header, and used only if all sparse information cannot fit into that extra header. There might even be many such extension headers, one after the other, until all sparse information has been recorded. */ struct sparse_header { /* byte offset */ struct sparse sp[SPARSES_IN_SPARSE_HEADER]; /* 0 */ char isextended; /* 504 */ /* 505 */ }; /* The old GNU format header conflicts with POSIX format in such a way that POSIX archives may fool old GNU tar's, and POSIX tar's might well be fooled by old GNU tar archives. An old GNU format header uses the space used by the prefix field in a POSIX header, and cumulates information normally found in a GNU extra header. With an old GNU tar header, we never see any POSIX header nor GNU extra header. Supplementary sparse headers are allowed, however. */ struct oldgnu_header { /* byte offset */ char unused_pad1[345]; /* 0 */ char atime[12]; /* 345 Incr. archive: atime of the file */ char ctime[12]; /* 357 Incr. archive: ctime of the file */ char offset[12]; /* 369 Multivolume archive: the offset of the start of this volume */ char longnames[4]; /* 381 Not used */ char unused_pad2; /* 385 */ struct sparse sp[SPARSES_IN_OLDGNU_HEADER]; /* 386 */ char isextended; /* 482 Sparse file: Extension sparse header follows */ char realsize[12]; /* 483 Sparse file: Real size*/ /* 495 */ }; /* OLDGNU_MAGIC uses both magic and version fields, which are contiguous. Found in an archive, it indicates an old GNU header format, which will be hopefully become obsolescent. With OLDGNU_MAGIC, uname and gname are valid, though the header is not truly POSIX conforming. */ #define OLDGNU_MAGIC "ustar " /* 7 chars and a null */ /* The standards committee allows only capital A through capital Z for user-defined expansion. Other letters in use include: 'A' Solaris Access Control List 'E' Solaris Extended Attribute File 'I' Inode only, as in 'star' 'N' Obsolete GNU tar, for file names that do not fit into the main header. 'X' POSIX 1003.1-2001 eXtended (VU version) */ /* This is a dir entry that contains the names of files that were in the dir at the time the dump was made. */ #define GNUTYPE_DUMPDIR 'D' /* Identifies the *next* file on the tape as having a long linkname. */ #define GNUTYPE_LONGLINK 'K' /* Identifies the *next* file on the tape as having a long name. */ #define GNUTYPE_LONGNAME 'L' /* This is the continuation of a file that began on another volume. */ #define GNUTYPE_MULTIVOL 'M' /* This is for sparse files. */ #define GNUTYPE_SPARSE 'S' /* This file is a tape/volume header. Ignore it on extraction. */ #define GNUTYPE_VOLHDR 'V' /* Solaris extended header */ #define SOLARIS_XHDTYPE 'X' /* Jörg Schilling star header */ struct star_header { /* byte offset */ char name[100]; /* 0 */ char mode[8]; /* 100 */ char uid[8]; /* 108 */ char gid[8]; /* 116 */ char size[12]; /* 124 */ char mtime[12]; /* 136 */ char chksum[8]; /* 148 */ char typeflag; /* 156 */ char linkname[100]; /* 157 */ char magic[6]; /* 257 */ char version[2]; /* 263 */ char uname[32]; /* 265 */ char gname[32]; /* 297 */ char devmajor[8]; /* 329 */ char devminor[8]; /* 337 */ char prefix[131]; /* 345 */ char atime[12]; /* 476 */ char ctime[12]; /* 488 */ /* 500 */ }; #define SPARSES_IN_STAR_HEADER 4 #define SPARSES_IN_STAR_EXT_HEADER 21 struct star_in_header { char fill[345]; /* 0 Everything that is before t_prefix */ char prefix[1]; /* 345 t_name prefix */ char fill2; /* 346 */ char fill3[8]; /* 347 */ char isextended; /* 355 */ struct sparse sp[SPARSES_IN_STAR_HEADER]; /* 356 */ char realsize[12]; /* 452 Actual size of the file */ char offset[12]; /* 464 Offset of multivolume contents */ char atime[12]; /* 476 */ char ctime[12]; /* 488 */ char mfill[8]; /* 500 */ char xmagic[4]; /* 508 "tar" */ }; struct star_ext_header { struct sparse sp[SPARSES_IN_STAR_EXT_HEADER]; char isextended; }; All characters in header blocks are represented by using 8-bit characters in the local variant of ASCII. Each field within the structure is contiguous; that is, there is no padding used within the structure. Each character on the archive medium is stored contiguously. Bytes representing the contents of files (after the header block of each file) are not translated in any way and are not constrained to represent characters in any character set. The 'tar' format does not distinguish text files from binary files, and no translation of file contents is performed. The 'name', 'linkname', 'magic', 'uname', and 'gname' are null-terminated character strings. All other fields are zero-filled octal numbers in ASCII. Each numeric field of width W contains W minus 1 digits, and a null. (In the extended GNU format, the numeric fields can take other forms.) The 'name' field is the file name of the file, with directory names (if any) preceding the file name, separated by slashes. The 'mode' field provides nine bits specifying file permissions and three bits to specify the Set UID, Set GID, and Save Text ("sticky") modes. Values for these bits are defined above. When special permissions are required to create a file with a given mode, and the user restoring files from the archive does not hold such permissions, the mode bit(s) specifying those special permissions are ignored. Modes which are not supported by the operating system restoring files from the archive will be ignored. Unsupported modes should be faked up when creating or updating an archive; e.g., the group permission could be copied from the _other_ permission. The 'uid' and 'gid' fields are the numeric user and group ID of the file owners, respectively. If the operating system does not support numeric user or group IDs, these fields should be ignored. The 'size' field is the size of the file in bytes; linked files are archived with this field specified as zero. The 'mtime' field represents the data modification time of the file at the time it was archived. It represents the integer number of seconds since January 1, 1970, 00:00 Coordinated Universal Time. The 'chksum' field represents the simple sum of all bytes in the header block. Each 8-bit byte in the header is added to an unsigned integer, initialized to zero, the precision of which shall be no less than seventeen bits. When calculating the checksum, the 'chksum' field is treated as if it were all blanks. The 'typeflag' field specifies the type of file archived. If a particular implementation does not recognize or permit the specified type, the file will be extracted as if it were a regular file. As this action occurs, 'tar' issues a warning to the standard error. The 'atime' and 'ctime' fields are used in making incremental backups; they store, respectively, the particular file's access and status change times. The 'offset' is used by the '--multi-volume' ('-M') option, when making a multi-volume archive. The offset is number of bytes into the file that we need to restart at to continue the file on the next tape, i.e., where we store the location that a continued file is continued at. The following fields were added to deal with sparse files. A file is "sparse" if it takes in unallocated blocks which end up being represented as zeros, i.e., no useful data. A test to see if a file is sparse is to look at the number blocks allocated for it versus the number of characters in the file; if there are fewer blocks allocated for the file than would normally be allocated for a file of that size, then the file is sparse. This is the method 'tar' uses to detect a sparse file, and once such a file is detected, it is treated differently from non-sparse files. Sparse files are often 'dbm' files, or other database-type files which have data at some points and emptiness in the greater part of the file. Such files can appear to be very large when an 'ls -l' is done on them, when in truth, there may be a very small amount of important data contained in the file. It is thus undesirable to have 'tar' think that it must back up this entire file, as great quantities of room are wasted on empty blocks, which can lead to running out of room on a tape far earlier than is necessary. Thus, sparse files are dealt with so that these empty blocks are not written to the tape. Instead, what is written to the tape is a description, of sorts, of the sparse file: where the holes are, how big the holes are, and how much data is found at the end of the hole. This way, the file takes up potentially far less room on the tape, and when the file is extracted later on, it will look exactly the way it looked beforehand. The following is a description of the fields used to handle a sparse file: The 'sp' is an array of 'struct sparse'. Each 'struct sparse' contains two 12-character strings which represent an offset into the file and a number of bytes to be written at that offset. The offset is absolute, and not relative to the offset in preceding array element. The header can hold four of these 'struct sparse' at the moment; if more are needed, they are not stored in the header. The 'isextended' flag is set when an 'extended_header' is needed to deal with a file. Note that this means that this flag can only be set when dealing with a sparse file, and it is only set in the event that the description of the file will not fit in the allotted room for sparse structures in the header. In other words, an extended_header is needed. The 'extended_header' structure is used for sparse files which need more sparse structures than can fit in the header. The header can fit 4 such structures; if more are needed, the flag 'isextended' gets set and the next block is an 'extended_header'. Each 'extended_header' structure contains an array of 21 sparse structures, along with a similar 'isextended' flag that the header had. There can be an indeterminate number of such 'extended_header's to describe a sparse file. 'REGTYPE' 'AREGTYPE' These flags represent a regular file. In order to be compatible with older versions of 'tar', a 'typeflag' value of 'AREGTYPE' should be silently recognized as a regular file. New archives should be created using 'REGTYPE'. Also, for backward compatibility, 'tar' treats a regular file whose name ends with a slash as a directory. 'LNKTYPE' This flag represents a file linked to another file, of any type, previously archived. Such files are identified in Unix by each file having the same device and inode number. The linked-to name is specified in the 'linkname' field with a trailing null. 'SYMTYPE' This represents a symbolic link to another file. The linked-to name is specified in the 'linkname' field with a trailing null. 'CHRTYPE' 'BLKTYPE' These represent character special files and block special files respectively. In this case the 'devmajor' and 'devminor' fields will contain the major and minor device numbers respectively. Operating systems may map the device specifications to their own local specification, or may ignore the entry. 'DIRTYPE' This flag specifies a directory or sub-directory. The directory name in the 'name' field should end with a slash. On systems where disk allocation is performed on a directory basis, the 'size' field will contain the maximum number of bytes (which may be rounded to the nearest disk block allocation unit) which the directory may hold. A 'size' field of zero indicates no such limiting. Systems which do not support limiting in this manner should ignore the 'size' field. 'FIFOTYPE' This specifies a FIFO special file. Note that the archiving of a FIFO file archives the existence of this file and not its contents. 'CONTTYPE' This specifies a contiguous file, which is the same as a normal file except that, in operating systems which support it, all its space is allocated contiguously on the disk. Operating systems which do not allow contiguous allocation should silently treat this type as a normal file. 'A' ... 'Z' These are reserved for custom implementations. Some of these are used in the GNU modified format, as described below. Other values are reserved for specification in future revisions of the P1003 standard, and should not be used by any 'tar' program. The 'magic' field indicates that this archive was output in the P1003 archive format. If this field contains 'TMAGIC', the 'uname' and 'gname' fields will contain the ASCII representation of the owner and group of the file respectively. If found, the user and group IDs are used rather than the values in the 'uid' and 'gid' fields. For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages 169-173 (section 10.1) for 'Archive/Interchange File Format'; and IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940 (section E.4.48) for 'pax - Portable archive interchange'.  File: tar.info, Node: Extensions, Next: Sparse Formats, Prev: Standard, Up: Tar Internals GNU Extensions to the Archive Format ==================================== _(This message will disappear, once this node revised.)_ The GNU format uses additional file types to describe new types of files in an archive. These are listed below. 'GNUTYPE_DUMPDIR' ''D'' This represents a directory and a list of files created by the '--incremental' ('-G') option. The 'size' field gives the total size of the associated list of files. Each file name is preceded by either a 'Y' (the file should be in this archive) or an 'N'. (The file is a directory, or is not stored in the archive.) Each file name is terminated by a null. There is an additional null after the last file name. 'GNUTYPE_MULTIVOL' ''M'' This represents a file continued from another volume of a multi-volume archive created with the '--multi-volume' ('-M') option. The original type of the file is not given here. The 'size' field gives the maximum size of this piece of the file (assuming the volume does not end before the file is written out). The 'offset' field gives the offset from the beginning of the file where this part of the file begins. Thus 'size' plus 'offset' should equal the original size of the file. 'GNUTYPE_SPARSE' ''S'' This flag indicates that we are dealing with a sparse file. Note that archiving a sparse file requires special operations to find holes in the file, which mark the positions of these holes, along with the number of bytes of data to be found after the hole. 'GNUTYPE_VOLHDR' ''V'' This file type is used to mark the volume header that was given with the '--label=ARCHIVE-LABEL' ('-V ARCHIVE-LABEL') option when the archive was created. The 'name' field contains the 'name' given after the '--label=ARCHIVE-LABEL' ('-V ARCHIVE-LABEL') option. The 'size' field is zero. Only the first file in each volume of an archive should have this type. For fields containing numbers or timestamps that are out of range for the basic format, the GNU format uses a base-256 representation instead of an ASCII octal number. If the leading byte is 0xff (255), all the bytes of the field (including the leading byte) are concatenated in big-endian order, with the result being a negative number expressed in two's complement form. If the leading byte is 0x80 (128), the non-leading bytes of the field are concatenated in big-endian order, with the result being a positive number expressed in binary form. Leading bytes other than 0xff, 0x80 and ASCII octal digits are reserved for future use, as are base-256 representations of values that would be in range for the basic format. You may have trouble reading a GNU format archive on a non-GNU system if the options '--incremental' ('-G'), '--multi-volume' ('-M'), '--sparse' ('-S'), or '--label=ARCHIVE-LABEL' ('-V ARCHIVE-LABEL') were used when writing the archive. In general, if 'tar' does not use the GNU-added fields of the header, other versions of 'tar' should be able to read the archive. Otherwise, the 'tar' program will give an error, the most likely one being a checksum error.  File: tar.info, Node: Sparse Formats, Next: Snapshot Files, Prev: Extensions, Up: Tar Internals Storing Sparse Files ==================== The notion of sparse file, and the ways of handling it from the point of view of GNU 'tar' user have been described in detail in *note sparse::. This chapter describes the internal format GNU 'tar' uses to store such files. The support for sparse files in GNU 'tar' has a long history. The earliest version featuring this support that I was able to find was 1.09, released in November, 1990. The format introduced back then is called "old GNU" sparse format and in spite of the fact that its design contained many flaws, it was the only format GNU 'tar' supported until version 1.14 (May, 2004), which introduced initial support for sparse archives in PAX archives (*note posix::). This format was not free from design flaws, either and it was subsequently improved in versions 1.15.2 (November, 2005) and 1.15.92 (June, 2006). In addition to GNU sparse format, GNU 'tar' is able to read and extract sparse files archived by 'star'. The following subsections describe each format in detail. * Menu: * Old GNU Format:: * PAX 0:: PAX Format, Versions 0.0 and 0.1 * PAX 1:: PAX Format, Version 1.0  File: tar.info, Node: Old GNU Format, Next: PAX 0, Up: Sparse Formats Old GNU Format -------------- The format introduced in November 1990 (v. 1.09) was designed on top of standard 'ustar' headers in such an unfortunate way that some of its fields overwrote fields required by POSIX. An old GNU sparse header is designated by type 'S' ('GNUTYPE_SPARSE') and has the following layout: Offset Size Name Data type Contents ---------------------------------------------------------------------------- 0 345 N/A Not used. 345 12 atime Number 'atime' of the file. 357 12 ctime Number 'ctime' of the file . 369 12 offset Number For multivolume archives: the offset of the start of this volume. 381 4 N/A Not used. 385 1 N/A Not used. 386 96 sp 'sparse_header'(4 entries) File map. 482 1 isextended Bool '1' if an extension sparse header follows, '0' otherwise. 483 12 realsize Number Real size of the file. Each of 'sparse_header' object at offset 386 describes a single data chunk. It has the following structure: Offset Size Data type Contents --------------------------------------------------------------------------- 0 12 Number Offset of the beginning of the chunk. 12 12 Number Size of the chunk. If the member contains more than four chunks, the 'isextended' field of the header has the value '1' and the main header is followed by one or more "extension headers". Each such header has the following structure: Offset Size Name Data type Contents ---------------------------------------------------------------------------- 0 21 sp 'sparse_header'(21 entries) File map. 504 1 isextended Bool '1' if an extension sparse header follows, or '0' otherwise. A header with 'isextended=0' ends the map.  File: tar.info, Node: PAX 0, Next: PAX 1, Prev: Old GNU Format, Up: Sparse Formats PAX Format, Versions 0.0 and 0.1 -------------------------------- There are two formats available in this branch. The version '0.0' is the initial version of sparse format used by 'tar' versions 1.14-1.15.1. The sparse file map is kept in extended ('x') PAX header variables: 'GNU.sparse.size' Real size of the stored file; 'GNU.sparse.numblocks' Number of blocks in the sparse map; 'GNU.sparse.offset' Offset of the data block; 'GNU.sparse.numbytes' Size of the data block. The latter two variables repeat for each data block, so the overall structure is like this: GNU.sparse.size=SIZE GNU.sparse.numblocks=NUMBLOCKS repeat NUMBLOCKS times GNU.sparse.offset=OFFSET GNU.sparse.numbytes=NUMBYTES end repeat This format presented the following two problems: 1. Whereas the POSIX specification allows a variable to appear multiple times in a header, it requires that only the last occurrence be meaningful. Thus, multiple occurrences of 'GNU.sparse.offset' and 'GNU.sparse.numbytes' are conflicting with the POSIX specs. 2. Attempting to extract such archives using a third-party's 'tar' results in extraction of sparse files in _condensed form_. If the 'tar' implementation in question does not support POSIX format, it will also extract a file containing extension header attributes. This file can be used to expand the file to its original state. However, posix-aware 'tar's will usually ignore the unknown variables, which makes restoring the file more difficult. *Note Extraction of sparse members in v.0.0 format: extracting sparse v0x, for the detailed description of how to restore such members using non-GNU 'tar's. GNU 'tar' 1.15.2 introduced sparse format version '0.1', which attempted to solve these problems. As its predecessor, this format stores sparse map in the extended POSIX header. It retains 'GNU.sparse.size' and 'GNU.sparse.numblocks' variables, but instead of 'GNU.sparse.offset'/'GNU.sparse.numbytes' pairs it uses a single variable: 'GNU.sparse.map' Map of non-null data chunks. It is a string consisting of comma-separated values "OFFSET,SIZE[,OFFSET-1,SIZE-1...]" To address the 2nd problem, the 'name' field in 'ustar' is replaced with a special name, constructed using the following pattern: %d/GNUSparseFile.%p/%f The real name of the sparse file is stored in the variable 'GNU.sparse.name'. Thus, those 'tar' implementations that are not aware of GNU extensions will at least extract the files into separate directories, giving the user a possibility to expand it afterwards. *Note Extraction of sparse members in v.0.1 format: extracting sparse v0x, for the detailed description of how to restore such members using non-GNU 'tar's. The resulting 'GNU.sparse.map' string can be _very_ long. Although POSIX does not impose any limit on the length of a 'x' header variable, this possibly can confuse some 'tar's.  File: tar.info, Node: PAX 1, Prev: PAX 0, Up: Sparse Formats PAX Format, Version 1.0 ----------------------- The version '1.0' of sparse format was introduced with GNU 'tar' 1.15.92. Its main objective was to make the resulting file extractable with little effort even by non-posix aware 'tar' implementations. Starting from this version, the extended header preceding a sparse member always contains the following variables that identify the format being used: 'GNU.sparse.major' Major version 'GNU.sparse.minor' Minor version The 'name' field in 'ustar' header contains a special name, constructed using the following pattern: %d/GNUSparseFile.%p/%f The real name of the sparse file is stored in the variable 'GNU.sparse.name'. The real size of the file is stored in the variable 'GNU.sparse.realsize'. The sparse map itself is stored in the file data block, preceding the actual file data. It consists of a series of decimal numbers delimited by newlines. The map is padded with nulls to the nearest block boundary. The first number gives the number of entries in the map. Following are map entries, each one consisting of two numbers giving the offset and size of the data block it describes. The format is designed in such a way that non-posix aware 'tar's and 'tar's not supporting 'GNU.sparse.*' keywords will extract each sparse file in its condensed form with the file map prepended and will place it into a separate directory. Then, using a simple program it would be possible to expand the file to its original form even without GNU 'tar'. *Note Sparse Recovery::, for the detailed information on how to extract sparse members without GNU 'tar'.  File: tar.info, Node: Snapshot Files, Next: Dumpdir, Prev: Sparse Formats, Up: Tar Internals Format of the Incremental Snapshot Files ======================================== A "snapshot file" (or "directory file") is created during incremental backups (*note Incremental Dumps::). It contains the status of the file system at the time of the dump and is used to determine which files were modified since the last backup. GNU 'tar' version 1.34 supports three snapshot file formats. The first format, called "format 0", is the one used by GNU 'tar' versions up to and including 1.15.1. The second format, called "format 1" is an extended version of this format, that contains more metadata and allows for further extensions. It was used by alpha release version 1.15.90. For alpha version 1.15.91 and stable releases version 1.16 up through 1.34, the "format 2" is used. GNU 'tar' is able to read all three formats, but will create snapshots only in format 2. This appendix describes all three formats in detail. 0. 'Format 0' snapshot file begins with a line containing a decimal number that represents a UNIX timestamp of the beginning of the last archivation. This line is followed by directory metadata descriptions, one per line. Each description has the following format: [NFS]DEV INODE NAME where: NFS A single plus character ('+'), if this directory is located on an NFS-mounted partition, otherwise empty. (That is, for non-NFS directories, the first character on the description line contains the start of the DEV field.) DEV Device number of the directory; INODE I-node number of the directory; NAME Name of the directory. Any special characters (white-space, backslashes, etc.) are quoted. 1. 'Format 1' snapshot file begins with a line specifying the format of the file. This line has the following structure: 'GNU tar-'TAR-VERSION'-'INCR-FORMAT-VERSION where TAR-VERSION is the version number of GNU 'tar' implementation that created this snapshot, and INCR-FORMAT-VERSION is the version number of the snapshot format (in this case '1'). Next line contains two decimal numbers, representing the time of the last backup. First number is the number of seconds, the second one is the number of nanoseconds, since the beginning of the epoch. Lines that follow contain directory metadata, one line per directory. Each line is formatted as follows: [NFS]MTIME-SEC MTIME-NSEC DEV INODE NAME where MTIME-SEC and MTIME-NSEC represent last modification time of this directory with nanosecond precision; NFS, DEV, INODE and NAME have the same meaning as with 'format 0'. 2. 'Format 2' snapshot file begins with a format identifier, as described for version 1, e.g.: GNU tar-1.34-2 This line is followed by newline. Rest of file consists of records, separated by null (ASCII 0) characters. Thus, in contrast to the previous formats, format 2 snapshot is a binary file. First two records are decimal integers, representing the time of the last backup. First number is the number of seconds, the second one is the number of nanoseconds, since the beginning of the epoch. These are followed by arbitrary number of directory records. Each "directory record" contains a set of metadata describing a particular directory. Parts of a directory record are delimited with ASCII 0 characters. The following table describes each part. The "Number" type in this table stands for a decimal integer in ASCII notation. (Negative values are preceded with a "-" character, while positive values have no leading punctuation.) Field Type Description --------------------------------------------------------------------------- nfs Character '1' if the directory is located on an NFS-mounted partition, or '0' otherwise; timestamp_sec Number Modification time, seconds; timestamp_nsec Number Modification time, nanoseconds; dev Number Device number; ino Number I-node number; name String Directory name; in contrast to the previous versions it is not quoted; contents Dumpdir Contents of the directory; *Note Dumpdir::, for a description of its format. Dumpdirs stored in snapshot files contain only records of types 'Y', 'N' and 'D'. The specific range of values allowed in each of the "Number" fields depends on the underlying C datatypes as determined when 'tar' is compiled. To see the specific ranges allowed for a particular 'tar' binary, you can use the '--show-snapshot-field-ranges' option: $ tar --show-snapshot-field-ranges This tar's snapshot file field ranges are (field name => [ min, max ]): nfs => [ 0, 1 ], timestamp_sec => [ -9223372036854775808, 9223372036854775807 ], timestamp_nsec => [ 0, 999999999 ], dev => [ 0, 18446744073709551615 ], ino => [ 0, 18446744073709551615 ], (This example is from a GNU/Linux x86_64 system.)  File: tar.info, Node: Dumpdir, Prev: Snapshot Files, Up: Tar Internals Dumpdir ======= Incremental archives keep information about contents of each dumped directory in special data blocks called "dumpdirs". Dumpdir is a sequence of entries of the following form: C FILENAME \0 where C is one of the "control codes" described below, FILENAME is the name of the file C operates upon, and '\0' represents a nul character (ASCII 0). The white space characters were added for readability, real dumpdirs do not contain them. Each dumpdir ends with a single nul character. The following table describes control codes and their meanings: 'Y' FILENAME is contained in the archive. 'N' FILENAME was present in the directory at the time the archive was made, yet it was not dumped to the archive, because it had not changed since the last backup. 'D' FILENAME is a directory. 'R' This code requests renaming of the FILENAME to the name specified with the 'T' command, that immediately follows it. 'T' Specify target file name for 'R' command (see below). 'X' Specify "temporary directory" name for a rename operation (see below). Codes 'Y', 'N' and 'D' require FILENAME argument to be a relative file name to the directory this dumpdir describes, whereas codes 'R', 'T' and 'X' require their argument to be an absolute file name. The three codes 'R', 'T' and 'X' specify a "renaming operation". In the simplest case it is: Rsource\0Tdest\0 which means "rename file 'source' to file 'dest'". However, there are cases that require using a "temporary directory". For example, consider the following scenario: 1. Previous run dumped a directory 'foo' which contained the following three directories: a b c 2. They were renamed _cyclically_, so that: a became b b became c c became a 3. New incremental dump was made. This case cannot be handled by three successive renames, since renaming 'a' to 'b' will destroy the existing directory. To correctly process it, GNU 'tar' needs a temporary directory, so it creates the following dumpdir (newlines have been added for readability): Xfoo\0 Rfoo/a\0T\0 Rfoo/b\0Tfoo/c\0 Rfoo/c\0Tfoo/a\0 R\0Tfoo/a\0 The first command, 'Xfoo\0', instructs the extractor to create a temporary directory in the directory 'foo'. Second command, 'Rfoo/aT\0', says "rename file 'foo/a' to the temporary directory that has just been created" (empty file name after a command means use temporary directory). Third and fourth commands work as usual, and, finally, the last command, 'R\0Tfoo/a\0' tells tar to rename the temporary directory to 'foo/a'. The exact placement of a dumpdir in the archive depends on the archive format (*note Formats::): * PAX archives In PAX archives, dumpdir is stored in the extended header of the corresponding directory, in variable 'GNU.dumpdir'. * GNU and old GNU archives These formats implement special header type 'D', which is similar to ustar header '5' (directory), except that it precedes a data block containing the dumpdir.  File: tar.info, Node: Genfile, Next: GNU Free Documentation License, Prev: Tar Internals, Up: Top Appendix F Genfile ****************** This appendix describes 'genfile', an auxiliary program used in the GNU tar testsuite. If you are not interested in developing GNU tar, skip this appendix. Initially, 'genfile' was used to generate data files for the testsuite, hence its name. However, new operation modes were being implemented as the testsuite grew more sophisticated, and now 'genfile' is a multi-purpose instrument. There are three basic operation modes: File Generation This is the default mode. In this mode, 'genfile' generates data files. File Status In this mode 'genfile' displays status of specified files. Synchronous Execution. In this mode 'genfile' executes the given program with '--checkpoint' option and executes a set of actions when specified checkpoints are reached. * Menu: * Generate Mode:: File Generation Mode. * Status Mode:: File Status Mode. * Exec Mode:: Synchronous Execution mode.  File: tar.info, Node: Generate Mode, Next: Status Mode, Up: Genfile F.1 Generate Mode ================= In this mode 'genfile' creates a data file for the test suite. The size of the file is given with the '--length' ('-l') option. By default the file contents is written to the standard output, this can be changed using '--file' ('-f') command line option. Thus, the following two commands are equivalent: genfile --length 100 > outfile genfile --length 100 --file outfile If '--length' is not given, 'genfile' will generate an empty (zero-length) file. The command line option '--seek=N' istructs 'genfile' to skip the given number of bytes (N) in the output file before writing to it. It is similar to the 'seek=N' of the 'dd' utility. You can instruct 'genfile' to create several files at one go, by giving it '--files-from' ('-T') option followed by a name of file containing a list of file names. Using dash ('-') instead of the file name causes 'genfile' to read file list from the standard input. For example: # Read file names from file file.list genfile --files-from file.list # Read file names from standard input genfile --files-from - The list file is supposed to contain one file name per line. To use file lists separated by ASCII NUL character, use '--null' ('-0') command line option: genfile --null --files-from file.list The default data pattern for filling the generated file consists of first 256 letters of ASCII code, repeated enough times to fill the entire file. This behavior can be changed with '--pattern' option. This option takes a mandatory argument, specifying pattern name to use. Currently two patterns are implemented: '--pattern=default' The default pattern as described above. '--pattern=zero' Fills the file with zeroes. If no file name was given, the program exits with the code '0'. Otherwise, it exits with '0' only if it was able to create a file of the specified length. Special option '--sparse' ('-s') instructs 'genfile' to create a sparse file. Sparse files consist of "data fragments", separated by "holes" or blocks of zeros. On many operating systems, actual disk storage is not allocated for holes, but they are counted in the length of the file. To create a sparse file, 'genfile' should know where to put data fragments, and what data to use to fill them. So, when '--sparse' is given the rest of the command line specifies a so-called "file map". The file map consists of any number of "fragment descriptors". Each descriptor is composed of two values: a number, specifying fragment offset from the end of the previous fragment or, for the very first fragment, from the beginning of the file, and "contents string", that specifies the pattern to fill the fragment with. File offset can be suffixed with the following quantifiers: 'k' 'K' The number is expressed in kilobytes. 'm' 'M' The number is expressed in megabytes. 'g' 'G' The number is expressed in gigabytes. Contents string can be either a fragment size or a pattern. Fragment size is a decimal number, prefixed with an equals sign. It can be suffixed with a quantifier, as discussed above. If fragment size is given, the fragment of that size will be filled with the currently selected pattern (*note -pattern: Generate Mode.) and written to the file. A pattern is a string of arbitrary ASCII characters. For each of them, 'genfile' will generate a "block" of data, filled with that character and will write it to the fragment. The size of block is given by '--block-size' option. It defaults to 512. Thus, if pattern consists of N characters, the resulting file fragment will contain 'N*BLOCK-SIZE' bytes of data. The last fragment descriptor can have only file offset part. In this case 'genfile' will create a hole at the end of the file up to the given offset. A dash appearing as a fragment descriptor instructs 'genfile' to read file map from the standard input. Each line of input should consist of fragment offset and contents string, separated by any amount of whitespace. For example, consider the following invocation: genfile --sparse --file sparsefile 0 ABCD 1M EFGHI 2000K It will create 3101184-bytes long file of the following structure: Offset Length Contents 0 4*512=2048 Four 512-byte blocks, filled with letters 'A', 'B', 'C' and 'D'. 2048 1046528 Zero bytes 1050624 5*512=2560 Five blocks, filled with letters 'E', 'F', 'G', 'H', 'I'. 1053184 2048000 Zero bytes The exit code of 'genfile --sparse' command is '0' only if created file is actually sparse. If it is not, the appropriate error message is displayed and the command exists with code '1'. The '--quite' ('-q') option suppresses this behavior. If '--quite' is given, 'genfile --sparse' exits with code '0' if it was able to create the file, whether the resulting file is sparse or not.  File: tar.info, Node: Status Mode, Next: Exec Mode, Prev: Generate Mode, Up: Genfile F.2 Status Mode =============== In status mode, 'genfile' prints file system status for each file specified in the command line. This mode is toggled by '--stat' ('-S') command line option. An optional argument to this option specifies output "format": a comma-separated list of 'struct stat' fields to be displayed. This list can contain following identifiers: name The file name. dev st_dev Device number in decimal. ino st_ino Inode number. mode[.NUMBER] st_mode[.NUMBER] File mode in octal. Optional NUMBER specifies octal mask to be applied to the mode before outputting. For example, '--stat mode.777' will preserve lower nine bits of it. Notice, that you can use any punctuation character in place of '.'. nlink st_nlink Number of hard links. uid st_uid User ID of owner. gid st_gid Group ID of owner. size st_size File size in decimal. blksize st_blksize The size in bytes of each file block. blocks st_blocks Number of blocks allocated. atime st_atime Time of last access. mtime st_mtime Time of last modification ctime st_ctime Time of last status change sparse A boolean value indicating whether the file is 'sparse'. Modification times are displayed in UTC as UNIX timestamps, unless suffixed with 'H' (for "human-readable"), as in 'ctimeH', in which case usual 'tar tv' output format is used. The default output format is: 'name,dev,ino,mode, nlink,uid,gid,size,blksize,blocks,atime,mtime,ctime'. For example, the following command will display file names and corresponding times of last access for each file in the current working directory: genfile --stat=name,atime *  File: tar.info, Node: Exec Mode, Prev: Status Mode, Up: Genfile F.3 Exec Mode ============= This mode is designed for testing the behavior of 'paxutils' commands when some of the files change during archiving. It supposes that the command being executed supports '--checkpoint' and '--checkpoint-action' options (*note Checkpoints: (tar)checkpoints.). The 'Exec Mode' is enabled by '--run' command line option (or its alias '-r'). The non-optional arguments supply the command line to be executed. 'Genfile' modifies this command line by inserting the following options between the command name and first argument: --checkpoint=N --checkpoint-action "echo=genfile checkpoint %u" --checkpoint-action "wait=SIGUSR1" Here, N stands for the checkpoint granularity (for GNU 'tar', it is the number of archive records read or written between each pair of checkpoints). The default value is 1. This value can be changed using the optional argument to the '--run' option. For example, to run actions on each 10th checkpoint: genfile --run=10 ... If the command line contains options, it must be preceded by a double-dash ('--'), which will prevent these options from being interpreted by 'genfile' itself. For example: genfile --run --checkpoint=2 --truncate foo -- tar -c -f a.tar . Notice also, that when running 'tar', its command line may not contain traditional options (cluster of letters without dash). A set of options is provided for defining checkpoint values and actions to be executed upon reaching them. Checkpoint values are introduced with the '--checkpoint' command line option. Argument to this option is the number of checkpoint in decimal. Any number of "actions" may be specified after a checkpoint. Available actions are '--cut FILE' '--truncate FILE' Truncate FILE to the size specified by previous '--length' option (or 0, if it is not given). '--append FILE' Append data to FILE. The size of data and its pattern are given by previous '--length' and 'pattern' options. '--touch FILE' Update the access and modification times of FILE. These timestamps are changed to the current time, unless '--date' option was given, in which case they are changed to the specified time. Argument to '--date' option is a date specification in an almost arbitrary format (*note Date input formats::). '--exec COMMAND' Execute given shell command. '--delete FILE' '--unlink FILE' Delete the named file or directory. If deleting the directory, it must be empty. Option '--verbose' instructs 'genfile' to print on standard output notifications about checkpoints being executed and to verbosely describe exit status of the command. While the command is being executed its standard output remains connected to descriptor 1. All messages it prints to file descriptor 2, except checkpoint notifications, are forwarded to standard error. In exec mode, 'genfile' exits with the exit status of the executed command.  File: tar.info, Node: GNU Free Documentation License, Next: Index of Command Line Options, Prev: Genfile, Up: Top Appendix G GNU Free Documentation License ***************************************** Version 1.3, 3 November 2008 Copyright (C) 2000-2021 Free Software Foundation, Inc. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference. 1. APPLICABILITY AND DEFINITIONS This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law. A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language. A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them. The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none. The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words. A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not "Transparent" is called "Opaque". Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only. The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text. The "publisher" means any person or entity that distributes copies of the Document to the public. A section "Entitled XYZ" means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as "Acknowledgements", "Dedications", "Endorsements", or "History".) To "Preserve the Title" of such a section when you modify the Document means that it remains a section "Entitled XYZ" according to this definition. The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License. 2. VERBATIM COPYING You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3. You may also lend copies, under the same conditions stated above, and you may publicly display copies. 3. COPYING IN QUANTITY If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects. If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages. If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public. It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document. 4. MODIFICATIONS You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version: A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission. B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement. C. State on the Title page the name of the publisher of the Modified Version, as the publisher. D. Preserve all the copyright notices of the Document. E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices. F. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below. G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice. H. Include an unaltered copy of this License. I. Preserve the section Entitled "History", Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence. J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission. K. For any section Entitled "Acknowledgements" or "Dedications", Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein. L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles. M. Delete any section Entitled "Endorsements". Such a section may not be included in the Modified Version. N. Do not retitle any existing section to be Entitled "Endorsements" or to conflict in title with any Invariant Section. O. Preserve any Warranty Disclaimers. If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles. You may add a section Entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties--for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard. You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one. The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version. 5. COMBINING DOCUMENTS You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers. The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work. In the combination, you must combine any sections Entitled "History" in the various original documents, forming one section Entitled "History"; likewise combine any sections Entitled "Acknowledgements", and any sections Entitled "Dedications". You must delete all sections Entitled "Endorsements." 6. COLLECTIONS OF DOCUMENTS You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects. You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document. 7. AGGREGATION WITH INDEPENDENT WORKS A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an "aggregate" if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document. If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate. 8. TRANSLATION Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail. If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title. 9. TERMINATION You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License. However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation. Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice. Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it. 10. FUTURE REVISIONS OF THIS LICENSE The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See . Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document. 11. RELICENSING "Massive Multiauthor Collaboration Site" (or "MMC Site") means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A "Massive Multiauthor Collaboration" (or "MMC") contained in the site means any set of copyrightable works thus published on the MMC site. "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization. "Incorporate" means to publish or republish a Document, in whole or in part, as part of another Document. An MMC is "eligible for relicensing" if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008. The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing. ADDENDUM: How to use this License for your documents ==================================================== To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page: Copyright (C) YEAR YOUR NAME. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the "with...Texts." line with this: with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST. If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation. If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.  File: tar.info, Node: Index of Command Line Options, Next: Index, Prev: GNU Free Documentation License, Up: Top Appendix H Index of Command Line Options **************************************** This appendix contains an index of all GNU 'tar' long command line options. The options are listed without the preceding double-dash. For a cross-reference of short command line options, see *note Short Option Summary::. [index] * Menu: * absolute-names: absolute. (line 10) * absolute-names, summary: Option Summary. (line 6) * acls, summary: Option Summary. (line 14) * add-file: files. (line 83) * after-date: after. (line 24) * after-date, summary: Option Summary. (line 17) * anchored: controlling pattern-matching. (line 78) * anchored, summary: Option Summary. (line 21) * append: append. (line 6) * append <1>: appending files. (line 6) * append, summary: Operation Summary. (line 6) * atime-preserve: Attributes. (line 10) * atime-preserve, summary: Option Summary. (line 25) * auto-compress: gzip. (line 161) * auto-compress, summary: Option Summary. (line 71) * backup: backup. (line 41) * backup, summary: Option Summary. (line 78) * block-number: verbose. (line 112) * block-number, summary: Option Summary. (line 84) * blocking-factor: Blocking Factor. (line 8) * blocking-factor, summary: Option Summary. (line 91) * bzip2, summary: Option Summary. (line 97) * catenate: concatenate. (line 6) * catenate, summary: Operation Summary. (line 11) * check-device, described: Incremental Dumps. (line 107) * check-device, summary: Option Summary. (line 103) * check-links, described: hard links. (line 31) * check-links, summary: Option Summary. (line 161) * checkpoint: checkpoints. (line 6) * checkpoint, defined: checkpoints. (line 13) * checkpoint, summary: Option Summary. (line 108) * checkpoint-action: checkpoints. (line 6) * checkpoint-action, defined: checkpoints. (line 22) * checkpoint-action, summary: Option Summary. (line 117) * clamp-mtime, summary: Option Summary. (line 178) * compare: compare. (line 6) * compare, summary: Operation Summary. (line 16) * compress: gzip. (line 124) * compress, summary: Option Summary. (line 170) * concatenate: concatenate. (line 6) * concatenate, summary: Operation Summary. (line 23) * confirmation, summary: Option Summary. (line 182) * create, additional options: create options. (line 6) * create, complementary notes: Basic tar. (line 11) * create, introduced: Creating the archive. (line 6) * create, summary: Operation Summary. (line 29) * create, using with --verbose: create verbose. (line 6) * create, using with --verify: verify. (line 24) * delay-directory-restore: Directory Modification Times and Permissions. (line 62) * delay-directory-restore, summary: Option Summary. (line 186) * delete: delete. (line 6) * delete, summary: Operation Summary. (line 34) * delete, using before -append: append. (line 47) * dereference: dereference. (line 6) * dereference, summary: Option Summary. (line 192) * diff, summary: Operation Summary. (line 39) * directory: directory. (line 11) * directory, summary: Option Summary. (line 199) * exclude: exclude. (line 6) * exclude <1>: exclude. (line 9) * exclude, potential problems with: problems with exclude. (line 6) * exclude, summary: Option Summary. (line 207) * exclude-backups: exclude. (line 114) * exclude-backups, summary: Option Summary. (line 212) * exclude-caches: exclude. (line 134) * exclude-caches, summary: Option Summary. (line 221) * exclude-caches-all: exclude. (line 142) * exclude-caches-all, summary: Option Summary. (line 236) * exclude-caches-under: exclude. (line 138) * exclude-caches-under, summary: Option Summary. (line 229) * exclude-from: exclude. (line 6) * exclude-from <1>: exclude. (line 20) * exclude-from, summary: Option Summary. (line 215) * exclude-ignore: exclude. (line 76) * exclude-ignore, summary: Option Summary. (line 241) * exclude-ignore-recursive: exclude. (line 81) * exclude-ignore-recursive, summary: Option Summary. (line 246) * exclude-tag: exclude. (line 151) * exclude-tag, summary: Option Summary. (line 251) * exclude-tag-all: exclude. (line 159) * exclude-tag-all, summary: Option Summary. (line 263) * exclude-tag-under: exclude. (line 155) * exclude-tag-under, summary: Option Summary. (line 257) * exclude-vcs: exclude. (line 85) * exclude-vcs, summary: Option Summary. (line 268) * exclude-vcs-ignores: exclude. (line 42) * exclude-vcs-ignores, summary: Option Summary. (line 275) * extract: extract. (line 6) * extract, additional options: extract options. (line 6) * extract, complementary notes: Basic tar. (line 49) * extract, summary: Operation Summary. (line 44) * extract, using with --listed-incremental: Incremental Dumps. (line 120) * file: file. (line 6) * file, short description: file. (line 15) * file, summary: Option Summary. (line 283) * file, tutorial: file tutorial. (line 6) * files-from: files. (line 14) * files-from, summary: Option Summary. (line 290) * force-local, short description: Device. (line 70) * force-local, summary: Option Summary. (line 297) * format, summary: Option Summary. (line 303) * full-time, summary: Option Summary. (line 328) * get, summary: Operation Summary. (line 50) * group: override. (line 99) * group, summary: Option Summary. (line 346) * group-map, summary: Option Summary. (line 356) * gunzip, summary: Option Summary. (line 366) * gzip: gzip. (line 99) * gzip, summary: Option Summary. (line 366) * hard-dereference, described: hard links. (line 59) * hard-dereference, summary: Option Summary. (line 375) * help: help tutorial. (line 6) * help, introduction: help. (line 26) * help, summary: Option Summary. (line 381) * hole-detection: sparse. (line 66) * hole-detection, summary: Option Summary. (line 387) * ignore-case: controlling pattern-matching. (line 85) * ignore-case, summary: Option Summary. (line 392) * ignore-command-error: Writing to an External Program. (line 111) * ignore-command-error, summary: Option Summary. (line 396) * ignore-failed-read: Ignore Failed Read. (line 7) * ignore-failed-read, summary: Option Summary. (line 400) * ignore-zeros: Ignore Zeros. (line 6) * ignore-zeros, short description: Blocking Factor. (line 152) * ignore-zeros, summary: Option Summary. (line 405) * incremental, summary: Option Summary. (line 411) * incremental, using with --list: Incremental Dumps. (line 185) * index-file, summary: Option Summary. (line 419) * info-script: Multi-Volume Archives. (line 83) * info-script, short description: Device. (line 121) * info-script, summary: Option Summary. (line 423) * interactive: interactive. (line 14) * interactive, summary: Option Summary. (line 432) * keep-directory-symlink, summary: Option Summary. (line 440) * keep-newer-files: Keep Newer Files. (line 6) * keep-newer-files, summary: Option Summary. (line 454) * keep-old-files: Keep Old Files. (line 9) * keep-old-files, introduced: Dealing with Old Files. (line 16) * keep-old-files, summary: Option Summary. (line 459) * label: Tape Files. (line 6) * label <1>: label. (line 6) * label, summary: Option Summary. (line 468) * level, described: Incremental Dumps. (line 75) * level, summary: Option Summary. (line 476) * list: list. (line 6) * list, summary: Operation Summary. (line 55) * list, using with --incremental: Incremental Dumps. (line 185) * list, using with --listed-incremental: Incremental Dumps. (line 185) * list, using with --verbose: list. (line 34) * list, using with file name arguments: list. (line 25) * listed-incremental, described: Incremental Dumps. (line 14) * listed-incremental, summary: Option Summary. (line 486) * listed-incremental, using with --extract: Incremental Dumps. (line 120) * listed-incremental, using with --list: Incremental Dumps. (line 185) * lzip: gzip. (line 112) * lzip, summary: Option Summary. (line 495) * lzma: gzip. (line 115) * lzma, summary: Option Summary. (line 500) * lzop: gzip. (line 118) * mode: override. (line 14) * mode, summary: Option Summary. (line 510) * mtime: override. (line 30) * mtime, summary: Option Summary. (line 517) * multi-volume: Multi-Volume Archives. (line 6) * multi-volume, short description: Device. (line 88) * multi-volume, summary: Option Summary. (line 532) * new-volume-script: Multi-Volume Archives. (line 83) * new-volume-script, short description: Device. (line 121) * new-volume-script, summary: Option Summary. (line 423) * new-volume-script, summary <1>: Option Summary. (line 538) * newer: after. (line 24) * newer, summary: Option Summary. (line 542) * newer-mtime: after. (line 35) * newer-mtime, summary: Option Summary. (line 551) * no-acls, summary: Option Summary. (line 557) * no-anchored: controlling pattern-matching. (line 78) * no-anchored, summary: Option Summary. (line 561) * no-auto-compress, summary: Option Summary. (line 565) * no-check-device, described: Incremental Dumps. (line 103) * no-check-device, summary: Option Summary. (line 570) * no-delay-directory-restore: Directory Modification Times and Permissions. (line 68) * no-delay-directory-restore, summary: Option Summary. (line 575) * no-ignore-case: controlling pattern-matching. (line 85) * no-ignore-case, summary: Option Summary. (line 581) * no-ignore-command-error: Writing to an External Program. (line 116) * no-ignore-command-error, summary: Option Summary. (line 584) * no-null, described: nul. (line 15) * no-null, summary: Option Summary. (line 588) * no-overwrite-dir, summary: Option Summary. (line 594) * no-quote-chars, summary: Option Summary. (line 599) * no-recursion: recurse. (line 11) * no-recursion, summary: Option Summary. (line 604) * no-same-owner: Attributes. (line 63) * no-same-owner, summary: Option Summary. (line 609) * no-same-permissions, summary: Option Summary. (line 616) * no-seek, summary: Option Summary. (line 622) * no-selinux, summary: Option Summary. (line 628) * no-unquote: Selecting Archive Members. (line 42) * no-unquote, summary: Option Summary. (line 632) * no-verbatim-files-from: files. (line 79) * no-verbatim-files-from, summary: Option Summary. (line 636) * no-wildcards: controlling pattern-matching. (line 41) * no-wildcards, summary: Option Summary. (line 650) * no-wildcards-match-slash: controlling pattern-matching. (line 91) * no-wildcards-match-slash, summary: Option Summary. (line 653) * no-xattrs, summary: Option Summary. (line 656) * null, described: nul. (line 11) * null, summary: Option Summary. (line 660) * numeric-owner: Attributes. (line 69) * numeric-owner, summary: Option Summary. (line 673) * occurrence, described: append. (line 34) * occurrence, summary: Option Summary. (line 691) * old-archive, summary: Option Summary. (line 706) * one-file-system: one. (line 14) * one-file-system, summary: Option Summary. (line 709) * one-top-level, summary: Option Summary. (line 714) * overwrite: Overwrite Old Files. (line 6) * overwrite, introduced: Dealing with Old Files. (line 32) * overwrite, summary: Option Summary. (line 725) * overwrite-dir: Overwrite Old Files. (line 28) * overwrite-dir, introduced: Dealing with Old Files. (line 6) * overwrite-dir, summary: Option Summary. (line 730) * owner: override. (line 67) * owner, summary: Option Summary. (line 735) * owner-map, summary: Option Summary. (line 745) * pax-option: PAX keywords. (line 6) * pax-option, summary: Option Summary. (line 755) * portability, summary: Option Summary. (line 761) * posix, summary: Option Summary. (line 765) * preserve-order: Same Order. (line 6) * preserve-order, summary: Option Summary. (line 768) * preserve-permissions: Setting Access Permissions. (line 10) * preserve-permissions, short description: Attributes. (line 109) * preserve-permissions, summary: Option Summary. (line 772) * quote-chars, summary: Option Summary. (line 783) * quoting-style: quoting styles. (line 36) * quoting-style, summary: Option Summary. (line 787) * read-full-records: Reading. (line 6) * read-full-records <1>: read full records. (line 6) * read-full-records, short description: Blocking Factor. (line 168) * read-full-records, summary: Option Summary. (line 794) * record-size, summary: Option Summary. (line 800) * recursion: recurse. (line 22) * recursion, summary: Option Summary. (line 808) * recursive-unlink: Recursive Unlink. (line 6) * recursive-unlink, summary: Option Summary. (line 813) * remove-files: remove files. (line 6) * remove-files, summary: Option Summary. (line 818) * restrict, summary: Option Summary. (line 823) * rmt-command, summary: Option Summary. (line 829) * rsh-command: Device. (line 73) * rsh-command, summary: Option Summary. (line 834) * same-order: Same Order. (line 6) * same-order, summary: Option Summary. (line 839) * same-owner: Attributes. (line 44) * same-owner, summary: Option Summary. (line 848) * same-permissions: Setting Access Permissions. (line 10) * same-permissions, short description: Attributes. (line 109) * same-permissions, summary: Option Summary. (line 772) * same-permissions, summary <1>: Option Summary. (line 855) * seek, summary: Option Summary. (line 859) * selinux, summary: Option Summary. (line 869) * show-defaults: defaults. (line 6) * show-defaults, summary: Option Summary. (line 873) * show-omitted-dirs: verbose. (line 105) * show-omitted-dirs, summary: Option Summary. (line 886) * show-snapshot-field-ranges: Snapshot Files. (line 111) * show-snapshot-field-ranges, summary: Option Summary. (line 891) * show-stored-names: list. (line 68) * show-stored-names, summary: Option Summary. (line 897) * show-transformed-names: transform. (line 45) * show-transformed-names, summary: Option Summary. (line 897) * skip-old-files, introduced: Dealing with Old Files. (line 28) * skip-old-files, summary: Option Summary. (line 906) * sort, summary: Option Summary. (line 919) * sparse: sparse. (line 24) * sparse, summary: Option Summary. (line 936) * sparse-version: sparse. (line 59) * sparse-version, summary: Option Summary. (line 942) * starting-file: Starting File. (line 6) * starting-file, summary: Option Summary. (line 948) * strip-components: transform. (line 25) * strip-components, summary: Option Summary. (line 955) * suffix: backup. (line 67) * suffix, summary: Option Summary. (line 966) * tape-length: Multi-Volume Archives. (line 33) * tape-length, short description: Device. (line 96) * tape-length, summary: Option Summary. (line 971) * test-label: label. (line 35) * test-label, summary: Option Summary. (line 982) * to-command: Writing to an External Program. (line 9) * to-command, summary: Option Summary. (line 987) * to-stdout: Writing to Standard Output. (line 14) * to-stdout, summary: Option Summary. (line 992) * totals: verbose. (line 45) * totals, summary: Option Summary. (line 998) * touch: Data Modification Times. (line 15) * touch <1>: Attributes. (line 33) * touch, summary: Option Summary. (line 1004) * transform: transform. (line 74) * transform, summary: Option Summary. (line 1011) * uncompress: gzip. (line 124) * uncompress, summary: Option Summary. (line 170) * uncompress, summary <1>: Option Summary. (line 1025) * ungzip: gzip. (line 99) * ungzip, summary: Option Summary. (line 366) * ungzip, summary <1>: Option Summary. (line 1029) * unlink-first: Unlink First. (line 6) * unlink-first, introduced: Dealing with Old Files. (line 51) * unlink-first, summary: Option Summary. (line 1033) * unquote: Selecting Archive Members. (line 39) * unquote, summary: Option Summary. (line 1039) * update: update. (line 6) * update <1>: how to update. (line 6) * update, summary: Operation Summary. (line 60) * usage: help. (line 53) * use-compress-program: gzip. (line 185) * use-compress-program, summary: Option Summary. (line 1043) * utc, summary: Option Summary. (line 1049) * verbatim-files-from: files. (line 74) * verbatim-files-from, summary: Option Summary. (line 1054) * verbose: verbose. (line 18) * verbose, introduced: verbose tutorial. (line 6) * verbose, summary: Option Summary. (line 1076) * verbose, using with --create: create verbose. (line 6) * verbose, using with --list: list. (line 34) * verify, short description: verify. (line 8) * verify, summary: Option Summary. (line 1084) * verify, using with --create: verify. (line 24) * version: help. (line 6) * version, summary: Option Summary. (line 1090) * volno-file: Multi-Volume Archives. (line 74) * volno-file, summary: Option Summary. (line 1096) * warning, explained: warnings. (line 12) * warning, summary: Option Summary. (line 1102) * wildcards: controlling pattern-matching. (line 38) * wildcards, summary: Option Summary. (line 1108) * wildcards-match-slash: controlling pattern-matching. (line 91) * wildcards-match-slash, summary: Option Summary. (line 1112) * xattrs, summary: Option Summary. (line 1115) * xattrs-exclude, summary: Option Summary. (line 1119) * xattrs-include, summary: Option Summary. (line 1123) * xform: transform. (line 74) * xform, summary: Option Summary. (line 1011) * xz: gzip. (line 104) * xz, summary: Option Summary. (line 1129) * zstd: gzip. (line 121)  File: tar.info, Node: Index, Prev: Index of Command Line Options, Up: Top Appendix I Index **************** [index] * Menu: * %s: Directory has been renamed from %s, warning message: warnings. (line 100) * %s: Directory has been renamed, warning message: warnings. (line 100) * %s: Directory is new, warning message: warnings. (line 102) * %s: directory is on a different device: not purging, warning message: warnings. (line 104) * %s: skipping existing file, warning message: warnings. (line 66) * -after-date and -update compared: after. (line 19) * -newer-mtime and -update compared: after. (line 19) * -quite, option: Generate Mode. (line 120) * .bzrignore: exclude. (line 63) * .cvsignore: exclude. (line 50) * .gitignore: exclude. (line 55) * .hgignore: exclude. (line 70) * A lone zero block at, warning message: warnings. (line 33) * abbreviations for months: Calendar date items. (line 36) * absolute file names: absolute. (line 6) * absolute file names <1>: Remote Tape Server. (line 17) * Adding archives to an archive: concatenate. (line 6) * Adding files to an Archive: appending files. (line 6) * ADMINISTRATOR: General-Purpose Variables. (line 6) * Age, excluding files by: after. (line 6) * ago in date strings: Relative items in date strings. (line 23) * all: warnings. (line 28) * alone-zero-block: warnings. (line 33) * alternative decompression programs: gzip. (line 62) * am in date strings: Time of day items. (line 21) * Appending files to an Archive: appending files. (line 6) * appending files to existing archive: append. (line 6) * Arch, excluding files: exclude. (line 85) * archive: Definitions. (line 6) * Archive creation: file. (line 34) * archive member: Definitions. (line 15) * Archive Name: file. (line 6) * Archive, creation of: create. (line 6) * Archives, Appending files to: appending files. (line 6) * archives, binary equivalent: PAX keywords. (line 148) * Archiving Directories: create dir. (line 6) * archiving files: Top. (line 23) * ARGP_HELP_FMT, environment variable: Configuring Help Summary. (line 21) * arguments to long options: Long Options. (line 31) * arguments to old options: Old Options. (line 26) * arguments to short options: Short Options. (line 13) * Attempting extraction of symbolic links as hard links, warning message: warnings. (line 72) * attributes, files: Attributes. (line 6) * authors of parse_datetime: Authors of parse_datetime. (line 6) * Avoiding recursion in directories: recurse. (line 6) * backup options: backup. (line 6) * backup suffix: backup. (line 67) * backups: backup. (line 41) * backups <1>: Backups. (line 6) * BACKUP_DIRS: General-Purpose Variables. (line 30) * BACKUP_FILES: General-Purpose Variables. (line 58) * BACKUP_HOUR: General-Purpose Variables. (line 10) * bad-dumpdir: warnings. (line 106) * basic operations: Operations. (line 6) * Bazaar, excluding files: exclude. (line 85) * Bazaar, ignore files: exclude. (line 37) * beginning of time, for POSIX: Seconds since the Epoch. (line 13) * bell, checkpoint action: checkpoints. (line 106) * Bellovin, Steven M.: Authors of parse_datetime. (line 6) * Berets, Jim: Authors of parse_datetime. (line 6) * Berry, K.: Authors of parse_datetime. (line 19) * binary equivalent archives, creating: PAX keywords. (line 148) * block: Blocking. (line 6) * Block number where error occurred: verbose. (line 112) * BLOCKING: General-Purpose Variables. (line 25) * Blocking Factor: Blocking Factor. (line 6) * blocking factor: Blocking Factor. (line 189) * Blocks per record: Blocking Factor. (line 6) * bug reports: Reports. (line 6) * Bytes per record: Blocking Factor. (line 6) * bzip2: gzip. (line 6) * cachedir: warnings. (line 40) * calendar date item: Calendar date items. (line 6) * case, ignored in dates: General date syntax. (line 60) * cat vs concatenate: concatenate. (line 63) * Changing directory mid-stream: directory. (line 6) * Character class, excluding characters from: wildcards. (line 34) * checkpoints, defined: checkpoints. (line 6) * Choosing an archive file: file. (line 6) * combined date and time of day item: Combined date and time of day items. (line 6) * comments, in dates: General date syntax. (line 60) * compress: gzip. (line 6) * Compressed archives: gzip. (line 6) * concatenate vs cat: concatenate. (line 63) * Concatenating Archives: concatenate. (line 6) * contains a cache directory tag, warning message: warnings. (line 40) * contiguous-cast: warnings. (line 70) * corrupted archives: Full Dumps. (line 8) * corrupted archives <1>: gzip. (line 151) * Creation of the archive: create. (line 6) * Current %s is newer or same age, warning message: warnings. (line 76) * CVS, excluding files: exclude. (line 85) * CVS, ignore files: exclude. (line 37) * Darcs, excluding files: exclude. (line 85) * DAT blocking: Blocking Factor. (line 199) * Data Modification time, excluding files by: after. (line 6) * Data modification times of extracted files: Data Modification Times. (line 6) * date and time of day format, ISO 8601: Combined date and time of day items. (line 6) * date format, ISO 8601: Calendar date items. (line 28) * date input formats: Date input formats. (line 6) * day in date strings: Relative items in date strings. (line 15) * day in date strings <1>: Relative items in date strings. (line 29) * day of week item: Day of week items. (line 6) * decompress-program: warnings. (line 80) * Deleting files from an archive: delete. (line 6) * Deleting from tape archives: delete. (line 17) * dereferencing hard links: hard links. (line 6) * Descending directories, avoiding: recurse. (line 6) * Device numbers, changing: Fixing Snapshot Files. (line 6) * Device numbers, using in incremental backups: Incremental Dumps. (line 89) * Directories, Archiving: create dir. (line 6) * Directories, avoiding recursion: recurse. (line 6) * Directory, changing mid-stream: directory. (line 6) * DIRLIST: General-Purpose Variables. (line 53) * displacement of dates: Relative items in date strings. (line 6) * doc-opt-col: Configuring Help Summary. (line 95) * door ignored, warning message: warnings. (line 45) * dot, checkpoint action: checkpoints. (line 130) * Double-checking a write operation: verify. (line 6) * dumps, full: Full Dumps. (line 8) * DUMP_BEGIN: User Hooks. (line 31) * DUMP_END: User Hooks. (line 35) * DUMP_REMIND_SCRIPT: General-Purpose Variables. (line 112) * dup-args: Configuring Help Summary. (line 52) * dup-args-note: Configuring Help Summary. (line 69) * echo, checkpoint action: checkpoints. (line 25) * Eggert, Paul: Authors of parse_datetime. (line 6) * End-of-archive blocks, ignoring: Ignore Zeros. (line 6) * End-of-archive info script: Multi-Volume Archives. (line 83) * entry: Naming tar Archives. (line 11) * Epoch, for POSIX: Seconds since the Epoch. (line 13) * Error message, block number of: verbose. (line 122) * Exabyte blocking: Blocking Factor. (line 199) * exclude: exclude. (line 12) * exclude-caches: exclude. (line 122) * exclude-from: exclude. (line 25) * exclude-tag: exclude. (line 145) * Excluding characters from a character class: wildcards. (line 34) * Excluding file by age: after. (line 6) * Excluding files by file system: exclude. (line 6) * Excluding files by name and pattern: exclude. (line 6) * Exec Mode, genfile: Exec Mode. (line 6) * exec, checkpoint action: checkpoints. (line 166) * existing backup method: backup. (line 59) * existing-file: warnings. (line 66) * exit status: Synopsis. (line 67) * Extracting contiguous files as regular files, warning message: warnings. (line 70) * extracting Nth copy of the file: append. (line 34) * extraction: Definitions. (line 22) * Extraction: extract. (line 6) * file archival: Top. (line 23) * file attributes: Attributes. (line 6) * file changed as we read it, warning message: warnings. (line 55) * file is on a different filesystem, warning message: warnings. (line 43) * file is the archive; not dumped, warning message: warnings. (line 51) * file is the archive; not dumped, warning message <1>: warnings. (line 51) * file is unchanged; not dumped, warning message: warnings. (line 49) * File lists separated by NUL characters: Generate Mode. (line 33) * file name: Definitions. (line 15) * File Name arguments, alternatives: files. (line 6) * File name arguments, using --list with: list. (line 25) * file name read contains nul character, warning message: warnings. (line 31) * file names, absolute: absolute. (line 6) * File names, excluding files by: exclude. (line 6) * File names, terminated by NUL: nul. (line 6) * File names, using hard links: hard links. (line 6) * File names, using symbolic links: dereference. (line 6) * File removed before we read it, warning message: warnings. (line 53) * File shrank by %s bytes, warning message: warnings. (line 41) * File system boundaries, not crossing: one. (line 6) * file-changed: warnings. (line 55) * file-ignored: warnings. (line 45) * file-removed: warnings. (line 53) * file-shrank: warnings. (line 41) * file-unchanged: warnings. (line 49) * FILELIST: General-Purpose Variables. (line 69) * filename-with-nuls: warnings. (line 31) * find, using with tar: files. (line 6) * find, using with tar <1>: recurse. (line 11) * first in date strings: General date syntax. (line 22) * format 0, snapshot file: Snapshot Files. (line 24) * format 1, snapshot file: Snapshot Files. (line 51) * format 2, snapshot file: Snapshot Files. (line 73) * Format Options: Format Variations. (line 6) * Format Parameters: Format Variations. (line 6) * Format, old style: old. (line 6) * fortnight in date strings: Relative items in date strings. (line 15) * full dumps: Full Dumps. (line 8) * future time stamps: Large or Negative Values. (line 6) * general date syntax: General date syntax. (line 6) * Generate Mode, genfile: Generate Mode. (line 6) * genfile: Genfile. (line 6) * genfile, create file: Generate Mode. (line 6) * genfile, creating sparse files: Generate Mode. (line 55) * genfile, generate mode: Generate Mode. (line 6) * genfile, reading a list of file names: Generate Mode. (line 22) * genfile, seeking to a given offset: Generate Mode. (line 18) * Getting program version number: help. (line 6) * git, excluding files: exclude. (line 85) * Git, ignore files: exclude. (line 37) * GNU archive format: gnu. (line 6) * GNU.sparse.major, extended header variable: PAX 1. (line 14) * GNU.sparse.map, extended header variable: PAX 0. (line 59) * GNU.sparse.minor, extended header variable: PAX 1. (line 17) * GNU.sparse.name, extended header variable: PAX 0. (line 67) * GNU.sparse.name, extended header variable, in v.1.0: PAX 1. (line 24) * GNU.sparse.numblocks, extended header variable: PAX 0. (line 14) * GNU.sparse.numbytes, extended header variable: PAX 0. (line 20) * GNU.sparse.offset, extended header variable: PAX 0. (line 17) * GNU.sparse.realsize, extended header variable: PAX 1. (line 24) * GNU.sparse.size, extended header variable: PAX 0. (line 10) * gnupg, using with tar: gzip. (line 209) * gpg, using with tar: gzip. (line 209) * gzip: gzip. (line 6) * hard links, dereferencing: hard links. (line 6) * header-col: Configuring Help Summary. (line 141) * hole detection: sparse. (line 66) * hook: User Hooks. (line 12) * hour in date strings: Relative items in date strings. (line 15) * ignore-archive: warnings. (line 51) * ignore-archive <1>: warnings. (line 51) * ignore-newer: warnings. (line 76) * Ignoring end-of-archive blocks: Ignore Zeros. (line 6) * Ignoring unknown extended header keyword '%s', warning message: warnings. (line 78) * implausibly old time stamp %s, warning message: warnings. (line 67) * Info script: Multi-Volume Archives. (line 83) * Interactive operation: interactive. (line 6) * ISO 8601 date and time of day format: Combined date and time of day items. (line 6) * ISO 8601 date format: Calendar date items. (line 28) * items in date strings: General date syntax. (line 6) * Labeling an archive: label. (line 6) * labeling archives: Tape Files. (line 6) * Labeling multi-volume archives: label. (line 6) * Labels on the archive media: label. (line 6) * language, in dates: General date syntax. (line 36) * language, in dates <1>: General date syntax. (line 40) * Large lists of file names on small machines: Same Order. (line 6) * large values: Large or Negative Values. (line 6) * last DAY: Day of week items. (line 15) * last in date strings: General date syntax. (line 22) * Laszlo Ersek: lbzip2. (line 6) * lbzip2: lbzip2. (line 6) * leap seconds: General date syntax. (line 65) * leap seconds <1>: Time of day items. (line 14) * leap seconds <2>: Seconds since the Epoch. (line 27) * Listing all tar options: help. (line 26) * listing member and file names: list. (line 45) * Listing volume label: label. (line 27) * Lists of file names: files. (line 6) * Local and remote archives: file. (line 70) * long options: Long Options. (line 6) * long options with mandatory arguments: Long Options. (line 31) * long options with optional arguments: Long Options. (line 39) * long-opt-col: Configuring Help Summary. (line 87) * lzip: gzip. (line 6) * lzma: gzip. (line 6) * lzop: gzip. (line 6) * MacKenzie, David: Authors of parse_datetime. (line 6) * Malformed dumpdir: 'X' never used, warning message: warnings. (line 106) * member: Definitions. (line 15) * member name: Definitions. (line 15) * members, multiple: multiple. (line 6) * Members, replacing with other members: append. (line 47) * Mercurial, excluding files: exclude. (line 85) * Mercurial, ignore files: exclude. (line 37) * Meyering, Jim: Authors of parse_datetime. (line 6) * Middle of the archive, starting in the: Starting File. (line 11) * midnight in date strings: Time of day items. (line 21) * minute in date strings: Relative items in date strings. (line 15) * minutes, time zone correction by: Time of day items. (line 29) * Modes of extracted files: Setting Access Permissions. (line 6) * Modification time, excluding files by: after. (line 6) * Modification times of extracted files: Data Modification Times. (line 6) * month in date strings: Relative items in date strings. (line 15) * month names in date strings: Calendar date items. (line 36) * months, written-out: General date syntax. (line 32) * MT: General-Purpose Variables. (line 74) * MT_BEGIN: Magnetic Tape Control. (line 10) * MT_OFFLINE: Magnetic Tape Control. (line 30) * MT_REWIND: Magnetic Tape Control. (line 20) * MT_STATUS: Magnetic Tape Control. (line 40) * Multi-volume archives: Multi-Volume Archives. (line 6) * Multi-volume archives in PAX format, extracting using non-GNU tars: Split Recovery. (line 17) * Multi-volume archives, extracting using non-GNU tars: Split Recovery. (line 6) * multiple members: multiple. (line 6) * Naming an archive: file. (line 6) * negative time stamps: Large or Negative Values. (line 6) * new-directory: warnings. (line 102) * next DAY: Day of week items. (line 15) * next in date strings: General date syntax. (line 22) * none: warnings. (line 29) * noon in date strings: Time of day items. (line 21) * now in date strings: Relative items in date strings. (line 33) * ntape device: Many. (line 6) * NUL-terminated file names: nul. (line 6) * Number of blocks per record: Blocking Factor. (line 6) * Number of bytes per record: Blocking Factor. (line 6) * numbered backup method: backup. (line 55) * numbers, written-out: General date syntax. (line 22) * Obtaining help: help. (line 26) * Obtaining total status information: verbose. (line 45) * Old GNU archive format: gnu. (line 6) * Old GNU sparse format: Old GNU Format. (line 6) * old option style: Old Options. (line 6) * old options with mandatory arguments: Old Options. (line 26) * Old style archives: old. (line 6) * Old style format: old. (line 6) * opt-doc-col: Configuring Help Summary. (line 127) * option syntax, traditional: Old Options. (line 6) * optional arguments to long options: Long Options. (line 39) * optional arguments to short options: Short Options. (line 22) * options for use with --extract: extract options. (line 6) * Options when reading archives: Reading. (line 6) * Options, archive format specifying: Format Variations. (line 6) * Options, format specifying: Format Variations. (line 6) * options, GNU style: Long Options. (line 6) * options, long style: Long Options. (line 6) * options, mixing different styles: Mixing. (line 6) * options, mnemonic names: Long Options. (line 6) * options, old style: Old Options. (line 6) * options, short style: Short Options. (line 6) * options, traditional: Short Options. (line 6) * ordinal numbers: General date syntax. (line 22) * Overwriting old files, prevention: Dealing with Old Files. (line 16) * parse_datetime: Date input formats. (line 6) * pattern, genfile: Generate Mode. (line 39) * PAX archive format: posix. (line 6) * Permissions of extracted files: Setting Access Permissions. (line 6) * Pinard, F.: Authors of parse_datetime. (line 19) * pm in date strings: Time of day items. (line 21) * POSIX archive format: posix. (line 6) * Progress information: verbose. (line 82) * Protecting old files: Dealing with Old Files. (line 36) * pure numbers in date strings: Pure numbers in date strings. (line 6) * RCS, excluding files: exclude. (line 85) * Reading file names from a file: files. (line 6) * Reading incomplete records: Reading. (line 6) * record: Blocking. (line 6) * Record Size: Blocking Factor. (line 6) * Record size = %lu blocks, warning message: warnings. (line 93) * record-size: warnings. (line 93) * Records, incomplete: Reading. (line 6) * Recursion in directories, avoiding: recurse. (line 6) * relative items in date strings: Relative items in date strings. (line 6) * Remote devices: file. (line 60) * remote tape drive: Remote Tape Server. (line 6) * Removing files from an archive: delete. (line 6) * rename-directory: warnings. (line 100) * Replacing members with other members: append. (line 47) * reporting bugs: Reports. (line 6) * RESTORE_BEGIN: User Hooks. (line 38) * RESTORE_END: User Hooks. (line 41) * Resurrecting files from an archive: extract. (line 6) * Retrieving files from an archive: extract. (line 6) * return status: Synopsis. (line 67) * rmargin: Configuring Help Summary. (line 159) * rmt: Remote Tape Server. (line 6) * RSH: General-Purpose Variables. (line 78) * RSH_COMMAND: General-Purpose Variables. (line 83) * Running out of space: Scarce. (line 8) * Salz, Rich: Authors of parse_datetime. (line 6) * SCCS, excluding files: exclude. (line 85) * short options: Short Options. (line 6) * short options with mandatory arguments: Short Options. (line 13) * short options with optional arguments: Short Options. (line 22) * short-opt-col: Configuring Help Summary. (line 79) * simple backup method: backup. (line 64) * SIMPLE_BACKUP_SUFFIX: backup. (line 67) * sleep, checkpoint action: checkpoints. (line 145) * SLEEP_MESSAGE: General-Purpose Variables. (line 121) * SLEEP_TIME: General-Purpose Variables. (line 106) * Small memory: Scarce. (line 8) * snapshot file field ranges: Snapshot Files. (line 111) * snapshot file, format 0: Snapshot Files. (line 24) * snapshot file, format 1: Snapshot Files. (line 51) * snapshot file, format 2: Snapshot Files. (line 73) * snapshot files, editing: Fixing Snapshot Files. (line 6) * snapshot files, fixing device numbers: Fixing Snapshot Files. (line 6) * socket ignored, warning message: warnings. (line 45) * Sparse Files: sparse. (line 6) * sparse files v.0.0, extracting with non-GNU tars: Sparse Recovery. (line 92) * sparse files v.0.1, extracting with non-GNU tars: Sparse Recovery. (line 92) * sparse files v.1.0, extracting with non-GNU tars: Sparse Recovery. (line 17) * Sparse files, creating using genfile: Generate Mode. (line 55) * sparse files, extracting with non-GNU tars: Sparse Recovery. (line 6) * sparse formats: Sparse Formats. (line 6) * sparse formats, defined: sparse. (line 52) * sparse formats, Old GNU: Old GNU Format. (line 6) * sparse formats, v.0.0: PAX 0. (line 6) * sparse formats, v.0.1: PAX 0. (line 51) * sparse formats, v.1.0: PAX 1. (line 6) * sparse versions: Sparse Formats. (line 6) * Specifying archive members: Selecting Archive Members. (line 6) * Specifying files to act on: Selecting Archive Members. (line 6) * Standard input and output: file. (line 39) * Standard output, writing extracted files to: Writing to Standard Output. (line 6) * Storing archives in compressed format: gzip. (line 6) * SVN, excluding files: exclude. (line 85) * Symbolic link as file name: dereference. (line 6) * symlink-cast: warnings. (line 72) * TAPE: file tutorial. (line 14) * tape blocking: Blocking Factor. (line 189) * tape marks: Many. (line 43) * tape positioning: Many. (line 26) * Tapes, using --delete and: delete. (line 17) * TAPE_FILE: General-Purpose Variables. (line 18) * tar: What tar Does. (line 6) * TAR: General-Purpose Variables. (line 126) * tar archive: Definitions. (line 6) * Tar archive formats: Formats. (line 6) * tar entry: Naming tar Archives. (line 11) * tar file: Naming tar Archives. (line 11) * tar to a remote device: file. (line 60) * tar to standard input and output: file. (line 39) * tar-snapshot-edit: Fixing Snapshot Files. (line 17) * tarcat: Tarcat. (line 6) * TAR_ARCHIVE, checkpoint script environment: checkpoints. (line 182) * TAR_ARCHIVE, info script environment variable: Multi-Volume Archives. (line 105) * TAR_ARCHIVE, to-command environment: Writing to an External Program. (line 79) * TAR_ATIME, to-command environment: Writing to an External Program. (line 52) * TAR_BLOCKING_FACTOR, checkpoint script environment: checkpoints. (line 185) * TAR_BLOCKING_FACTOR, info script environment variable: Multi-Volume Archives. (line 108) * TAR_BLOCKING_FACTOR, to-command environment: Writing to an External Program. (line 82) * TAR_CHECKPOINT, checkpoint script environment: checkpoints. (line 188) * TAR_CTIME, to-command environment: Writing to an External Program. (line 61) * TAR_FD, info script environment variable: Multi-Volume Archives. (line 122) * TAR_FILENAME, to-command environment: Writing to an External Program. (line 40) * TAR_FILETYPE, to-command environment: Writing to an External Program. (line 24) * TAR_FORMAT, checkpoint script environment: checkpoints. (line 195) * TAR_FORMAT, info script environment variable: Multi-Volume Archives. (line 118) * TAR_FORMAT, to-command environment: Writing to an External Program. (line 88) * TAR_GID, to-command environment: Writing to an External Program. (line 70) * TAR_GNAME, to-command environment: Writing to an External Program. (line 49) * TAR_MODE, to-command environment: Writing to an External Program. (line 37) * TAR_MTIME, to-command environment: Writing to an External Program. (line 58) * TAR_OPTIONS, environment variable: using tar options. (line 30) * TAR_REALNAME, to-command environment: Writing to an External Program. (line 43) * TAR_SIZE, to-command environment: Writing to an External Program. (line 64) * TAR_SUBCOMMAND, checkpoint script environment: checkpoints. (line 191) * TAR_SUBCOMMAND, info script environment variable: Multi-Volume Archives. (line 114) * TAR_UID, to-command environment: Writing to an External Program. (line 67) * TAR_UNAME, to-command environment: Writing to an External Program. (line 46) * TAR_VERSION, checkpoint script environment: checkpoints. (line 179) * TAR_VERSION, info script environment variable: Multi-Volume Archives. (line 102) * TAR_VERSION, to-command environment: Writing to an External Program. (line 76) * TAR_VOLUME, info script environment variable: Multi-Volume Archives. (line 111) * TAR_VOLUME, to-command environment: Writing to an External Program. (line 85) * this in date strings: Relative items in date strings. (line 33) * time of day item: Time of day items. (line 6) * time stamp %s is %s s in the future, warning message: warnings. (line 67) * time zone correction: Time of day items. (line 29) * time zone item: General date syntax. (line 40) * time zone item <1>: Time zone items. (line 6) * timestamp: warnings. (line 67) * today in date strings: Relative items in date strings. (line 33) * tomorrow in date strings: Relative items in date strings. (line 29) * totals, checkpoint action: checkpoints. (line 140) * ttyout, checkpoint action: checkpoints. (line 111) * TZ: Specifying time zone rules. (line 6) * Ultrix 3.1 and write failure: Remote Tape Server. (line 40) * Unknown file type '%c', extracted as normal file, warning message: warnings. (line 74) * Unknown file type; file ignored, warning message: warnings. (line 45) * unknown-cast: warnings. (line 74) * unknown-keyword: warnings. (line 78) * unpacking: Definitions. (line 22) * Updating an archive: update. (line 6) * usage-indent: Configuring Help Summary. (line 155) * Using encrypted archives: gzip. (line 209) * ustar archive format: ustar. (line 6) * v7 archive format: old. (line 6) * VCS, excluding files: exclude. (line 85) * VCS, excluding patterns from ignore files: exclude. (line 37) * VCS, ignore files: exclude. (line 37) * Verbose operation: verbose. (line 18) * Verifying a write operation: verify. (line 6) * Verifying the currency of an archive: compare. (line 6) * version control system, excluding files: exclude. (line 85) * Version of the tar program: help. (line 6) * version-control Emacs variable: backup. (line 49) * VERSION_CONTROL: backup. (line 41) * volno file: Multi-Volume Archives. (line 74) * VOLNO_FILE: General-Purpose Variables. (line 89) * Volume label, listing: label. (line 27) * Volume number file: Multi-Volume Archives. (line 74) * wait, checkpoint action: checkpoints. (line 151) * week in date strings: Relative items in date strings. (line 15) * Where is the archive?: file. (line 6) * Working directory, specifying: directory. (line 6) * Writing extracted files to standard output: Writing to Standard Output. (line 6) * Writing new archives: file. (line 34) * xdev: warnings. (line 43) * xdev <1>: warnings. (line 104) * XLIST: General-Purpose Variables. (line 95) * xsparse: Sparse Recovery. (line 13) * year in date strings: Relative items in date strings. (line 15) * yesterday in date strings: Relative items in date strings. (line 29) * zstd: gzip. (line 6)