usr.bin/compress/compress.1

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.Dd March 4, 2021
.Dt COMPRESS 1
.Os
.Sh NAME
.Nm compress ,
.Nm uncompress
.Nd compress and expand data
.Sh SYNOPSIS
.Nm
.Op Fl fv
.Op Fl b Ar bits
.Op Ar
.Nm
.Fl c
.Op Fl b Ar bits
.Op Ar file
.Nm uncompress
.Op Fl f
.Op Ar
.Nm uncompress
.Fl c
.Op Ar file
.Sh DESCRIPTION
The
.Nm
utility reduces the size of files using adaptive Lempel-Ziv coding.
Each
.Ar file
is renamed to the same name plus the extension
.Pa .Z .
A
.Ar file
argument with a
.Pa .Z
extension will be ignored except it will cause an
error exit after other arguments are processed.
If compression would not reduce the size of a
.Ar file ,
the file is ignored.
.Pp
The
.Nm uncompress
utility restores compressed files to their original form, renaming the
files by deleting the
.Pa .Z
extensions.
A file specification need not include the file's
.Pa .Z
extension.
If a file's name in its file system does not have a
.Pa .Z
extension, it will not be uncompressed and it will cause
an error exit after other arguments are processed.
.Pp
If renaming the files would cause files to be overwritten and the standard
input device is a terminal, the user is prompted (on the standard error
output) for confirmation.
If prompting is not possible or confirmation is not received, the files
are not overwritten.
.Pp
As many of the modification time, access time, file flags, file mode,
user ID, and group ID as allowed by permissions are retained in the
new file.
.Pp
If no files are specified or a
.Ar file
argument is a single dash
.Pq Sq Fl ,
the standard input is compressed or uncompressed to the standard output.
If either the input and output files are not regular files, the checks for
reduction in size and file overwriting are not performed, the input file is
not removed, and the attributes of the input file are not retained
in the output file.
.Pp
The options are as follows:
.Bl -tag -width ".Fl b Ar bits"
.It Fl b Ar bits
The code size (see below) is limited to
.Ar bits ,
which must be in the range 9..16.
The default is 16.
.It Fl c
Compressed or uncompressed output is written to the standard output.
No files are modified.
The
.Fl v
option is ignored.
Compression is attempted even if the results will be larger than the
original.
.It Fl f
Files are overwritten without prompting for confirmation.
Also, for
.Nm compress ,
files are compressed even if they are not actually reduced in size.
.It Fl v
Print the percentage reduction of each file.
Ignored by
.Nm uncompress
or if the
.Fl c
option is also used.
.El
.Pp
The
.Nm
utility uses a modified Lempel-Ziv algorithm.
Common substrings in the file are first replaced by 9-bit codes 257 and up.
When code 512 is reached, the algorithm switches to 10-bit codes and
continues to use more bits until the
limit specified by the
.Fl b
option or its default is reached.
.Pp
After the limit is reached,
.Nm
periodically checks the compression ratio.
If it is increasing,
.Nm
continues to use the existing code dictionary.
However, if the compression ratio decreases,
.Nm
discards the table of substrings and rebuilds it from scratch.
This allows
the algorithm to adapt to the next "block" of the file.
.Pp
The
.Fl b
option is unavailable for
.Nm uncompress
since the
.Ar bits
parameter specified during compression
is encoded within the output, along with
a magic number to ensure that neither decompression of random data nor
recompression of compressed data is attempted.
.Pp
The amount of compression obtained depends on the size of the
input, the number of
.Ar bits
per code, and the distribution of common substrings.
Typically, text such as source code or English is reduced by 50\-60%.
Compression is generally much better than that achieved by Huffman
coding (as used in the historical command pack), or adaptive Huffman
coding (as used in the historical command compact), and takes less
time to compute.
.Pp
If
.Ar file
is a soft or hard link
.Nm
will replace it with a compressed copy of the file pointed to by the link.
The link's target file is left uncompressed.
.Sh EXIT STATUS
.Ex -std compress uncompress
.Pp
The
.Nm compress
utility exits 2 if attempting to compress a file would not reduce its size
and the
.Fl f
option was not specified and if no other error occurs.
.Sh EXAMPLES
Create a file
.Pa test_file
with a single line of text:
.Bd -literal -offset indent
echo "This is a test" > test_file
.Ed
.Pp
Try to reduce the size of the file using a 10-bit code and show the exit status:
.Bd -literal -offset indent
$ compress -b 10 test_file
$ echo $?
2
.Ed
.Pp
Try to compress the file and show compression percentage:
.Bd -literal -offset indent
$ compress -v test_file
test_file: file would grow; left unmodified
.Ed
.Pp
Same as above but forcing compression:
.Bd -literal -offset indent
$ compress -f -v test_file
test_file.Z: 79% expansion
.Ed
.Pp
Compress and uncompress the string
.Ql hello
on the fly:
.Bd -literal -offset indent
$ echo "hello" | compress | uncompress
hello
.Ed
.Sh SEE ALSO
.Xr gunzip 1 ,
.Xr gzexe 1 ,
.Xr gzip 1 ,
.Xr zcat 1 ,
.Xr zmore 1 ,
.Xr znew 1
.Rs
.%A Welch, Terry A.
.%D June, 1984
.%T "A Technique for High Performance Data Compression"
.%J "IEEE Computer"
.%V 17:6
.%P pp. 8-19
.Re
.Sh STANDARDS
The
.Nm compress
and
.Nm uncompress
utilities conform to
.St -p1003.1-2001 .
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.3 .
.Sh BUGS
The program does not handle links well and has no link-handling options.
.Pp
Some of these might be considered otherwise-undocumented features.
.Pp
.Nm compress :
If the utility does not compress a file because doing so would not
reduce its size, and a file of the same name except with an
.Pa .Z
extension exists, the named file is not really ignored as stated above;
it causes a prompt to confirm the overwriting of the file with the extension.
If the operation is confirmed, that file is deleted.
.Pp
.Nm uncompress :
If an empty file is compressed (using
.Fl f ) ,
the resulting
.Pa .Z
file is also empty.
That seems right, but if
.Nm uncompress
is then used on that file, an error will occur.
.Pp
Both utilities: If a
.Sq Fl
argument is used and the utility prompts the user, the standard input
is taken as the user's reply to the prompt.
.Pp
Both utilities:
If the specified file does not exist, but a similarly-named one with (for
.Nm compress )
or without (for
.Nm uncompress )
a
.Pa .Z
extension does exist, the utility will waste the user's time by not
immediately emitting an error message about the missing file and
continuing.
Instead, it first asks for confirmation to overwrite
the existing file and then does not overwrite it.