cron/crontab/crontab.5

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.Dd June 6, 2018
.Dt CRONTAB 5
.Os
.Sh NAME
.Nm crontab
.Nd tables for driving cron
.Sh DESCRIPTION
A
.Nm
file contains instructions to the
.Xr cron 8
daemon of the general form: ``run this command at this time on this date''.
Each user has their own crontab, and commands in any given crontab will be
executed as the user who owns the crontab.
Uucp and News will usually have
their own crontabs, eliminating the need for explicitly running
.Xr su 1
as part of a cron command.
.Pp
Blank lines and leading spaces and tabs are ignored.
Lines whose first
non-space character is a pound-sign (#) are comments, and are ignored.
Note that comments are not allowed on the same line as cron commands, since
they will be taken to be part of the command.
Similarly, comments are not
allowed on the same line as environment variable settings.
.Pp
An active line in a crontab will be either an environment setting or a cron
command.
An environment setting is of the form,
.Bd -literal
    name = value
.Ed
.Pp
where the spaces around the equal-sign (=) are optional, and any subsequent
non-leading spaces in
.Em value
will be part of the value assigned to
.Em name .
The
.Em value
string may be placed in quotes (single or double, but matching) to preserve
leading or trailing blanks.
The
.Em name
string may also be placed in quote (single or double, but matching)
to preserve leading, trailing or inner blanks.
.Pp
Several environment variables are set up
automatically by the
.Xr cron 8
daemon.
.Ev SHELL
is set to
.Pa /bin/sh ,
.Ev PATH
is set to
.Pa /sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin ,
and
.Ev LOGNAME
and
.Ev HOME
are set from the
.Pa /etc/passwd
line of the crontab's owner.
.Ev HOME ,
.Ev PATH
and
.Ev SHELL
may be overridden by settings in the crontab;
.Ev LOGNAME
may not.
.Pp
(Another note: the
.Ev LOGNAME
variable is sometimes called
.Ev USER
on
.Bx
systems...
On these systems,
.Ev USER
will be set also).
.Pp
In addition to
.Ev LOGNAME ,
.Ev HOME ,
.Ev PATH ,
and
.Ev SHELL ,
.Xr cron 8
will look at
.Ev MAILTO
if it has any reason to send mail as a result of running
commands in ``this'' crontab.
If
.Ev MAILTO
is defined (and non-empty), mail is
sent to the user so named.
.Ev MAILTO
may also be used to direct mail to multiple recipients
by separating recipient users with a comma.
If
.Ev MAILTO
is defined but empty (MAILTO=""), no
mail will be sent.
Otherwise mail is sent to the owner of the crontab.
This
option is useful if you decide on
.Pa /bin/mail
instead of
.Pa /usr/lib/sendmail
as
your mailer when you install cron --
.Pa /bin/mail
does not do aliasing, and UUCP
usually does not read its mail.
.Pp
The format of a cron command is very much the V7 standard, with a number of
upward-compatible extensions.
Each line has five time and date fields,
followed by a user name
(with optional ``:<group>'' and ``/<login-class>'' suffixes)
if this is the system crontab file,
followed by a command.
Commands are executed by
.Xr cron 8
when the minute, hour, and month of year fields match the current time,
.Em and
when at least one of the two day fields (day of month, or day of week)
matches the current time (see ``Note'' below).
.Xr cron 8
examines cron entries once every minute.
The time and date fields are:
.Bd -literal -offset indent
field         allowed values
-----         --------------
minute        0-59
hour          0-23
day of month  1-31
month         1-12 (or names, see below)
day of week   0-7 (0 or 7 is Sun, or use names)
.Ed
.Pp
A field may be an asterisk (*), which always stands for ``first\-last''.
.Pp
Ranges of numbers are allowed.
Ranges are two numbers separated
with a hyphen.
The specified range is inclusive.
For example,
8-11 for an ``hours'' entry specifies execution at hours 8, 9, 10
and 11.
.Pp
Lists are allowed.
A list is a set of numbers (or ranges)
separated by commas.
Examples: ``1,2,5,9'', ``0-4,8-12''.
.Pp
Step values can be used in conjunction with ranges.
Following
a range with ``/<number>'' specifies skips of the number's value
through the range.
For example, ``0-23/2'' can be used in the hours
field to specify command execution every other hour (the alternative
in the V7 standard is ``0,2,4,6,8,10,12,14,16,18,20,22'').
Steps are
also permitted after an asterisk, so if you want to say ``every two
hours'', just use ``*/2''.
.Pp
Names can also be used for the ``month'' and ``day of week''
fields.
Use the first three letters of the particular
day or month (case does not matter).
Ranges or
lists of names are not allowed.
.Pp
The ``sixth'' field (the rest of the line) specifies the command to be
run.
The entire command portion of the line, up to a newline or %
character, will be executed by
.Pa /bin/sh
or by the shell
specified in the
.Ev SHELL
variable of the cronfile.
Percent-signs (%) in the command, unless escaped with backslash
(\\), will be changed into newline characters, and all data
after the first % will be sent to the command as standard
input.
.Pp
Note: The day of a command's execution can be specified by two
fields \(em day of month, and day of week.
If both fields are
restricted (ie, are not *), the command will be run when
.Em either
field matches the current time.
For example,
``30 4 1,15 * 5''
would cause a command to be run at 4:30 am on the 1st and 15th of each
month, plus every Friday.
.Pp
Instead of the first five fields,
a line may start with
.Sq @
symbol followed either by one of eight special strings or by a numeric value.
The recognized special strings are:
.Bd -literal -offset indent
string		meaning
------		-------
@reboot		Run once, at startup of cron.
@yearly		Run once a year, "0 0 1 1 *".
@annually	(same as @yearly)
@monthly	Run once a month, "0 0 1 * *".
@weekly		Run once a week, "0 0 * * 0".
@daily		Run once a day, "0 0 * * *".
@midnight	(same as @daily)
@hourly		Run once an hour, "0 * * * *".
@every_minute	Run once a minute, "*/1 * * * *".
@every_second	Run once a second.
.Ed
.Pp
The
.Sq @
symbol followed by a numeric value has a special notion of running
a job that much seconds after completion of previous invocation of
the job.
Unlike regular syntax, it guarantees not to overlap two or more
invocations of the same job.
The first run is scheduled specified amount of seconds after cron
has started.
.Sh EXAMPLE CRON FILE
.Bd -literal

# use /bin/sh to run commands, overriding the default set by cron
SHELL=/bin/sh
# mail any output to `paul', no matter whose crontab this is
MAILTO=paul
#
# run five minutes after midnight, every day
5 0 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
# run at 2:15pm on the first of every month -- output mailed to paul
15 14 1 * *     $HOME/bin/monthly
# run at 10 pm on weekdays, annoy Joe
0 22 * * 1-5	mail -s "It's 10pm" joe%Joe,%%Where are your kids?%
23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
5 4 * * sun     echo "run at 5 after 4 every sunday"
# run at 5 minutes intervals, no matter how long it takes
@300		svnlite up /usr/src
.Ed
.Sh SEE ALSO
.Xr crontab 1 ,
.Xr cron 8
.Sh EXTENSIONS
When specifying day of week, both day 0 and day 7 will be considered Sunday.
.Bx
and
.Tn ATT
seem to disagree about this.
.Pp
Lists and ranges are allowed to co-exist in the same field.
"1-3,7-9" would
be rejected by
.Tn ATT
or
.Bx
cron -- they want to see "1-3" or "7,8,9" ONLY.
.Pp
Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".
.Pp
Names of months or days of the week can be specified by name.
.Pp
Environment variables can be set in the crontab.
In
.Bx
or
.Tn ATT ,
the
environment handed to child processes is basically the one from
.Pa /etc/rc .
.Pp
Command output is mailed to the crontab owner
.No ( Bx
cannot do this), can be
mailed to a person other than the crontab owner (SysV cannot do this), or the
feature can be turned off and no mail will be sent at all (SysV cannot do this
either).
.Pp
All of the
.Sq @
directives that can appear in place of the first five fields
are extensions.
.Sh AUTHORS
.An Paul Vixie Aq Mt paul@vix.com
.Sh BUGS
If you are in one of the 70-odd countries that observe Daylight
Savings Time, jobs scheduled during the rollback or advance may be
affected if
.Xr cron 8
is not started with the
.Fl s
flag.
In general, it is not a good idea to schedule jobs during
this period if
.Xr cron 8
is not started with the
.Fl s
flag, which is enabled by default.
See
.Xr cron 8
for more details.
.Pp
For US timezones (except parts of AZ and HI) the time shift occurs at
2AM local time.
For others, the output of the
.Xr zdump 8
program's verbose
.Fl ( v )
option can be used to determine the moment of time shift.