CRONTAB(5)

NAME
crontab - tables for driving cron

DESCRIPTION

A crontab file contains instructions to the 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 su(1) as part of a cron command.

Note that comments on the same line as cron commands are not interpreted as comments in the cron sense, but are considered part of the command and passed to the shell. This is similarly true for comments on the same line as environment variable settings.

An active line in a crontab will be either an environment setting or a cron command. An environment setting is of the form,

    name = value

where the spaces around the equal-sign (=) are optional, and any subsequent non-leading spaces in value will be part of the value assigned to name. The value string may be placed in quotes (single or double, but matching) to preserve leading or trailing blanks. To define an empty variable, quotes can be used.

The value string is not parsed for environmental substitutions or replacement of variables or tilde(~) expansion, thus lines like

    PATH=$HOME/bin:$PATH
    PATH=~/bin:/usr/bin

will not work as you might expect. And neither will this work.

    A=1
    B=2
    C=$A $B

There will not be any substitution for the defined variables in the last value. However, with most shells you can also try e.g.,:

    P=PATH=/a/b/c:$PATH
    33 22 1 2 3 eval $P && some commands

Several environment variables are set up automatically by the cron(8) daemon. SHELL is set to /usr/bin/sh, and LOGNAME and HOME are set from the /etc/passwd line of the crontab's owner. HOME and SHELL may be overridden by settings in the crontab; LOGNAME may not.

(Another note: the LOGNAME variable is sometimes called USER on BSD systems... on these systems, USER will be set also.)

In addition to LOGNAME, HOME, and SHELL, cron(8) will look at MAILTO if it has any reason to send mail as a result of running commands in “this” crontab. If MAILTO is defined (and non-empty), mail is sent to the user so named. If 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 /usr/bin/mail instead of /usr/lib/sendmail as your mailer when you install cron -- /usr/bin/mail doesn't do aliasing, and UUCP usually doesn't read its mail.

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 command, followed by a newline character ('\n'). The system crontab (/etc/crontab) uses the same format, except that the username for the command is specified after the time and date fields and before the command. The fields may be separated by spaces or tabs. The maximum permitted length for the command field is 998 characters.

Commands are executed by cron(8) when the minute, hour, and month of year fields match the current time, and when at least one of the two day fields (day of month, or day of week) match the current time (see “Note” below). cron(8) examines cron entries once every minute. The time and date fields are:
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)

A field may be an asterisk (*), which always stands for “first-last”.

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.

Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: “1,2,5,9”, “0-4,8-12”.

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”.

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 doesn't matter). Examples for the month are: “Jan-Mar” or “Jan,Mar” . Examples for the day of week are; “Mon-Wed” or “Mon-Wed”. Names are in English language.

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 /usr/bin/sh or by the shell specified in the 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.

Note: The day of a command's execution can be specified by two fields — day of month, and day of week. If both fields are restricted (i.e., aren't *), the command will be run when 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. One can, however, achieve the desired result by adding a test to the command (see the last example in EXAMPLE CRON FILE below).

Instead of the first five fields, one of eight special strings may appear:
string | meaning
@reboot | Run once, at startup.
@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 * * * *".

Please note that startup, as far as @reboot is concerned, is the time when the cron(8) daemon startup. In particular, it may be before some system daemons, or other facilities, were startup. This is due to the boot order sequence of the machine.

EXAMPLE CRON FILE

    # use /usr/bin/sh to run commands, no matter what /etc/passwd says
    SHELL=/usr/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"
    0 */4 1 * mon   echo "run every 4th hour on the 1st and on every Monday"
    0 0 */2 * sun   echo "run at midn on every Sunday that's an uneven date"
    # Run on every second Saturday of the month
    0 4 8-14 * *    test $(date +\%u) -eq 6 && echo "2nd Saturday"
    # Same thing, efficient too:
    0 4 * * * Sat   d=$(date +e) && test $d -ge 8 -a $d -le 14 && echo "2nd Saturday"
    #Execute early the next morning following the first
    #Thursday of each month
    57 2 * * 5 case $(date +d) in 0[2-8]) echo "After 1st Thursday"; esac

All the above examples run non-interactive programs. If you wish to run a program that interacts with the user's desktop you have to make sure the proper environment variable DISPLAY is set.

    # Execute a program and run a notification every day at 10:00 am
    0 10 * * *  $HOME/bin/program | DISPLAY=:0 notify-send "Program run" "$(cat)"

EXAMPLE SYSTEM CRON FILE

The following lists the content of a regular system-wide crontab file. Unlike a user's crontab, this file has the username field, as used by /etc/crontab.

    # /etc/crontab: system-wide crontab
    # Unlike any other crontab you don't have to run the `crontab'
    # command to install the new version when you edit this file
    # and files in /etc/cron.d.  These files also have username fields,
    # that none of the other crontabs do.

    SHELL=/bin/sh
    PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

    # Example of job definition:
    # .---------------- minute (0 - 59)
    # |  .------------- hour (0 - 23)
    # |  |  .---------- day of month (1 - 31)
    # |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
    # |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
    # m h dom mon dow usercommand
    17 * * * *  root  cd / && run-parts --report /etc/cron.hourly
    25 6 * * *  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
    47 6 * * 7  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
    52 6 1 * *  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
    #

Note that all the system-wide tasks will run, by default, from 6 am to 7 am. In the case of systems that are not powered on during that period of time, only the hourly tasks will be executed unless the defaults above are changed.

YET ANOTHER EXAMPLE

In that example one can see that numbers can be prepended some 0, in order to line up columns.

    17  * * * *  root  cd / && run-parts --report /etc/cron.hourly
    25 16 * * *  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
    47 06 * * 7  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
    52 06 1 * *  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )

SEE ALSO

cron(8), crontab(1)

EXTENSIONS

When specifying day of week, both day 0 and day 7 will be considered Sunday. BSD and AT&T seem to disagree about this.

Lists and ranges are allowed to co-exist in the same field. "1-3,7-9" would be rejected by AT&T or BSD cron -- they want to see "1-3" or "7,8,9" ONLY.

Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".

Names of months or days of the week can be specified by name.

Environment variables can be set in the crontab. In BSD or AT&T, the environment handed to child processes is basically the one from /etc/rc.

Command output is mailed to the crontab owner (BSD can't do this), can be mailed to a person other than the crontab owner (SysV can't do this), or the feature can be turned off and no mail will be sent at all (SysV can't do this either).

All of the “@” commands that can appear in place of the first five fields are extensions.

LIMITATIONS

The cron daemon runs with a defined timezone. It currently does not support per-user timezones. All the tasks: system's and user's will be run based on the configured timezone. Even if a user specifies the TZ environment variable in his crontab this will affect only the commands executed in the crontab, not the execution of the crontab tasks themselves. If one wants to specify a particular timezone for crontab tasks, one may check the date in the child script, for example:

    # m h  dom mon dow   command

    TZ=UTC
    0 * * * * [ "$(date +\%R)" = 00:00 ] && run_some_script

POSIX specifies that the day of month and the day of week fields both need to match the current time if either of them is a *. However, this implementation only checks if the first character is a *. This is why "0 0 */2 * sun" runs every Sunday that's an uneven date while the POSIX standard would have it run every Sunday and on every uneven date.

The crontab syntax does not make it possible to define all possible periods one can imagine. For example, it is not straightforward to define the last weekday of a month. To have a task run in a time period that cannot be defined using crontab syntax, the best approach would be to have the program itself check the date and time information and continue execution only if the period matches the desired one.

If the program itself cannot do the checks then a wrapper script would be required. Useful tools that could be used for date analysis are ncal(1) or calendar(1). For example, to run a program the last Saturday of every month you could use the following wrapper code:

    0 4 * * Sat   [ "$(date +\%e)" = "$(LANG=C ncal | sed -n 's/^Sa .* \([0-9]\+\) *$/\1/p')" ] && echo "Last Saturday" && program_to_run

The maximum permitted length for the command field is 998 characters.

The recommended length should be less than 80 characters, like lines in readable source files. When one must use a long command, it is worth creating a dedicated script, which in turn can contain valuable comments.

USING EVAL TO WRAP MISC ENVIRONMENT SETTINGS

The following tip is kindly provided by 積丹尼 Dan Jacobson:

    CONTENT_TYPE="text/plain; charset=UTF-8"
    d=eval LANG=zh_TW.UTF-8 w3m -dump
    26 22 16 1-12 * $d https://www.ptt.cc/bbs/transgender/index.html

it won't work without the eval. Saying

    d=LANG=zh_TW.UTF-8 w3m -dump

will get

    /bin/sh: LANG=zh_TW.UTF-8: command not found

DIAGNOSTICS

cron requires that each entry in a crontab end in a newline character. If the last entry in a crontab is missing a newline (i.e. terminated by EOF), cron will consider the crontab (at least partially) broken. A warning will be written to syslog.

AUTHORS

Paul Vixie <paul@vix.com>
    Wrote this manpage (1994).

Steve Greenland <stevegr@debian.org>
    Maintained the package (1996-2005).

Javier Fernández-Sanguino Peña <jfs@debian.org>
    Maintained the package (2005-2014).

Christian Kastner <ckk@debian.org>
    Maintained the package (2010-2016).

Georges Khaznadar <georgesk@debian.org>
    Maintained the package (2022-2024).

COPYRIGHT
Copyright © 1994 Paul Vixie

Distribute freely, except: don't remove my name from the source or documentation (don't take credit for my work), mark your changes (don't get me blamed for your possible bugs), don't alter or remove this notice. May be sold if buildable source is provided to buyer. No warranty of any kind, express or implied, is included with this software; use at your own risk, responsibility for damages (if any) to anyone resulting from the use of this software rests entirely with the user.

Since year 1994, many modifications were made in this manpage, authored by Debian Developers which maintained cron(TM); above is a short list, more information can be found in the file /usr/share/doc/cron/copyright.