CRONTAB(5) File Formats Manual CRONTAB(5)
NAME
crontab - tables for driving cron
DESCRIPTION
A crontab file contains instructions to the cron(8) daemon of the gen-
eral form: ``run this command at this time on this date''. Each user
has their own crontab, and commands in any given crontab will be exe-
cuted 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 inter-
preted 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 subse-
quent 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 must be used.
The value string is not parsed for environmental substitutions or re-
placement 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 num-
ber 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 0-31
month 0-12 (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 spec-
ify command execution every other hour (the alternative in the V7 stan-
dard 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). Ranges or lists of names are not allowed.
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 charac-
ters, 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 cur-
rent 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 exam-
ple in EXAMPLE CRON FILE below).
Instead of the first five fields, one of eight special strings may ap-
pear:
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 pe-
riod of time, only the hourly tasks will be executed unless the de-
faults 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 monts 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 them-
selves. 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 * * * * [ "$(date +\%R)" = 00:00 ] && run_some_script
POSIX specifies that the day of month and the day of week fields both
need to match the current time if either of them is a *. However, this
implementation only checks if the first character is a *. This is why
"0 0 */2 * sun" runs every Sunday that's an uneven date while the POSIX
standard would have it run every Sunday and on every uneven date.
The crontab syntax does not make it possible to define all possible pe-
riods one can imagine. For example, it is not straightforward to de-
fine the last weekday of a month. To have a task run in a time period
that cannot be defined using crontab syntax, the best approach would be
to have the program itself check the date and time information and con-
tinue execution only if the period matches the desired one.
If the program itself cannot do the checks then a wrapper script would
be required. Useful tools that could be used for date analysis are
ncal or calendar For example, to run a program the last Saturday of
every month you could use the following wrapper code:
0 4 * * Sat [ "$(date +\%e)" = "$(LANG=C ncal | sed -n 's/^Sa .* \([0-9]\+\) *$/\1/p')" ] && echo "Last Saturday" && program_to_run
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.
AUTHOR
Paul Vixie <paul@vix.com> is the author of cron and original creator of
this manual page. This page has also been modified for Debian by Steve
Greenland, Javier Fernandez-Sanguino, Christian Kastner, Christian
Pekeler, Georges Khaznadar.
4th Berkeley Distribution 19 April 2010 CRONTAB(5)
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