crontab(5) - Man pages

CRONTAB(5) crontab User Manual 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 │ 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 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). 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
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 )