CRONTAB(5) - phpMan

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 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 usu-
ally have their own crontabs, eliminating the need for explicitly running su(1) as part of
a cron command.

Blank lines and leading spaces and tabs are ignored. Lines whose first non-space charac-
ter is a hash-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.

An active line in a crontab will be either an environment setting or a cron command. The
crontab file is parsed from top to bottom, so any environment settings will affect only
the cron commands below them in the file. 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 must 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:/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.

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

(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 and MAILFROM 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. MAILTO may also
be used to direct mail to multiple recipients by separating recipient users with a comma.
If MAILTO is defined but empty (MAILTO=""), no mail will be sent. Otherwise mail is sent
to the owner of the crontab.

If MAILFROM is defined, the sender email address is set to MAILFROM. Otherwise mail is
sent as "root (Cron Daemon)".

On the Debian GNU/Linux system, cron supports the pam_env module, and loads the environ-
ment specified by /etc/environment and /etc/security/pam_env.conf. It also reads locale
information from /etc/default/locale. However, the PAM settings do NOT override the set-
tings described above nor any settings in the crontab file itself.

By default, cron will send mail using the mail "Content-Type:" header of "text/plain" with
the "charset=" parameter set to the charmap / codeset of the locale in which crond(8) is
started up - i.e. either the default system locale, if no LC_* environment variables are
set, or the locale specified by the LC_* environment variables ( see locale(7)). You can
use different character encodings for mailed cron job output by setting the CONTENT_TYPE
and CONTENT_TRANSFER_ENCODING variables in crontabs, to the correct values of the mail
headers of those names.

The format of a cron command is very much the V7 standard, with a number of upward-compat-
ible 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 spec-
ified 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 /bin/sh
or by the shell specified in the SHELL variable of the crontab file. 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. There is no
way to split a single command line onto multiple lines, like the shell's trailing "\".

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., don't start with *), 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
The following lists an example of a user crontab file.

# use /bin/bash to run commands, instead of the default /bin/sh
SHELL=/bin/bash
# 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"

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
# You can also override PATH, but by default, newer versions inherit it from the environment
#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.