crontab(5)

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.

Blank lines and leading spaces and tabs are ignored. Lines whose first
non-space character 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, com-
ments are not allowed on the same line as environment variable set-
tings.

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 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:/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 set to
"/usr/bin:/bin". 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 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 multi-
ple 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.

On the Debian GNU/Linux system, cron supports the pam_env module, and
loads the environment specified by /etc/environment and /etc/secu-
rity/pam_env.conf. It also reads locale information from /etc/de-
fault/locale. However, the PAM settings do NOT override the settings
described above nor any settings in the crontab file itself. Note in
particular that if you want a PATH other than "/usr/bin:/bin", you will
need to set it in the crontab file.

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 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 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 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 /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 charac-
ters, 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 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
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
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.