/usr/bin/crontab [-u username] [filename]

/usr/bin/crontab { -e | -l | -r } [username]

/usr/bin/crontab -u username { -e | -l | -r }

/usr/xpg4/bin/crontab [filename]

/usr/xpg4/bin/crontab { -e | -l | -r } [username]

/usr/xpg4/bin/crontab -u username { -e | -l | -r }

/usr/xpg6/bin/crontab [filename]

/usr/xpg6/bin/crontab { -e | -l | -r } [username]

/usr/xpg6/bin/crontab -u username { -e | -l | -r }

If crontab is invoked with filename, this overwrites an existing crontab entry for the user that invokes it, or for the user specified with the -u option.

Users: Access to crontab is denied:

The rules for allow and deny apply to root only if the allow/deny files exist.

The allow/deny files consist of one user name per line.

minute (0-59),
hour (0-23),
day of the month (1-31),
month of the year (1-12),
day of the week (0-6 with 0=Sunday).

Each of these patterns can be either an asterisk (meaning all legal values) or a list of elements separated by commas. An element is either a number or two numbers separated by a hyphen (meaning an inclusive range).

A range or asterisk can optionally be followed by a step value as /<number>. For example, 2-59/3 can be used in the minutes field to specify every three minutes starting at 2 past the hour, or */2 in the hours field means every two hours.

Time specified here is interpreted in the currently active timezone. At the top of the crontab file this is the timezone which is set system-wide in /etc/default/init. A user can add a line such as:

TZ=timezone

...and all subsequent entries will be interpreted using that timezone, until a new TZ=timezone line is encountered. The specification of days can be made by two fields (day of the month and day of the week). Both are adhered to if specified as a list of elements. See EXAMPLES.

The sixth field of a line in a crontab file is a string that is executed by the shell at the specified times. A percent character in this field (unless escaped by \e\|) is translated to a NEWLINE character.

Only the first line (up to a `\|%\|' or end of line) of the command field is executed by the shell. Other lines are made available to the command as standard input. Any blank line or line beginning with a `\|#\|' is a comment and is ignored.

The shell is invoked from your $HOME directory. As with $TZ, both $SHELL and $HOME can be set by having a line such as:

SHELL=/usr/bin/someshell

...or:

HOME=somedirectory

...which will take precedence for all the remaining entries in the crontab or until there is another HOME or SHELL entry. It is invoked with an arg0 of the basename of the $SHELL that is currently in effect. A user who wants to have his .profile or equivalent file executed must explicitly do so in the crontab file. cron supplies a default environment for every shell, defining HOME, LOGNAME, SHELL, TZ, and PATH. The default PATH for user cron jobs is /usr/bin; while root cron jobs default to /usr/sbin:/usr/bin. The default PATH can be set in /etc/default/cron (see cron(8)). The TZ, HOME, and SHELL environment variables are set to match those that are in effect in the crontab file at the time.

If you do not redirect the standard output and standard error of your commands, any generated output or errors are mailed to you.

HOME=/var/tmp

SHELL=/usr/bin/ksh

TZ=Iceland

RANDOM_DELAY=2

Each of these variables affects all of the lines that follow it in the crontab file, until it is reset by a subsequent line resetting that variable. Hence, it is possible to have multiple timezones supported within a single crontab file.

The lines that are not setting these environment variables are the same as crontab entries that conform to the UNIX standard and are described elsewhere in this man page.

If some form of daylight savings or summer/winter time is in effect, then jobs scheduled during the switchover period could be executed once, twice, or not at all.

This example cleans up core files every weekday morning at 3:15 am:

15 3 * * 1-5 find $HOME -namecore 2>/dev/null | xargs rm -f

Example 2 Mailing a Birthday Greeting

This example mails a birthday greeting:

0 12 14 2 * mailx john%Happy Birthday!%Time for lunch.

Example 3 Specifying Days of the Month and Week

This example runs a command on the first and fifteenth of each month, as well as on every Monday:

0 0 1,15 * 1

To specify days by only one field, the other field should be set to *. For example:

0 0 * * 1

would run a command only on Mondays.

Example 4 Using step values:

This example runs a job every hour during the night and every 3 hours during working hours.

0 8-18/3,19-7 * * *

and to run a job every 2 minutes, use:

*/2 * * * *

Example 5 Using Environment Variables

The following entries take advantage of crontab support for certain environment variables.

TZ=GMT
HOME=/local/home/user
SHELL=/usr/bin/ksh
0 0 * * * echo $(date) > midnight.GMT
TZ=PST
0 0 * * * echo $(date) > midnight.PST
TZ=EST
HOME=/local/home/myuser
SHELL=/bin/csh

The preceding entries allow two jobs to run. The first one would run at midnight in the GMT timezone and the second would run at midnight in the PST timezone. Both would be run in the directory /local/home/user using the Korn shell. The file concludes with TZ, HOME, and SHELL entries that return those variable to their default values.

When updating cron, check first for existing crontab entries that can be scheduled close to the time of the update. Such entries can be lost if the update process completes after the scheduled event. This can happen because, when cron is notified by crontab to update the internal view of a user's crontab file, it first removes the user's existing internal crontab and any internal scheduled events. Then it reads the new crontab file and rebuilds the internal crontab and events. This last step takes time, especially with a large crontab file, and can complete after an existing crontab entry is scheduled to run if it is scheduled too close to the update. To be safe, start a new job at least 60 seconds after the current date and time.

If an authorized user other than root modifies another user's crontab file, the resulting behavior can be unpredictable. Instead, the authorized user should first use su(8) to become superuser to the other user's login before making any changes to the crontab file.

Care should be taken when adding TZ, SHELL and HOME variables to the crontab file when the crontab file could be shared with applications that do not expect those variables to be changed from the default. Resetting the values to their defaults at the bottom of the file will minimize the risk of problems.