Executing processes at regular intervals

The crontab(C) command lets you execute routine jobs (called ``cron'' jobs) on a regular basis. For example, it could be used to periodically back up your files, or to periodically clean up tmp and log files. To submit a cron job, details of the job must be added to a cronfile. This is a normal file, but its contents are formatted in a special way:

   Minutes   Hours   Day_of_Month   Month   Day_of_week   Command

Field	Allowable values
Minutes	0-59
Hours	0-23
Day of month	1-31
Month of year	1-12
Day of week	0-6 (0=Sunday)
Command	any non-interactive command

A field can be a number, a range of numbers (for example 10-20), a list of numbers separated by commas, or an asterisk (all values). For example, an asterisk in the ``Hours'' field means ``every hour''; an asterisk in the ``Month'' field means ``every month''.

Let us assume that you want to write a cronfile to issue reminders and perform regular tasks:

• You need to attend a meeting at 10A.M. every Monday, and you want to remind yourself of this at 9:45A.M. on Monday mornings.

• You want to find and remove any old files beginning with ``#'' in your home directory at 4:30P.M. on the first day of every month.

• You want to echo the date and time to your terminal at 9:00A.M. Monday to Friday.

   45  9    *   *   1    echo "Weekly status meeting" > /dev/tty06
   30  16   1   *   *    find $HOME -name '#*' -atime +3 -exec rm -f {} \;
   0   9    *   *   1-5  echo date > /dev/tty06

   $ crontab cronfile

To display the current cron job, type crontab -l. Redirect the output to a file and edit it, then re-submit the new file. This replaces the old cron job.

To remove the current cron job, type crontab -r. If you submit a second cronfile before the first one is executed, the first one will be overwritten by the second.

As with at and batch, access to crontab can be turned on and off by the root user by adding user names to /usr/lib/cron/cron.allow and /usr/lib/cron/cron.deny respectively.