Linux in a Nutshell, Fourth Edition

Name

crontab

Synopsis

                  crontab [options] [file]

View, install, or uninstall your current crontab file. A privileged user can run crontab for another user by supplying -u user. A crontab file is a list of commands, one per line, that will execute automatically at a given time. Numbers are supplied before each command to specify the execution time. The numbers appear in five fields, as follows:

                  Minute              0-59
Hour             0-23
Day of month      1-31
Month            1-12
                 Jan, Feb, Mar, ...
Day of week              0-6, with 0 = Sunday
                 Sun, Mon, Tue, ...

Use a comma between multiple values, a hyphen to indicate a range, and an asterisk to indicate all possible values. For example, assuming these crontab entries:

59 3 * * 5     find / -print | backup_program
0 0 1,15 * *   echo "Timesheets due" | mail user

the first command backs up the system files every Friday at 3:59 a.m., and the second command mails a reminder on the 1st and 15th of each month.

The superuser can always issue the crontab command. Other users must be listed in the file /etc/cron.allow if it exists; otherwise, they must not be listed in /etc/cron.deny. If neither file exists, only the superuser can issue the command.

Options

The -e, -l, and -r options are not valid if any files are specified.

-e

Edit the user’s current crontab file (or create one).

-l

Display the user’s crontab file on standard output.

-r

Delete the user’s crontab file.

-u user

Indicate which user’s crontab file will be acted upon.