What are cron and crontab, and how do I use them?

Note:
On most systems, you must get permission from the system administrator before you can submit job requests to cron. On some shared systems, because there is only one crontab file, only the administrator has access to the crontab command. To request that cron be enabled on your Webserve account, see the Indiana University Webmaster's Cron page.

The cron daemon is a long-running process that executes commands at specific dates and times. You can use this to schedule activities, either as one-time events or as recurring tasks.

To schedule one-time only tasks with cron, use the at or batch command. For more information, see In Unix, what are the at and batch commands, and how do I use them to submit non-interactive job requests?

For commands that need to be executed repeatedly (e.g., hourly, daily, or weekly), you can use the crontab command. The crontab command creates a crontab file containing commands and instructions for the cron daemon to execute. You can use the crontab command with the following options:

crontab -a filename
Install filename as your crontab file. On many systems, this command is executed simply as crontab filename (i.e., without the -a option).
crontab -e
Edit your crontab file, or create one if it doesn't already exist.
crontab -l Display your crontab file.
crontab -r Remove your crontab file.
crontab -v
Display the last time you edited your crontab file. (This option is available on only a few systems.)
crontab -u user
Used in conjunction with other options, this option allows you to modify or view the crontab file of user. When available, only administrators can use this option.

Each entry in a crontab file consists of six fields, specifying in the following order:

  minute(s) hour(s) day(s) month(s) weekday(s) command(s)

The fields are separated by spaces or tabs. The first five are integer patterns and the sixth is the command to execute. The following table briefly describes each of the fields:

Field Value Description
minute 0-59
The exact minute that the command sequence executes
hour 0-23
The hour of the day that the command sequence executes
day 1-31
The day of the month that the command sequence executes
month 1-12
The month of the year that the command sequence executes
weekday 0-6
The day of the week that the command sequence executes (Sunday = 0, Monday = 1, Tuesday = 2, and so forth)
command Special
The complete sequence of commands to execute. The command string must conform to Bourne shell syntax. Commands, executables (such as scripts), or combinations are acceptable.

Each of the patterns from the first five fields may be either * (an asterisk), meaning all legal values, or a list of elements separated by commas. An element is either a number or an inclusive range, indicated by two numbers separated by a minus sign (e.g., 10-12). You can specify days with two fields: day of the month and day of the week. If you specify both of them as a list of elements, cron will observe both of them, for example:

  0 0 1,15 * 1 /mydir/myprogram

The cron daemon would run the program myprogram in the mydir directory on the first and fifteenth of each month, as well as on every Monday. To specify days by only one field, the other field should be set to *, for example:

  0 0 * * 1 /mydir/myprogram

In the above example, the program would run only on Mondays.

If a cron job specified in your crontab entry produces any error messages when it runs, you will get a mail message reporting the errors.

For more information, consult the following relevant man pages:

  man crontab
  man cron
  man at
  man batch

At Indiana University, for personal or departmental Linux or Unix systems support, see At IU, how do I get support for Linux or Unix?

This is document afiz in the Knowledge Base.
Last modified on 2015-08-13.

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