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Quick Reference of Cron

Last modified: Aug, 2011

View cron job entries

To list cron job entries of root

sudo crontab -l

To view current user’s cron job entries

crontab -l

To view other users’:

sudo crontab -u other_username -l

Edit cron job entries

To edit root’s:

sudo crontab -e

Entry syntax

The basic template is as follows

*     *     *   *    *        command_to_be_executed
-     -     -   -    -
|     |     |   |    |
|     |     |   |    +----- day of week (0 - 6) (Sunday=0)
|     |     |   +------- month (1 - 12)
|     |     +--------- day of        month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)

Besides, there are some special usages. For example,

*/10 * * * * command_to_be_executed

means executing this command every 10 minutes; and

@hourly command_to_be_executed

means to execute command every hour, which is equivalent to

0 * * * * command_to_be_executed

Similarly, @daily = 0 0 * * *, @monthly = 0 0 1 * *, @yearly = 0 0 1 1 *, and @reboot means executing at boot every time self-explanatorily.

Log Cron Jobs

Cron job output may already be logged in “/var/log/syslog”. To log cron job output to a separate file such as “/var/log/cron.log”, change the rsyslog settings as decribed in this post.

Reference

There are tons of cron tutorials online. Here are just a sip.