cron is a utility that you can use to schedule and automate tasks. By defining items in the cron table, called crontab, you can schedule any script or program to run on almost any sort of schedule. A few examples:
- Download RSS feeds daily at 6:30am,
- Run a program 5 minutes after midnight on mondays, wednesdays and fridays,
- Schedule something to run every five minutes, or
- Do something just once a month.
Each user has their own crontab, the scheduled scripts run as that user so take this in account with regards to permissions. To edit the crontab use the following command:
$ crontab -e
You can list what your currnet crontab is using the following command:
$ crontab -l
The following is the format entries in a crontab must be. Note all lines starting with # are ignored, comments.
MIN HOUR MDAY MON DOW COMMAND 5 * * * * echo 'Hello'
|HOUR||Hour [24-hour clock]||0-23|
|MDAY||Day of Month||1-31|
|MON||Month||1-12 OR jan,feb,mar,apr …|
|DOW||Day of Week||
0-6 OR |
|COMMAND||Command to be run||Any valid command-line|
Here are a few examples, to see what some entries look like.
# Run command at 7:00am each weekday [mon-fri] 00 07 * * 1-5 mail_pager.script 'Wake Up' # Run command on 1st of each month, at 5:30pm 30 17 1 * * pay_rent.scrip # Run command at 8:00am,10:00am and 2:00pm every day 00 8,10,14 * * * do_something.script # Run command every 5 minutes during market hours /5 6-13 * mon-fri get_stock_quote.script # Run command every 3-hours while awake 0 7-23/3 * * * drink_water.script
Special Characters in Crontab
You can use an asterisk in any category to mean for every item, such as every day or every month.
You can use commas in any category to specify multiple values. For example:
You can use dashes to specify ranges. For example:
You can use forward slash to specify a repeating range. For example:
*/5 for every five minutes, hours, days
There are several special entries, some which are just shortcuts, that you can use instead of specifying the full cron entry.
The most useful of these is probably @reboot which allows you to run a command each time the computer gets reboot. This could be useful if you want to start up a server or daemon under a particular user, or if you do not have access to the rc.d/init.d files.
# restart freevo servers @reboot freevo webserver start @reboot freevo recordserver start
The complete list:
|@reboot||Run once, at startup.||None|
|@yearly||Run once a year||0 0 1 1 *|
|@annually||(same as @yearly)||0 0 1 1 *|
|@monthly||Run once a month||0 0 1 * *|
|@weekly||Run once a week||0 0 * * 0|
|@daily||Run once a day||0 0 * * *|
|@midnight||(same as @daily)||0 0 * * *|
|@hourly||Run once an hour||0 * * * *|
If there is any output from your script or command it will be sent to that user’s e-mail account, on that box. Using the default mailer which must be setup properly.
You can set the variable
MAILTO in the crontab to specify a separate e-mail address to use. For example:
Redirect Output to /dev/null
You can redirect the output from a cron script to /dev/null which just throws it away. By redirecting to /dev/null you will not receive anything from the script, even if it is throwing errors.
* * * * * /script/every_minute.pl > /dev/null 2>&1
If you want to run cron at a different timezone than your system time. You can set the
TZ parameter in
/etc/default/cron. For example, I want it to run in Pacific Time zone, so I set:
Missed Schedule Time
Cron does not run a command if it was missed. Your computer must be running for cron to run the job at the time it is scheduled. For example, if you have a 1:00am scheduled job and your computer was off at that time, it will not run the missed job in the morning when you turn it on.