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Cron and crontab

Cron allows to run jobs (programs, scripts) at specified times.

OpenWrt comes with a cron system by default, provided by busybox.

Adding and editing cron jobs

You can edit the current config with:

crontab -e 

:!: There should be a end-of-line character on the last line of the crontab file. If in doubt, just leave an empty line at the end.

Example of time specification:

*/5 * * * * every 5 minutes
12 */3 * * * every 3 hours at 12 minutes
57 11 15 1,6,12 * At 11:57 Hrs on 15th of Jan, June & Dec.
25 6 * * 1-5 At 6:25 AM every weekday (Mon-Fri)
0 0 4,12,26 * * At midnight on 4th, 12th and 26th of every month
5,10 9,14 10 * 0,4 At 9:05AM, 9:10AM, 2:05PM and 2:10PM every Sunday and Thursday

0 (zero) is treated as Sunday. If you set the day of the week to 7, busybox will go bonkers and run your command every day.

Activating cron

Cron is not enabled by default, so your jobs won't be run. To activate cron:

/etc/init.d/cron start
/etc/init.d/cron enable

:!: If there is no crontab defined (i.e. /etc/crontabs/ is empty), then cron won't start! Make sure you already defined cron jobs before trying to start cron.


Tips and tricks

Check that the jobs are run

Each time a job is run by cron, a line is printed in OpenWrt logs. Run logread to check whether your jobs are correctly run.

Periodic reboot of a router

A simple solution for some hard-to-solve problems (memory leak, performance degradation, …) is to reboot the router periodically, for instance every night.

However, this is not as simple as it seems, becomes router usually have no real-time clock. This could lead to a never-ending loop of reboot.

In the boot process the clock is initially set by sysfixtime to the most recent timestamp of any file found in /etc. The most recent file is possibly a status file or config file, modified maybe 30 seconds before the reboot initiated by cron. So, in the boot process the clock gets set backwards a few seconds to that file's timestamp. Then cron starts and notices a few seconds later that the required boot moment has again arrived and reboots again… (At the end of the boot process ntpd starts, and it may also take a while before ntpd gets and sets the correct time, so cron may start the reboot in between.)

One solution for cron is to use a delay and touch a file in /etc before reboot:

# Reboot at 4:30am every day
# Note: To avoid infinite reboot loop, wait 70 seconds
# and touch a file in /etc so clock will be set
# properly to 4:31 on reboot before cron starts.
30 4 * * * sleep 70 && touch /etc/banner && reboot

:!: On many platforms shutdown does not work; it will just halt the CPU but it won't power off the device. There is usually no programmable circuitry to actually power off the unit. reboot does work, in case you should want to reboot the router periodically.

Alarm clock

If you have Daylight saving time you could write yourself a nice alarm clock ;-) When DST starts in central Europe, clocks advance from 02:00 CET to 03:00 CEST on last Sunday in March. Six day before that, you could make your WOL wake you 10 minutes earlier. Later won't work, you'll be late ;-) When DST ends in central Europe, clocks retreat from 03:00 CEST to 02:00 CET on last Sunday in October.

#min hour day month dayofweek command
59 05 * * 1 /usr/bin/wol -h xx:xx:xx:xx:xx:xx # Mo
#crontab must (as fstab) end with the last line as space or a comment

doc/howto/cron.txt · Last modified: 2016/04/06 22:00 by Bernini