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cron cheatsheet

System wide crontab

/etc/crontab

cron.d

Other crontabs can be put into /etc/cron.d/

Create a user specific crontab

As user:

  • crontab -e

The user specific crontabs are saved in /var/spool/cron/crontabs/

Deactivate cron

  • service cron stop

Send output to specific email address

Per default cron sends output to the owner of the crontab file.

To redirect use the MAILTO variable. Put on top of the crontab:

MAILTO="my@example.com"

Every 5 minutes

*/5 * * * *   root   /path/to/command

Use @reboot

# use sleep, because network and other services are'nt up yet
@reboot         sleep 60 && mycommand

# test
@reboot         sleep 60 && ls -la 2>&1 >> /home/myuser/reboot.log

Quiet cron (Output redirection)

script.sh > /dev/null 2>&1

Quiet cron, but send errors

script.sh > /dev/null

Log output into logfile (stdout & stderr), but swallow stdout output

-> All output is appended to logfile, but in case of errors a cron email ist sent

04 03  * * *   root    /etc/script.sh > >(tee --append /var/log/script.log > /dev/null) 2> >(tee --append /var/log/script.log >&2)

Run jobs with very low priority

nice -n 19 ionice -c3 script.sh

Dont start a cronjob again before it's finished

http://blog.dustinkirkland.com/2011/02/introducing-run-one-and-run-this-one.html

sudo apt-get install run-one

run-one script.sh

Script aborted or crashed?

rm ~/.cache/run-one/*

Check which scripts are execute by e.g. cron.daily

run-parts --test /etc/cron.daily