Cron jobs are a very useful tool for scheduling commands however I find the Crontab (Cron Table) syntax nearly impossible to remember unless I’m working with it daily.
Most of my Cron jobs are fairly standard, for example backup a particular directory every hour. While configuring a new job I looked around to remember how to execute a command at a particular time every day. Most of the online editors I tried are more complex than the syntax itself. Thankfully I came across an interesting post from 2007 that mentioned Special Words. It turns out that you can use a set of keywords as well as numbers when defining a job:
@reboot Run once at startup @yearly Run once a year @monthly Run once a month @weekly Run once a week @daily Run once a day @hourly Run once an hour
To run a command daily I can simply use:
@daily <some command>
But when is @daily? Using the run-parts command we can find out when each keyword will be executed, in this case 6.25am. A strange time but works for me!
$ grep run-parts /etc/crontab 17 * * * * root cd / && run-parts --report /etc/cron.hourly 25 6 * * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily ) 47 6 * * 7 root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly ) 52 6 1 * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )