I wanted to trigger a cron that ran a task between a certain window, but wanted it to run at a random time to minimize clashes with other systems.
This is what I came up with in pure bash.
0 0 * * * root sleep $(( $RANDOM \% 43200 + 1)) && certbot -q renewWhich equates to at midnight as the user root, sleep for a random amount of time between 1 and 43200 seconds (43200 seconds = 12 hours) and then run the certbot renewal command.
You need the % escaping with, \ as cron will convert % to a newline if you don’t, and that will cause your script to fail.
The
commandfield (the rest of the line) is the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/sh or by the shell specified in theSHELLvariable of thecrontab. Percent signs (‘%’) in the command, unless escaped with a backslash (‘\’), will be changed into newline characters, and all data after the first ‘%’ will be sent to the command as standard input.