Cron is a job scheduler that allows users to run commands periodically.
Cron is a daemon that runs periodic scheduled jobs.
Use the crontab command to edit the table of scheduled jobs.
Use at to schedule a job for one execution only at a specific date.
Common pitfalls
If a command works when you type it in a terminal but not from a crontab, here are some common reasons:
Cron provides a limited environment, e.g., a minimal
$PATH.Try adding this entry to your crontab to see what cron's environment is:
* * * * * { date; pwd; echo "env:"; env; echo "set:"; set; } > ~/cron.env
Cron uses
/bin/sh, which may not be the shell you normally use.Cron treats the
%character specially (it is turned into a newline in the command).This will affect the
datecommand especially:* * * * * date "+%F %T" # bad: cron sees `date "+` * * * * * date "+\%F \%T" # good
