How do you schedule recurring tasks in Linux using cron?

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Cron is the standard Linux task scheduler for running commands at specified times. The cron daemon runs continuously, checking for scheduled jobs.

Edit your crontab with crontab -e. View it with crontab -l. Remove it with crontab -r. Each user has their own crontab.

Cron syntax has five time fields followed by the command. The fields are minute, hour, day of month, month, and day of week. Values range from zero to fifty-nine for minutes, zero to twenty-three for hours, one to thirty-one for day of month, one to twelve for month, and zero to six for day of week with zero being Sunday.

Use asterisk to match any value. Use comma to list multiple values like 1,15 for the first and fifteenth. Use hyphen for ranges like 1-5 for Monday through Friday. Use slash for intervals like */15 for every fifteen minutes.

Examples illustrate the syntax. Running a job at 2:30 AM daily would be 30 2 * * * /path/to/script. Running every hour at minute zero would be 0 * * * * command. Running every Monday at 9 AM would be 0 9 * * 1 command. Running every fifteen minutes would be */15 * * * * command.

System crontabs exist in /etc/crontab and /etc/cron.d/ directory. These include an extra field for the user to run as. Directories like /etc/cron.daily, /etc/cron.weekly, and /etc/cron.monthly contain scripts that run at those intervals.

Cron jobs run with a minimal environment. If your script needs specific environment variables or paths, set them explicitly in the script or at the top of the crontab.

Output handling matters. By default, cron emails output to the user. Redirect output to a file or /dev/null to prevent mail accumulation. Use command >> /var/log/myjob.log 2>&1 to log both stdout and stderr.

The at command schedules one-time jobs. Use echo "command" | at 2pm tomorrow to run once at a specific time.

Systemd timers are a modern alternative on systems using systemd. They offer more features like dependency handling and better logging, but cron remains widely used and understood.