Computer Science › Operating Systems

Cron

Scheduling commands to run at fixed times.

Also known as: crontab, cron job, cron expression, scheduled task

Cron is the classic Unix scheduler. It runs commands automatically at fixed times or intervals. You list jobs in a file called a crontab.

crontab -e          # edit your jobs
crontab -l          # list them

# minute  hour  day-of-month  month  day-of-week  command
  30      2     *             *      *            /usr/local/bin/backup.sh
  */15    *     *             *      *            /usr/bin/python3 /srv/app/poll.py
  0       9     *             *      1-5          /srv/app/report.sh

The five fields are minute (0-59), hour (0-23), day of month (1-31), month (1-12) and day of week (0-7, where 0 and 7 are Sunday). * means every, */15 means every 15, 1-5 is a range, and 1,15 is a list.

Things that go wrong

• Different environment. Cron runs with a minimal PATH and no shell profile, so commands that work in your terminal “aren’t found”. Use full paths and set variables in the script (PATH).
• Silent failures. Output is mailed or discarded. Redirect to a log: ... >> /var/log/job.log 2>&1, and check the exit code.
• Time zones. Cron uses the server’s zone, and daylight saving can skip or repeat a run. Prefer UTC for servers (time zones).
• Overlapping runs if a job takes longer than its interval. Use a lock, or flock.
• Not idempotent. A job that runs twice (or is retried) shouldn’t double-send emails (idempotence).
• Single machine. If that server is down or there are several, jobs are missed or duplicated.
• No dependencies, retries or monitoring. Fine for simple tasks.

For application-level scheduling, see scheduled jobs. Data pipelines use orchestrators that also take cron-style schedules (pipeline scheduling).