Mastering Time: Demystifying Cron Syntax for DevOps

Cron is the heartbeat of Linux infrastructure. It powers backups, email digests, and log rotation. Yet, looking at 0 23 * * 1-5 still causes senior engineers to double-check documentation.

The Five Fields

A standard crontab entry has five fields followed by the command. Memorize the order: Minute, Hour, Day-of-Month, Month, Day-of-Week.

Common Pitfalls

• Timezone Confusion: Servers usually run on UTC. If you set a cron to run at 9 AM your local time, it might run at 2 AM server time. Always check date on the server.
• Day of Week Mismatch: In some systems, 0 is Sunday. In others, 7 is Sunday. It is safer to use short names like SUN or MON to avoid ambiguity.
• Environment Variables: Cron runs with a minimal shell. It doesn't know your $PATH or your aliases. Always use absolute paths (e.g., /usr/bin/python3 instead of just python).

Testing Without Waiting

Don't wait until midnight to see if your backup script triggers. Use a Crontab Generator to validate your syntax strings. It translates the asterisks into plain English (e.g., "At 23:00 on every day-of-week from Monday through Friday") so you can catch logic errors instantly.