What Is a Cron Job? Scheduled Tasks Explained

Q: How do I create a cron job on Linux or macOS?

Run crontab -e in your terminal to open the crontab editor (uses $EDITOR , typically nano or vi). Each line is one job in the format schedule command . Save and exit — the cron daemon picks up changes immediately. View your current crontab with crontab -l . Remove all your cron jobs with crontab -r (be careful — this deletes all jobs without confirmation). System-wide cron jobs live in /etc/cron.d/ and /etc/crontab .

Q: Why isn't my cron job running?

The most common causes: (1) Wrong path — cron runs with a minimal PATH; use absolute paths for commands ( /usr/bin/python3 not python3 ). (2) No output captured — cron silently discards output by default; add >> /tmp/myjob.log 2>&1 to see errors. (3) Permission issue — the script isn't executable ( chmod +x script.sh ). (4) Syntax error — verify your expression with a cron expression generator before deploying. (5) cron daemon not running — check with systemctl status cron or service cron status .

A cron job is a scheduled task that runs automatically at specified times on Unix-based systems. Learn what cron jobs are, how the syntax works, and how to schedule tasks on Linux, macOS, and in the cloud.

Cron Jobs Explained Simply

A cron job is a command or script that the cron daemon runs automatically on a schedule you define. The schedule is specified using a 5-field expression that controls the minute, hour, day, month, and weekday the job runs.

The cron daemon: A background process (crond) that wakes up every minute, checks the crontab, and runs any scheduled jobs
crontab: The configuration file listing jobs and their schedules — edit with crontab -e
Expression format: minute hour day month weekday command
Runs silently: Output is emailed to the user unless redirected — use >> /var/log/myjob.log 2>&1 to capture it

Build a Cron Expression →

Cron Expression Syntax

Five fields: * * * * * = every minute. Fields left to right: minute (0–59), hour (0–23), day-of-month (1–31), month (1–12), day-of-week (0–7, 0 and 7 both = Sunday)
* = any value: 0 9 * * * = 9:00 AM every day
*/n = every n units: */15 * * * * = every 15 minutes
a-b = range: 0 9 * * 1-5 = 9 AM weekdays only
a,b,c = list: 0 9,17 * * * = 9 AM and 5 PM daily
Common examples: 0 0 * * * = midnight daily; 0 0 1 * * = 1st of each month; 0 0 * * 0 = every Sunday midnight

Common Uses for Cron Jobs

Database backups: 0 2 * * * — run a backup script at 2 AM daily, before business hours
Log rotation: 0 0 * * 0 — compress and archive log files weekly
Report generation: 0 8 * * 1 — email a weekly report every Monday at 8 AM
Cache warming: */5 * * * * — refresh cached data every 5 minutes
SSL certificate renewal: 0 12 * * * — Let's Encrypt's certbot runs daily to check and renew certificates
Data sync: 0 */4 * * * — sync data from an external API every 4 hours

Frequently Asked Questions

How do I create a cron job on Linux or macOS?

Run crontab -e in your terminal to open the crontab editor (uses $EDITOR, typically nano or vi). Each line is one job in the format schedule command. Save and exit — the cron daemon picks up changes immediately. View your current crontab with crontab -l. Remove all your cron jobs with crontab -r (be careful — this deletes all jobs without confirmation). System-wide cron jobs live in /etc/cron.d/ and /etc/crontab.

What timezone does cron use?

By default, cron runs in the system's local timezone. To use a different timezone for a specific crontab, add TZ=America/New_York (or any IANA timezone name) as a line before the jobs that should use it. Cloud schedulers like AWS EventBridge and GitHub Actions run in UTC by default — always specify times in UTC or use a timezone-aware scheduler. A common mistake is setting 0 9 * * * intending 9 AM local time, only to find jobs run at the wrong time on a UTC server.

Why isn't my cron job running?

The most common causes: (1) Wrong path — cron runs with a minimal PATH; use absolute paths for commands (/usr/bin/python3 not python3). (2) No output captured — cron silently discards output by default; add >> /tmp/myjob.log 2>&1 to see errors. (3) Permission issue — the script isn't executable (chmod +x script.sh). (4) Syntax error — verify your expression with a cron expression generator before deploying. (5) cron daemon not running — check with systemctl status cron or service cron status.