What Is a Cron Job and How Do You Set One Up? Linux Scheduling Guide

Want automatic nightly backups, weekly log cleanup, or a 5-minute health check on your server? Cron is Linux’s standard tool. This guide covers crontab syntax, practical examples, and common pitfalls.

What is cron?

Cron is a Linux daemon that runs commands at scheduled times. It reads crontab files every minute and triggers due commands.

Crontab syntax

A cron line is 5 time fields plus a command:

* * * * * command
│ │ │ │ │
│ │ │ │ └── Day of week (0-7, 0 and 7 = Sunday)
│ │ │ └──── Month (1-12)
│ │ └────── Day of month (1-31)
│ └──────── Hour (0-23)
└────────── Minute (0-59)

Practical examples

Expression	Meaning
* * * * *	Every minute
*/5 * * * *	Every 5 minutes
0 * * * *	Top of every hour
0 3 * * *	Daily at 03:00
0 3 * * 1	Mondays at 03:00
0 3 1 * *	1st of every month at 03:00
30 2 * * 1-5	Weekdays at 02:30
@reboot	At server boot
@daily	Daily at 00:00
@weekly	Sundays at 00:00

To test complex expressions: crontab.guru

How to edit crontab?

crontab -e        # Edit current user's
sudo crontab -e   # Edit root's
crontab -l        # List
crontab -r        # Delete ALL (careful!)

First time, you’ll be asked to pick an editor — nano (easy) or vim.

Practical examples

Nightly backup

# Every night at 03:00, WordPress backup
0 3 * * * /usr/local/bin/backup-wp.sh >> /var/log/backup.log 2>&1

>> /var/log/backup.log 2>&1 saves output to a log including errors.

Hourly log cleanup

0 * * * * find /var/log/myapp -name "*.log" -mtime +30 -delete

Delete logs older than 30 days.

Health check every 5 minutes

*/5 * * * * curl -s -o /dev/null -w "%{http_code}" https://yoursite.com >> /var/log/health.log

Database backup

30 2 * * * mysqldump -u root -p'PASSWORD' dbname | gzip > /backup/db-$(date +\%Y\%m\%d).sql.gz

⚠️ % is special in crontab — escape with \%.

Certificate renewal

0 4 * * * certbot renew --quiet && systemctl reload nginx

For SSL details, see our Let’s Encrypt post.

How to verify cron ran?

# Is cron running?
sudo systemctl status cron

# Cron log
sudo tail -f /var/log/syslog | grep CRON
# or
sudo tail -f /var/log/cron

To test, schedule for 1-2 minutes ahead and watch the log.

Common pitfalls

• Cron didn’t run: write commands with absolute paths (/usr/bin/php, not just php)
• No env vars: cron runs with minimal env → set PATH= at the script’s top
• Forgetting %: date +%Y%m%d breaks in cron → use \%
• No output visible: without mail config, STDOUT is lost → use >> log 2>&1
• PHP/Node script timeouts: runs as CLI, no web timeout — but background long scripts
• Time zone: if server is UTC, times shift → sudo timedatectl set-timezone Europe/Istanbul

systemd timer alternative (modern)

systemd timers are more modern than cron with cleaner log integration:

/etc/systemd/system/backup.service:

[Unit]
Description=Daily backup

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup-wp.sh

/etc/systemd/system/backup.timer:

[Unit]
Description=Daily backup timer

[Timer]
OnCalendar=*-*-* 03:00:00
Persistent=true

[Install]
WantedBy=timers.target

sudo systemctl enable --now backup.timer
sudo systemctl list-timers

Conclusion

Cron is the foundation of server automation. Learned in 5 minutes, used every day. Backups, cleanup, monitoring — all cron.

Related: Server Backups · Linux Disk Extension