Cron on Linux: crontab Syntax and Schedule Examples

Cron is the Linux job scheduler. It runs commands and scripts on a schedule: backups at night, temporary file clean-up once a week, a certificate check every day.

Managing jobs

Every user has their own job table — the crontab.

crontab -e    # open the table in an editor
crontab -l    # show current jobs
crontab -r    # delete all of the user's jobs

To edit another user’s jobs, add -u: sudo crontab -u www-data -e.

Line format

* * * * * command
│ │ │ │ │
│ │ │ │ └─ day of week (0–7, where 0 and 7 are Sunday)
│ │ │ └─── month (1–12)
│ │ └───── day of month (1–31)
│ └─────── hour (0–23)
└───────── minute (0–59)

Each field accepts:

• * — any value;
• 5 — a specific value;
• 1,15 — a list;
• 1-5 — a range;
• */10 — every 10 units.

Schedule examples

Schedule	When it runs
* * * * *	every minute
*/5 * * * *	every 5 minutes
0 * * * *	at the start of every hour
30 3 * * *	daily at 03:30
0 9 * * 1-5	on weekdays at 09:00
0 0 1 * *	at midnight on the first day of each month
0 4 * * 0	on Sundays at 04:00

There are shortcuts for common cases: @reboot (at system start-up), @hourly, @daily, @weekly, @monthly.

Practical jobs

A daily site backup at 02:00:

0 2 * * * tar -czf /backup/site-$(date +\%F).tar.gz /var/www/site

Directory sync every 15 minutes:

*/15 * * * * rsync -a /var/www/ backup@203.0.113.20:/backup/www/

Run a script after the server reboots:

@reboot /opt/scripts/start.sh

Output and logs

By default cron tries to email a command’s output, and on a server without a mail service it is lost. Redirect the output to a file:

0 2 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1

2>&1 sends error messages to the same file. The system journal shows that jobs were started:

journalctl -u cron --since today      # Debian, Ubuntu
journalctl -u crond --since today     # AlmaLinux, Rocky Linux, RHEL

Preventing overlapping runs

If a job takes longer than the interval between runs, copies start piling up. The flock utility starts the command only if the previous instance has finished:

*/5 * * * * flock -n /tmp/sync.lock /opt/scripts/sync.sh

Common mistakes

• The command works in a terminal but not in cron. Cron has a minimal environment and a short PATH. Use full paths to programs (/usr/bin/php) or set PATH on the first line of the crontab.
• The % character. In a crontab it means a newline. Escape it in commands: date +\%F.
• Relative paths. A job starts in the user’s home directory. Use absolute paths or cd /path && command.
• The script is not executable. Run chmod +x script.sh and add #!/bin/bash as the first line.
• Time zone. Cron uses system time. Check it with timedatectl.

System jobs

Besides user tables there are system ones:

• /etc/crontab and files in /etc/cron.d/ — the same format with an extra user name field before the command;
• /etc/cron.daily/, /etc/cron.weekly/, /etc/cron.monthly/ — scripts in these directories run at the corresponding interval.

Cron and systemd timers

On modern distributions some jobs are handled by systemd timers. They can run jobs missed while the server was down and keep a detailed log, but need two files instead of one line. For simple schedules on a VPS, cron is still the quickest option. List active timers with systemctl list-timers.