Scheduling with cron ⏰

Good admins are lazy in the best way: anything that has to happen regularly (backups, cleanups, reports) gets handed to cron, a service that runs your commands on a schedule. It has been doing this job on Unix since the 1970s.

Two families, one idea

Heads up: the service is cron/crond, but the command you type is crontab. Typing cron -e won't work.

The five time fields

Every cron line is five time fields followed by the command. Read the fields left to right:

That line means “at 02:30, Monday to Friday.” Day of week: 0 or 7 = Sunday, 1 = Monday … 6 = Saturday.

Your own schedule: crontab

crontab -e          # edit your schedule (opens an editor)
crontab -l          # list it
crontab -l > my-crontab.bak    # back it up!
crontab -r          # REMOVE it, instantly, no "are you sure?"
sudo crontab -u alex -l        # root can see anyone's

Never edit the files in /var/spool/cron directly. Always go through crontab -e. It checks your syntax before installing, and it tells cron to reload. If you make a typo, it says something like bad minute and asks if you want to fix it.

crontab -e                   # opens vi: press i, type, Esc, :wq
sudo dnf install nano
EDITOR=nano crontab -e       # or use nano this once

crontab -e                   # first time: pick 1 for nano
select-editor                # change your mind later

System-wide schedules

Jobs for the whole system live in /etc, and only root can edit them. There are three flavors:

1. /etc/crontab and 2. /etc/cron.d/

Same format as your crontab, but with one extra field: which user runs the job:

*/10 * * * *   root   /usr/local/bin/check-disk.sh
 time fields   USER   command

Instead of editing /etc/crontab itself, it's cleaner to drop a small file into /etc/cron.d/. Each app or task gets its own file, and you can remove it later without touching anything else:

echo '*/10 * * * * root date >> /var/log/heartbeat.log' | sudo tee /etc/cron.d/heartbeat

3. The drop-in folders

Don't care about the exact minute? Put an executable script into one of these folders and it runs roughly that often:

/etc/cron.hourly/    /etc/cron.daily/    /etc/cron.weekly/    /etc/cron.monthly/

The script needs chmod +x. On Ubuntu its name also can't contain a dot (the same run-parts rule). Look around: Ubuntu keeps things like logrotate and man-db in cron.daily. On Rocky, anacron runs these folders, so a daily job still happens after the laptop was switched off overnight.

Why isn't my cron job running?! 🐛

The same traps get everyone. Check this list first:

1. Test the command by hand first. If it doesn't work in your terminal, it won't work in cron.
2. Use full paths. Cron runs with a tiny PATH and starts in your home folder. Write /home/student/backup.sh, not backup.sh. which tar tells you where a program lives.
3. Where did the output go? Cron tries to email it. Most servers have no mail set up, so it quietly disappears. Send it to a file instead: … >> /home/student/job.log 2>&1 (2>&1 includes the error messages too, see lesson 11).
4. The % trap. In a crontab, % means “new line.” date +%F breaks, so write date +\%F, or better, put the command in a script.
5. The /bin/sh trap (Ubuntu). Cron runs jobs with /bin/sh, which is dash on Ubuntu but bash on Rocky. Bash-only tricks like <( ) or [[ ]] fail on Ubuntu with Syntax error. Put SHELL=/bin/bash on the first line of your crontab (lesson 12).
6. File names in cron.d / cron.daily: no dots on Ubuntu, and they need the right owner and permissions.
7. Is cron even running? systemctl status crond (Rocky) or systemctl status cron (Ubuntu).
8. Check cron's log. sudo cat /var/log/cron on Rocky, grep CRON /var/log/syslog on Ubuntu. If there's no CMD line, cron never tried to run it.

The modern alternative is systemd timers (systemctl list-timers). Both families use them for built-in chores like package-list refreshes. Cron is simpler to learn, and you'll find it on every Linux server.

Try it

The practice terminal runs a real cron clock. Jobs fire every minute, and a small ⏱ note appears in the title bar when they do. Don't want to wait? The playground-only timewarp 10m command jumps the clock forward and runs every job that would have been due.

Quick check