# cron and crontab

> How cron runs scheduled jobs, the five crontab time fields, where system cron files live and how systemd timers compare.

cron is the classic Unix daemon that runs commands on a schedule. Each user has a crontab, edited with crontab -e, in which every line has five time fields (minute, hour, day of month, month, day of week) followed by the command. System jobs live in /etc/crontab and /etc/cron.d/.

## The time fields

'30 2 * * *' means 02:30 every day; '*/10 * * * *' every ten minutes; '0 9 * * 1' 09:00 every Monday. Day of week 0 and 7 are Sunday.

## System files

Files in /etc/cron.d/ and /etc/crontab have an extra field, the user name, between the time and the command. Directories such as /etc/cron.daily hold scripts run once a day.

## Common pitfalls

cron runs with a minimal environment: use full paths or set PATH in the crontab. Output is mailed to the owner or lost; redirect it to a log file. The % character has a special meaning in crontab lines and must be escaped as \%.

## systemd timers

On systemd distributions, .timer units are an alternative with logging in the journal, randomised delays and catch-up of missed runs (Persistent=true).

## How to add a daily job

1. Open your crontab: crontab -e
2. Add a line, for example: 30 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1
3. Save and exit; cron picks up the change automatically.
4. List the jobs: crontab -l

## Cautions

- 'crontab -r' deletes the whole crontab without asking.
- Test the command by hand with the same user before scheduling it.

## Sources

- [crontab(5)](https://man7.org/linux/man-pages/man5/crontab.5.html)
- [crontab(5) (Debian)](https://manpages.debian.org/bookworm/cron/crontab.5.en.html)
- [systemd.timer(5)](https://man7.org/linux/man-pages/man5/systemd.timer.5.html)

Last updated: 2026-10-08
URL: https://raincleaner.eu/wiki/linux/cron