crontab — schedule recurring jobs

What it does

crontab is the control panel for cron, the daemon that runs commands on a schedule. Each user owns a crontab file: a list of lines, one per job, each with five time fields (minute, hour, day-of-month, month, day-of-week) followed by the command to run. You edit it with `crontab -e`, list it with `crontab -l`, and remove it with `crontab -r`. The daemon checks these files every minute and fires anything that matches. It is how log rotation, backups, certificate renewal, and cleanups keep running without anyone touching them.

Why it matters

Any task that should happen on a schedule — nightly backups, clearing /tmp, renewing a TLS cert, a daily report — is a cron job. The value is reliability: once the line is in, it runs every time, whether or not you remember. The gotcha is that cron does not load your shell profile: it runs with a bare environment (limited PATH, no your aliases, no your exports), which is why jobs that work when you type them fail mysteriously in cron. Understanding that one mismatch saves hours of debugging.

Shortcuts, dry runs, and system crontabs

Two conveniences are worth knowing. First, the @ shorthands: instead of `0 0 * * *` write @daily (or @midnight), instead of `0 0 1 1 *` write @yearly (or @annually), and @hourly, @weekly, @monthly and @reboot work the same way. @reboot fires when the cron daemon starts, which can be before the network or other services are up, so do not put anything in it that needs the network. Second, `crontab -n file` checks syntax without installing: it prints 'The syntax of the crontab file was successfully checked.' or refuses with the offending line, which is the safe way to test a schedule a script generated. Both forms are also worth knowing because cron is not the only scheduler on the box: /etc/crontab and /etc/cron.d/ hold system-wide jobs whose lines carry a sixth field naming the user to run as, and /etc/cron.allow and /etc/cron.deny decide who may use crontab at all.

Example

$ crontab -l

# m h  dom mon dow   command
PATH=/usr/local/bin:/usr/bin:/bin
HOME=/home/kmail

# Nightly backup at 02:30
30 2 * * *  /home/kmail/scripts/backup.sh >> /var/log/backup.log 2>&1

$ echo '0 9 * * 1-5  /home/kmail/scripts/daily-report.sh' | crontab -

$ crontab -l | tail -2
0 9 * * 1-5  /home/kmail/scripts/daily-report.sh

$ crontab -e

# opens in your $EDITOR; add a line and save
# This job clears the temp dir at 04:00 every Sunday:
0 4 * * 0  /usr/bin/find /tmp/kmail -type f -mtime +2 -delete

$ printf '0 3 * * /bin/true\n' | crontab -n -

"-":0: bad day-of-week
errors in crontab file, can't install.

Common flags

-e

edit the current user's crontab in $EDITOR

-l

list the current user's crontab to stdout

-r

remove (delete) the current user's crontab entirely

-i

interactive — prompt before deleting when used with -r

-n

dry run — check a crontab's syntax without installing it

file | -

install a crontab from a file, or from stdin with `crontab -`

-u user

target another user's crontab (root only; e.g. crontab -u www-data -l)

*

wildcard in a time field matches every value (e.g. `* * * * *` = every minute)