How to Create a Cron Job with crontab

Cron runs commands automatically on a schedule. Each user has their own list of jobs, called a crontab.

The original notes

Allowed values for each field

The order of the fields

Minute - Hour - Day of Month - Month - Day of Week

* * * * * Command goes here

A * means “every” value for that field, so * * * * * runs the command every minute.

How to find this table on your own system

info crontab | grep -A 6 -i 'allowed values'

This searches the crontab documentation for the “allowed values” table and prints the six lines after the match. The table lives in the crontab file-format manual page, so if the command above prints nothing on your system, open that page directly:

man 5 crontab

Notes for today

Each field explained

1. Minute (0-59): the minute past the hour.
2. Hour (0-23): 24-hour clock, so 0 is midnight and 14 is 2pm.
3. Day of month (1-31).
4. Month (1-12): you can also use the first three letters of the name, such as jan or dec.
5. Day of week (0-7): 0 and 7 are both Sunday, 1 is Monday. Names such as mon or fri also work.

Special characters you can use in any field:

• * every value
• , a list, for example 1,15 (the 1st and 15th)
• - a range, for example 1-5 (Monday to Friday in the day-of-week field)
• / a step, for example */15 in the minute field (every 15 minutes)

Add, list and remove jobs

Edit your crontab (it opens in your default editor, often vi):

crontab -e

Add one job per line, save and quit. Cron picks up the change automatically; you do not need to restart anything.

List your current jobs:

crontab -l

Be careful with crontab -r: it deletes your whole crontab without asking. The r key sits next to e, so it is easy to type by mistake.

Jobs that need root (for example changing system settings) belong in root’s crontab. Use sudo crontab -e, or crontab -e when logged in as root.

Examples

Run a backup script every day at 02:30:

30 2 * * * /home/user/backup.sh

Run a script every 15 minutes:

*/15 * * * * /home/user/check.sh

Run at 09:00 Monday to Friday:

0 9 * * 1-5 /home/user/report.sh

Run at midnight on the first day of every month:

0 0 1 * * /home/user/monthly.sh

The script paths above are only examples; use the full path to your own script.

Check it worked

• Run crontab -l to confirm the line was saved.

• Make the job write to a log so you can see that it ran:

  */5 * * * * /home/user/check.sh >> /tmp/check.log 2>&1

  >> appends normal output to the file and 2>&1 sends errors to the same place.

Common mistakes

• Relying on your login environment. Cron runs with a very small PATH and does not load your shell settings. Use full paths to commands and scripts, for example /usr/bin/php rather than php. Run which php to find the full path.
• Script not executable. Make it executable with chmod +x script.sh, or call it through the interpreter, for example /bin/bash /home/user/backup.sh.
• The % character. In a crontab line, % is treated as a newline. Escape it as \%, which matters for commands such as date +%Y-%m-%d.
• Day of month and day of week together. If both fields are set (neither is *), the job runs when either one matches, not only when both match.
• Timezone. Cron uses the server’s local time. Check it with date.