Crontab Cheatsheet

Crontab (cron table) is a Unix-based job scheduling program that allows users to schedule jobs (commands or scripts) to run periodically at fixed times, dates, or intervals.

Format of a Cron Job

A cron job is defined by a line in the crontab, with the format:

* * * * * command-to-be-executed
- - - - -
| | | | | 
| | | | +---- Day of the week (0 - 7) [Both 0 and 7 represent Sunday]
| | | +------ Month (1 - 12)
| | +-------- Day of the month (1 - 31)
| +---------- Hour (0 - 23)
+------------ Minute (0 - 59)

Common Operators

• *: represents every possible value for the field.
• -: represents a range of values.
• ,: specifies a list of values.
• /: specifies increments.

Examples

1. Run my-command at 5:30 every morning:

30 5 * * * my-command

2. Run my-command every 15 minutes:

*/15 * * * * my-command

3. Run my-command at 2:30pm on the first day of every month:

30 14 1 * * my-command

4. Run my-command at 10 pm from Monday to Friday:

0 22 * * 1-5 my-command

5. Run my-command every month in January, March, and May at 3:15pm:

15 15 * 1,3,5 * my-command

Crontab Commands

• crontab -e: Edit your crontab.
• crontab -l: List your crontab.
• crontab -r: Remove your crontab.
• crontab -u <username> -e: Edit crontab for another user (requires appropriate permissions).

Special Strings

These strings represent special schedules:

• @reboot: Run once at startup.
• @yearly or @annually: Run once a year, i.e., "0 0 1 1 *".
• @monthly: Run once a month, i.e., "0 0 1 * *".
• @weekly: Run once a week, i.e., "0 0 * * 0".
• @daily or @midnight: Run once a day, i.e., "0 0 * * *".
• @hourly: Run once an hour, i.e., "0 * * * *".

For example, to run my-command at startup:

@reboot my-command

Tips

• Ensure that any scripts or commands being called by your cron jobs have absolute paths to avoid confusion.
• If you're storing output or logs, it's also wise to use absolute paths.
• It can be helpful to redirect the output of your cron jobs to a log for debugging purposes. E.g., 30 5 * * * my-command >> /path/to/logfile 2>&1.