Here’s a cheat sheet for using Cron, a time-based job scheduler in Unix-like operating systems:
Cron Basics
- Access the Cron Table:
crontab -e: Edit the current user’s crontab file.
- List Cron Jobs:
crontab -l: Display the current user’s crontab entries.
- Remove All Cron Jobs:
crontab -r: Remove all crontab entries.
Cron Syntax
Cron Format:
┌───────────── minute (0 - 59)
│ ┌───────────── hour (0 - 23)
│ │ ┌───────────── day of month (1 - 31)
│ │ │ ┌───────────── month (1 - 12)
│ │ │ │ ┌───────────── day of week (0 - 6, Sunday = 0 or 7)
│ │ │ │ │
│ │ │ │ │
* * * * *Wildcard Characters:
*: Matches any value.,: Specifies a list of values.-: Specifies a range of values./: Specifies a step value.
Examples
- Run a Job Every Hour:
0 * * * * command
- Run a Job Every Day at Midnight:
0 0 * * * command
- Run a Job Every Monday at 2:30 PM:
30 14 * * 1 command
- Run a Job Every 15 Minutes:
*/15 * * * * command
- Run a Job Every Weekday at 8:00 AM:
0 8 * * 1-5 command
Special Strings
- @reboot:
- Run the job once after the system reboots.
- @yearly or @annually:
- Run the job once a year (0 0 1 1 *).
- @monthly:
- Run the job once a month (0 0 1 * *).
- @weekly:
- Run the job once a week (0 0 * * 0).
- @daily or @midnight:
- Run the job once a day (0 0 * * *).
- @hourly:
- Run the job once an hour (0 * * * *).
Miscellaneous
- Redirect Output:
0 * * * * command > /path/to/output.log 2>&1: Redirect both standard output and standard error to a file.
- Logging Output to Syslog:
0 * * * * command 2>&1 | /usr/bin/logger -t cronjob_tag: Log output to syslog.
- Environment Variables:
SHELL=/bin/bashPATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin[email protected]
- Prevent Email Notifications:
0 * * * * command > /dev/null 2>&1: Suppress email notifications.
This cheat sheet provides a quick reference for creating and managing Cron jobs in Unix-like systems. Adjust the values based on your specific scheduling needs.