CRON JOB | CHEAT SHEET

Cron Jobs

  • Cron – A time based job scheduling service
  • Crontab -A program to create, read, update, and delete your schedules.
  • Use cron to schedule and automate tasks.

A crontab or table is used to specify when commands will be executed by cron. Any line starting with “#” within the editor will be ignored, as It’s for comments.

 *   *   *   *   *   Command
 |    |    |    |    |
 |    |    |    |    + — Day of the week (0-6)
 |    |    |    +   —- Month of the Year (1-12)
 |    |    +    ——–Day of the month (1-31)
 |    +   ————-Hour  (0-23)
 +   —————–Minute (0 – 59)
Example : # Run every monday at 7 AM
 0   7   *   *   1    /opt/sales/bin/weekly-report
 |     |    |    |    |
 |     |    |    |    + — Day of the week (0-6)
 |     |    |    +   —- Month of the Year (1-12)
 |     |    +    ——–Day of the month (1-31)
 |     +   ————-Hour  (0-23)
 +   —————–Minute (0 – 59)

If any output is generated by a cron job, it will be mailed by you.
Example: #Run at 2:00 every day and send the output to a file log

0    2    *    *    *    /root/backupdb > /tmp/db.log 2>&1

If you want to run a command to run every thirty minutes simply use a 0,30 or 0/2 in the minutes tab. If you want to run a command every 5 minutes at the top of an hour, use 0-4 in the minutes tab.

Examples:
0 0 1 1 *                                                     yearly
0 0 1 * *                                                     monthly  
0 0 * * 0                                                     weekly
0 0 * * *                                                     daily
0 * * * *                                                     hourly

*/15 * * * *                                             every 15 mins
0 */6 * * *                                              every 6 hours

@reboot                                                  every reboot

Quickly adding a task to crontab;

echo "0 */6 * * * echo Follow the white rabbit..." | crontab

After the quick add, you will see the following:

root@ubuntu:~# echo "0 */6 * * * echo Follow the White Rabbit..." | crontab
root@ubuntu:~# crontab -l
0 */6 * * * echo Follow the White Rabbit...

Using the Crontab Command:

# crontab file  – Install a new crontab from file
 # crontab -l    – List your cron jobs
 # crontab -e  – Edit your cron jobs, will take you to the editor
 # crontab -r   – Remove all of your cron jobs

For additional information, review the man page or use crontab –help

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