Syntax
Linux crontab has six fields. 1-5 fields defines the date and time of execution. The 6’th fields are used for command or script to be executed.The Linux crontab syntax are as following:
[Minute] [hour] [Day_of_the_Month] [Month_of_the_Year] [Day_of_the_Week] [command]
- Astrics (*) – Matches anything
- Define range – You can define range using the hypen like: 1-10 or 20-30 or sun-fri or feb-apr
- Define multiple range – You can define multiple ranges with command seprated like: jan-mar,jul-sep
How to Add/Edit Crontab
To add or update job in crontab, use below command. It will open crontab file in the editor where a job can be added/updated.
crontab -e
By default, it will edit crontab entries of current logged in user. To edit other user crontab use command:
crontab -u username -e
To view crontab entries of current user:
crontab -l
Use -u followed by username to view crontab entries of the specified user.
crontab -u username -l
20 Useful Crontab Examples
1. Schedule a cron to execute at 2am daily.
0 2 * * * /bin/sh script.sh
2. Schedule a cron to execute twice a day.
Below example command will execute at 5 AM and 5 PM daily. You can specify multiple time stamp by comma separated.
0 5,17 * * * /scripts/script.sh
3. Schedule a cron to execute on every minutes.
Generally, we don’t require any script to execute on every minute but in some case, you may need to configure it.
* * * * * /scripts/script.sh
4. Schedule a cron to execute on every Sunday at 5 PM.
0 17 * * sun /scripts/script.sh
5. Schedule a cron to execute on every 10 minutes.
*/10 * * * * /scripts/monitor.sh
6. Schedule a cron to execute on selected months.
* * * jan,may,aug * /script/script.sh
7. Schedule a cron to execute on selected days.
0 17 * * sun,fri /script/script.sh
8. Schedule a cron to execute on first sunday of every month.
To schedule a script to execute a script on first Sunday only is not possible by time parameter, But we can use the condition in command fields to do it.
0 2 * * sun [ $(date +%d) -le 07 ] && /script/script.sh
9. Schedule a cron to execute on every four hours.
0 */4 * * * /scripts/script.sh
10. Schedule a cron to execute twice on every Sunday and Monday.
To schedule a task to execute twice on Sunday and Monday only.
0 4,17 * * sun,mon /scripts/script.sh
11. Schedule a cron to execute on every 30 Seconds.
To schedule a task to execute on every 30 seconds is not possible by time parameters, But it can be done by schedule same cron twice like below.
* * * * * /scripts/script.sh
* * * * * sleep 30; /scripts/script.sh
12. Schedule a multiple tasks in single cron.
To configure multiple tasks with single cron, Can be done by separating tasks by the semicolon ( ; ).
* * * * * /scripts/script.sh; /scripts/scrit2.sh
13. Schedule tasks to execute on yearly ( @yearly ).
@yearly timestamp is similar to “0 0 1 1 *”. It will execute task on the first minute of every year.
@yearly /scripts/script.sh
14. Schedule tasks to execute on monthly ( @monthly ).
@monthly timestamp is similar to “0 0 1 * *”. It will execute a task in the first minute of the month.
@monthly /scripts/script.sh
15. Schedule tasks to execute on Weekly ( @weekly ).
@weekly timestamp is similar to “0 0 1 * mon”. It will execute a task in the first minute of the week.
@weekly /bin/script.sh
16. Schedule tasks to execute on daily ( @daily ).
@daily timestamp is similar to “0 0 * * *”. It will execute a task in the first minute of every day.
@daily /scripts/script.sh
17. Schedule tasks to execute on hourly ( @hourly ).
@hourly timestamp is similar to “0 * * * *”. It will execute a task in the first minute of every hour.
@hourly /scripts/script.sh
18. Schedule tasks to execute on system reboot ( @reboot ).
@reboot is useful for those tasks which you want to run on your system startup. It will be same as system startup scripts.
@reboot /scripts/script.sh
19. Redirect Cron Results to specified email account.
By default, cron sends details to the current user where cron is scheduled. If you want to redirect it to your other account, can be done by setup MAIL variable like below
# crontab -l
MAIL=bob
0 2 * * * /script/backup.sh
20. Taking backup of all crons to plain text file.
I recommend keeping a backup of all jobs entry in a file. This will help you to recover crons in case of accidental deletion.
Check current scheduled cron:
# crontab -l
MAIL=rahul
0 2 * * * /script/backup.sh
Backup cron to text file:
# crontab -l > cron-backup.txt
# cat cron-backup.txt
MAIL=rahul
0 2 * * * /script/backup.sh
Removing current scheduled cron:
# crontab -r
# crontab -l
no crontab for root
Restore crons from text file:
# crontab cron-backup.txt
# crontab -l
MAIL=rahul
0 2 * * * /script/backup.sh