Cron is named after “chronos” the Greek word for time and it is a daemon that runs scheduled jobs and scripts. Crontab or ‘Cron table’ is a file which contains all the jobs details with their schedule time of run.
Any userid which has an entry in the /usr/lib/cron/cron.allow file can run the Crontab. The same user id can run the Crontab if this cron.allow file does not exit and also if this userid does not exist in /usr/lib/cron/cron.deny file. If the cron.deny file has no entry in it, all userid can use the Crontab.
The cron daemon continuously checks every minute for /etc/crontab file and /var/spool/cron/ directory and /etc/cron.* directories. Updates to these files are loaded to the memory by the cron daemon. In case cron daemon is not running for a newly build OS, root user can start the cron service using /sbin/service/crond start. To stop the service, use the command /sbin/service/crond stop
As per their schedules, the scripts will get executed in /etc/cron.hourly/, /etc/cron.daily/, /etc/cron.weekly/, and /etc/cron.monthly/ directories on an hourly, daily, weekly or monthly basis respectively. Except these schedule any other scripts can be run on /etc/cron.d/ directory.
/var/spool/cron/ directory contains user defined crontab files and these files are executed with same userid which are created it.
- Crontab -r – deletes the crontab file
- Crontab -v – shows the time when you last edited the file
- Crontab -l – view the content of the crontab file
- Crontab -e – opens the crontab file in edit mode
- Crontab -u username -l – to view crontab entries of other users.
- Crontab -u username -e – to view crontab entries of other users.
- Crontab -l > crontab.current – to copy all the content of latest cron entries to a text file. This crontab.current file can be edited in vi editor mode
- Crontab crontab.current – To load the updated content to cron memory.
To specify day, date and time each crontab file follows this standard format.
* * * * * /path/script_name to run
| | | | |
| | | | L—– day of week [0 – 6] [Sunday=0]
| | | L——- month [1 – 12]
| | L——— day of month [1 – 31]
| L———– hour [0 – 23]
L————- min [0 – 59]
Different ways of scheduling the cron job
* * * * * /path/Script_name — To run every minute
0 * * * * /path/Script_name — To run every hour
15 2 * * * /path/Script_name — To run at 2:15 AM
5 20 * * * /path/Script_name — To run at 8:05 PM
0 11 2,6 * * /path/Script_name — To run at 11 am of 2nd and 6th of every month
0 0 * * * /path/Script_name — To run daily at mid night
* * 5,6,7 * * /path/Script_name — To run every minute on May, June and July
30 4 3 2 * /path/Script_name — To run every year 3rd Feb at 4:30 AM
* * * * * /path/Script_name; /path/2nd_Script_name — To run 2 tasks on a single cron [use semicolon]
@yearly /path/Script_name — To run on first minute of every year
@monthly /path/Script_name — To run on first minute of every month
@weekly /path/Script_name — To run on first minute of every week
@daily /path/Script_name — To run on first minute of every day
@hourly /path/Script_name — To run on first minute of every hour
@reboot /path/Script_name — To run after reboot on system start up
46 0-23/2 * * * /path/Script_name — To run at 46 minutes starting at 2 am, 4am, 6am etc