Editing Linux 101 hacks (section)

=='''Hack 74. Crontab'''==


Using cron you can execute a shell-script or Linux commands at a specific time and date. For example a sysadmin can schedule a backup job that can run every day.


'''How to add a job to the cron?'''


<nowiki># </nowiki>crontab –e


0 5 <nowiki>* </nowiki><nowiki>* </nowiki><nowiki>* </nowiki>/root/bin/backup.sh

This will execute /root/bin/backup.sh at 5 a.m every day.


'''Description of Cron fields.'''


Following is the format of the crontab file.


{minute} {hour} {day-of-month} {month} {day-of-week} {full-path-to-shell-script}


o minute: Allowed range 0 – 59 o hour: Allowed range 0 – 23


o day-of-month: Allowed range 0 – 31


o month: Allowed range 1 – 12. 1 = January. 12 = December. o Day-of-week: Allowed range 0 – 7. Sunday is either 0 or 7.


'''Crontab examples'''


# Run at 12:01 a.m. 1 minute after midnight everyday. This is a good time to run backup when the system is not under load. 

# 
## 0 <nowiki>* </nowiki><nowiki>* </nowiki><nowiki>* </nowiki>/root/bin/backup.sh 

# Run backup every weekday (Mon – Fri) at 11:59 p.m. 

# 
## 11 <nowiki>* </nowiki><nowiki>* </nowiki>1,2,3,4,5 /root/bin/backup.sh 

Following will also do the same.


59 11 <nowiki>* </nowiki><nowiki>* </nowiki>1-5 /root/bin/backup.sh


# Execute the command every 5 minutes. 

<nowiki>*/5 </nowiki><nowiki>* </nowiki><nowiki>* </nowiki><nowiki>* </nowiki><nowiki>* </nowiki>/root/bin/check-status.sh 


# Execute at 1:10 p.m on 1st of every month 

# 
## 13 1 <nowiki>* </nowiki><nowiki>* </nowiki>/root/bin/full-backup.sh 

# Execute 11 p.m on weekdays. 

# 
## 23 <nowiki>* </nowiki><nowiki>* </nowiki>1-5 /root/bin/incremental-backup.sh 

'''Crontab Options '''


Following are the available options with crontab:


# crontab –e : Edit the crontab file. This will create a crontab, if it doesn’t exist 

o crontab –l : Display the crontab file. o crontab -r : Remove the crontab file.


o crontab -ir : This will prompt user before deleting a crontab.