Linux Crontab ReferenceCrontab basics
1. Crontab usage: [user@linux1 ~] crontab --help
crontab: usage: crontab [-u user] file crontab [-u user] [ -e | -l | -r ] (default operation is replace, per 1003.2) -e (edit user's crontab) -l (list user's crontab) -r (delete user's crontab) -i (prompt before deleting user's crontab) -s (selinux context) 2. Edit/View crontab in default editor [user@linux1 ~] crontab -e
3. Crontab syntax * * * * * command to be executed
- - - - - | | | | | | | | | +----- day of week (0 - 6) (Sunday=0) | | | +------- month (1 - 12) | | +--------- day of month (1 - 31) | +----------- hour (0 - 23) +------------- min (0 - 59) 4. Crontab examples 0 * * * * rm /home/user/tmp/* # Will run every hour
0 18 * * * rm /home/user/tmp/* # Will run every day at 6pm 0 18 2 * * rm /home/user/tmp/* # Will run on the 2nd day each month at 6pm 0 18 2 1 * rm /home/user/tmp/* # Will run every jan 2nd at 6pm 45 2 * * 6 /home/user/backup.sh # Will run every saturday at 2:45am 0 23 * * 1-5 /home/user/backup.sh # Will run at 11pm every monday to friday (no sat/sun) 0 12 * * 0,2,4 /home/user/backup.sh # Will run at noon every sun,tues,thurs 0 12 * 1 * /home/user/backup.sh # Will run at noon every day, only in month of january Crontab Tweaks 1. Change default editor for cron [user@linux1 ~] crontab -e This will set the crontab to open in your default editor
[user@linux1 ~] export EDITOR=nano; This will set the crontab to open in nano
[user@linux1 ~] export EDITOR=vi; This will set the crontab to open in vi
2. Check if crond is running [user@linux1 ~] ps aux|grep crond
The first entry shows that your cron daemon is running.
root 701 0.0 0.1 5092 1076 ? Ss 08:23 0:00 crond root 724 0.0 0.0 4044 668 pts/8 S+ 08:24 0:00 grep crond 3. Restart crond if needed. To restart crond, if needed become root, then run crond (no & required) [user@linux1 ~] su
[root@linux1 ~] crond 4. Launch a GUI App from cron 30 8 * * 1-5 export DISPLAY=:0 && pidgin
5. Kill a process by name from cron 30 17 * * 1-5 killall pidgin
6. Disable email 30 18 * * * 7z a /home/user/nightly_backup.zip /home/user/files/ > /dev/null 2>&1
7. Pipe to log 30 18 * * * 7z a /home/user/nightly_backup.zip /home/user/files/ > /home/user/cronlogs/backup_7z.log
8. Append to log 30 18 * * * 7z a /home/user/nightly_backup.zip /home/user/files/ >> /home/user/cronlogs/backup_7z.log 2>&1
9. Comment out a line #30 18 * * * 7z a /home/user/nightly_backup.zip /home/user/files/ >> /home/user/cronlogs/backup_7z.log 2>&1 Tags: linux | Related Articles |
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