Crontab With Example

Video crontab- masteringlinux.com

Cron– It is nothing but a job scheduler. Or,

Cron is a unix, solaris utility that allows tasks to be automatically run in the background at regular intervals by the cron daemon.

Crontab (CRON Table) is a file which contains the schedule of cron entries to be run and at specified times.

Crontab Commands

export EDITOR=vi, to specify a editor to open crontab file.

crontab -e Edit your crontab file, or create one if it doesn’t already exist.
crontab -l Display your crontab file.
crontab -r Remove your crontab file.
crontab -v Display the last time you edited your crontab file. (This option is only available on a few systems.)

Crontab Syntax
A crontab file has five fields for specifying day, date and time followed by the command to be run at that interval.
* * * * * 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)

 

Examples

1. Run the script every day at 12:00.

0 12 * * * /home/your_username/run-me.sh >/dev/null 2>&1

2. Run the script every day at 12:00, 14:00 and 16:00.

0 12,14,16 * * * /home/your_username/run-me.sh >/dev/null 2>&1

3. Run the script every Sunday at 13:30.

30 13 * * 0 /home/your_username/run-me.sh >/dev/null 2>&1

4. Run the script every Saturday at 12:00, 14:00 and 16:00.

0 12,14,16 * * 6 /home/your_username/run-me.sh >/dev/null 2>&1

5. Run the script on the first, fifteenth and twentieth of every month.

0 0 1,15,20 * * /home/your_username/run-me.sh >/dev/null 2>&1

6. Run the script every day from 3 to 3 hours: 00:00, 03:00, 06:00 etc.

0 */3 * * * /home/your_username/run-me.sh >/dev/null 2>&1

7. A line in crontab file like below removes the tmp files from /home/someuser/tmp each day at 6:30 PM.
30 18 * * * rm /home/someuser/tmp/*
8. This runs every fifteen minutes
*/15 * * * * /notesbit/work/scripts/crons/denyhack1 > /dev/null 2>&1
*/20 * * * * /notesbit/work/scripts/crons/denyhack2 > /dev/null 2>&1
9. This runs every twenty five minutes
*/25 * * * * /notesbit/work/scripts/crons/denyhack3 > /dev/null 2>&1
*/35 * * * * /notesbit/work/scripts/crons/denyhack4 > /dev/null 2>&1
10.  This runs every day at 2:14 PM
08 14 * * * /root/work/scripts/crons/stkinc NUAN > /dev/null 2>&1

 

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