A cronjob is a linux scheduler utility
which is part of “crond” linux service daemon used to schedule execution
of configured tasks. There are two methods to configure the
task & time of execution
- Using the system cronjob file edit command with option –e i.e, “crontab –e”
- Using a flat file in which the list of tasks and time of execution are saved and passed as argument to the cronjob comand i.e “crontab <filename>”
Syntax
of a cronjob entry in a cronjob configuration file
<minute> <hour>
<day of month> <month> <day of week> <command to execute>
|
Field
|
Allowed Values
|
|
Minute
|
0
to 59
|
|
Hour
|
0
to 23
|
|
Day
of Month
|
1
to 31
|
|
Month
|
1
to 12 or Names first 3 characters i.e, Jan , Feb .. Dec allowed
|
|
Day
of Week
|
0
to 7 or Names first 3 characters i.e,
Sun , Mon .. Sat allowed
|
Fields
support value ranges , lists and step values as shown in below examples
|
Cronjob schedule value
samples
|
Description
|
|
5
0 * * *
|
Run
at 5th minute after midnight
|
|
15
14 1 * *
|
Run
at 14:15 hrs on the 1st of every month
|
|
0
22 * * 1-5
|
Run
at 10 PM on weekdays
|
|
5
4 * * sun
|
Run
at 4.05 AM every Sunday
|
|
30
02 10 1,2,3 *
|
Run
on Jan,Feb,Mar 10th at 02.30 AM
|
|
*/5
* * * *
|
Run
every 5 minutes
|
|
0
* * * *
|
Run
once a hour at the beginning of the hour
|
|
*
* * * *
|
Run
every minute
|
Verify the contents of cronjob file . Below file has 2 tasks configured
$ cat /opt/shareolite/cronjobs
*/2 * * * * sh Shareolite.sh 2>&1
00 02 * * * sh /opt/shareolite/2.sh >> /opt/shareolite.log 2>&1
Initialize cronjob file using the below command
$ crontab /opt/shareolite/cronjobs
Check the cronjob status by executing below command
$ crontab -l
*/2 * * * * sh Shareolite.sh 2>&1
00 02 * * * sh /opt/shareolite/2.sh >> /opt/shareolite.log 2>&1
Remove
the cronjob by executing below command
$ crontab –r
For
a linux user , only one cronjob can be initialised. Tasks requiring root user
privileges are usually configured under root user. On OS reboot , tasks are
reinitialised automatically and manual initialisation is not required.
