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Crontab

Written by config on . Posted in CentOS, Linux, Ubuntu

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What is Crontab ?
One can enter the list of commands that he wishes to execute at a particular time in future, or they can be scheduled to execute on a daily basis. Crontab is a general text file which holds this list. The commands entered in crontab and the times to execute them is handled by cron daemon. It runs in background. Crontab’s man page explains about itself pretty well. Later we have lined up a crontab example for better understanding.

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Working with Crontab

A crontab file is maintained for each user by system. For creating and editing a crontab file, one can use text editor that a system specifies. Most linux distros officially comes with vi as default text editor. The text editor is started by executing the crontab command with -e option. One must type following command in terminal to get started with crontab:

#crontab -e

It will invoke vi text editor with a black window. The desired schedules and/for commands go there. Crontab commands are separated by line space, that is, one ‘cron-job’ per line. Every cron command is called a cron-job. Sections are separated buy a space and final section having one or more than one space. There cannot be spaces within sections. The schedule goes in the sections 1-5.

How do we lay down a cron job ?

minute[0-59], hour[0-23, 0= midnight],
day[1-31],month[1-12],weekday[0-6, 0=Sunday],

command

01 04 1 1 1 /usr/bin/Directory/Command

The above command will execute “Command” at 04:01AM on any Monday which falls on January 1st. If we want to use every instance of the specified time period, we can use asterisk(*). It will now be repeated at every hour, week, month, etc..

08 06 * * * /usr/bin/TheDirectory/ACommand

Above instruction will execute /usr/bin/TheDirectory/ACommand at 6:08 every day for uninterruptedly every month.

If one wishes to execute multiple instances of some command in a particular time period, values can be separated by comma and that should do. If separated via dash, it will run them continuously.

02,15 06,07 1-15 1,6 * /usr/bin/MyDirectory/MyCommand

Above instruction will be executed when minute hand strikes 02 and 15 after hour hand reaches 6:00 AM and 7:00 AM on 1st,all the way to 15 of January and June, every year.

It is always recommended that the full path of desired commands are used, as we used in above examples. Once cron file is properly edited and saved, it will work like a charm.

We have a -l option which displays the current crontab on standard output. While option -r can remove the current crontab. To edit the current crontab, option -e is used which opens appropriate editor. Upon exit from editor, all the changes made to crontab will be checked for errors and accuracy, if error free, changes will be installed automatically.

One more noteworthy technique, a double-ampersand can be inserted between commands in an event of multiple commands. It executes mentioned commands consecutively.

15 03 * * * /usr/bin/command1 && /usr/bin/command2

This will execute command1 and command 2 at 03:15 daily.

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