Understanding crontab

If we need to run a specific task repetitively, then the solution is to use crontab. The syntax of the command is as follows:

    $ crontab -e

This will open a new editor. The following diagram is the syntax to add tasks. The fields to use for repeating tasks at a particular time are explained here:

Finally, to save the jobs, use the following:

    Press Esc then type :wq
    

The preceding operations will save the job and quit crontab.

The following are a few examples of the crontab command:

• Use the following command to run a script every hour at the fifth minute, every day:

    5 * * * *      $HOME/bin/daily.job >> $HOME/tmp/out  2>&1

• Use the following command to run 5 minutes after midnight every day:

    5 0 * * *      $HOME/bin/daily.job >> $HOME/tmp/out  2>&1

• Use the following command to run at 2.15 p.m. on the first of every month–the output is mailed to Paul:

    15 14 1 * * *     $HOME/bin/monthly

• Use the following command to run at 10 P.M. on weekdays, and send the email to ganesh@abc.com:

    0 22 * *  1-5   sendmail ganesh@abc.com  < ~/work/email.txt

• The sendmail utility is used for sending emails. We can also use the mail utility as follows:

    sendmail user@example.com  < /tmp/email.txt

• The following commands are self-explanatory from the text of the echo command:

    23 0-23/2  *  *  *  echo "run 23 minutes after midn, 2 am, 4 am, everyday"
    5  4  *  *  sun    echo "run at 5 minutes after 4 am every Sunday"

The following are a few more crontab command examples: