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 what time are explained here:

Finally, to save the jobs, use the following:

wq # save and quite crontab job

The following are a few examples of the crontab command:

• Use the following command to run a script every 5 minutes, 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, send the e-mail to <ganesh@abc.com>:

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

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

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

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

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

The following are a few more crontab command examples:

We can add macros in the crontab file. Use the following to restart my_program after each reboot:

@reboot /bin/my_program
@reboot echo `hostname` was rebooted at `date` | mail -s "Reboot notification" ganesh.admin@some-corp.com

The following is the summary of a few more macros: