Linux Automation [at, Cron]

Understanding “at”

Many a time, we need to schedule a task for a future time, say in the evening at 8 p.m. on a specific day. We can use theatcommand in such a situation.

Sometimes, we need to repeat the same task at a specific time, periodically, every day, or every month. In such situations, we can use thecrontabcommand.

Let’s learn more about theuseof theatcommand. To use theatcommand, the syntax is as follows:

$ at time date

The following are examples of the at command:

TheCtrl+Dcommand will save theatjob. The task will be executed at 11.15 A.M. This command will log messages to thelog.txtfile at 11.15 a.m.:

$ at 11.15 AMat >  echo "Hello World" > $HOME/log.txtat >  Control + D

The following command will send an email on March 31, 2015, at 10 A.M.:

$ at 10am mar 31 2015at> echo "taxes due" | mail jonat> ^D

The followingcommandwill make the task run on May 20 at 11 A.M.:

$ at 11 am may 20

All the jobs that are scheduled by theatcommand can be listed using the following command:

$ atq

To remove a specific job listed by theatqcommand, we can use the following command:

$ atrm  job-id

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 followingdiagramis 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 thecrontabcommand:

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 totest@example.com:

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

Thesendmailutility 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 theechocommand:

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:

Min

Hour

Day / month

Month

Day / week

Execution time

1,6,12

00:45 hrs on the fifth day of January, June, and December.

1-5

00 P.M. every weekday (Monday-Friday), only in October.

1,10,15

Midnight on the first, tenth, and fifteenth days of the month.

5,10

At 12.05 and 12.10 every Monday, and on the tenth day of every month.

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" admin@example.com

The following is a summary of a few more macros:

Entry

Description

Equivalent To

@reboot

Run once at start-up

None

@weekly

Run once a week

0 0 * * 0

@daily

Run once a day

0 0 * * *

@midnight

(same as @daily)

0 0 * * *

@hourly

Run once an hour

0 * * * *