How To Schedule Background Jobs Using Crontab

The cron daemon is a slap-up user tool to automate tasks that don't require human intervention. Users pre-specify jobs to run inwards the background at item times, for example, every Monday, Midweek in addition to Fri at 2am.

To purpose cron, each user creates his ain crontab ('cron table') file. The ascendancy to examine one's crontab file is crontab -l.

$ crontab -l
MAILTO=commandolinux@gmail.com
0 ii * * 1,3,5 /home/peter/backups/backupWP.sh 2>&1

The MAILTO delineate specifies the e-mail address to which cron sends the output of ascendancy execution. Please holler to my before post on how to ready an SMTP server to forwards your emails.

The minute crontab delineate specifies that the backupWP.sh script should last executed at 2am every Monday, Midweek in addition to Friday. The syntax may expect complicated. Fortunately, yous tin purpose the on-line Crontab Generator to arts and crafts the crontab statements. If yous desire to larn the syntax, click here instead.

Create crontab

Your crontab file is initially empty. To exercise the file from scratch, run the crontab ascendancy in addition to type inwards the crontab statements.

$ crontab

Alternatively, pose the statements into a temporary file, say /tmp/cron, in addition to run this command:

$ truthful cat /tmp/cron | crontab -

Edit crontab

If yous desire to modify crontab contents afterwards they are created, run this command:

$ crontab -e

The ascendancy opens the crontab file inwards your default text editor. It is the well-nigh versatile means to modify crontab. You tin purpose it to create, modify, in addition to delete crontab statements. Don't forget to salvage the file afterwards yous complete editing.

The downside for this edit ascendancy is the fourth dimension in addition to overhead of starting the text editor. You tin append a novel declaration straight past times using the ascendancy inwards the adjacent section.

Add to crontab

When I was novel to crontab, I made the error of trying to append a declaration past times running crontab without whatsoever argument. That truly replaced everything inwards the crontab file amongst the novel input.

The right ascendancy to append a novel declaration is:

$ (crontab -l; echo "30 04 * * iv /home/peter/backups/report.sh 2>&1") | crontab -

The fob is to run ii commands inwards a subshell grouped past times the circular brackets. The get-go command, crontab -l, fetches the existing crontab statements. The echo ascendancy echoes the novel declaration to last appended. The collective output from both commands are piped to crontab criterion input.

Empty crontab

To erase all crontab contents, execute the next command:

$ crontab -r

Conclusion

You may purpose crontab to schedule regular maintenance in addition to backup tasks. Once it is laid up, the crontab file tends to last static. But, if yous always demand to add together roughly other task, or alter the scheduled times, the commands introduced inwards this post volition come upwards inwards handy.

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