IT/Software/System Config/cron: Difference between revisions

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Revision as of 05:34, 25 September 2020

Cron is a "daemon to execute scheduled commands"^[1]

To edit your crontab you can run

crontab -e

The first time you will need to select an editor. We normally use Nano the first choice.

If you make a mistake you can run the select-editor(or sudo select-editor) command to change it.

To edit root privilege crontabs you need.

sudo crontab -e

Cron jobs are in typically in the following format

* * * * * /path/to/script.sh

The time is in the format of

Minute, Hour, Day of the month, Month, Day of the week.

The following line would execute script.sh at minute 0 of the 8th hour, every day, every month and every day of the week.

Or in simpler terms, it runs every day at 8:00 AM.

0 8 * * * /path/to/script.sh

Now let's say we want to run our script every 5 minutes.

We could try doing the following but that would only run the script 5 minutes after every hour.

5 * * * * /path/to/script.sh

In cron we can use the "/" character to indicate step values so to run every 5 minutes we can do the following.

*/5 * * * * /path/to/script.sh

Cron also has several special times that can be used in place of the regular time format.

For the following, the script will be run on the first minute of the selected time frame.

So, for example, to run at minute 0 of every hour we can do the following.

@hourly /path/to/script.sh

We also have one more special time to run scripts at reboot.

crontab guru is a helpful tool for confirming your cron formatting is correct.

@@ Line 5: / Line 5: @@
 To edit your crontab you can run
   crontab -e
+The first time you will need to select an editor.  We normally use Nano the first choice.
+If you make a mistake you can run the select-editor(or sudo select-editor) command to change it.
 To edit root privilege crontabs you need.
   sudo crontab -e