Crontabs Demystified

Crontabs are a great way of scheduling tasks. The crontab command itself is pretty straight forward. In its basic usage, you can do:

  • crontab -l – List existing crontabs
  • crontab -e – Edit crontabs
  • crontab -r – Remove all crontabs
  • crontab -i – Same as -r, but with a warning prompt

Notice how e and r are a next to each other on the keyboard? Maybe a bit too close to each other for comfort?

I’ve lost count of the number of times I’ve accidentally removed my crontab when I wanted to edit it. Worse still, there’s no way to restore them without resorting to a backup.

Here are a couple of ways to guard against accidental deletion.

Automated Backups

The first approach is a simple solution. Add a line like the last line below:

#minute (0-59),
#|      hour (0-23),
#|      |       day of the month (1-31),
#|      |       |       month of the year (1-12),
#|      |       |       |       day of the week (0-6 with 0=Sunday).
#|      |       |       |       |       commands
 0      *       *       *       *       crontab -l > /path/to/somewhere/mycron.crontab

This writes your cron to a backup file every hour, making it easy to restore if you accidentally wipe it.

If you do delete it, you can reinstall the deleted crontab with the following:

crontab /path/to/somewhere/mycron.crontab

Meta and simple 🙂

Crontab Installation

The second approach is to never edit a crontab directly. Instead, keep the crontab in its own file. This has multiple advantages:

  • Never any risk of accidental deletion
  • You can use version control on it
  • You can manage multiple crontabs and switch between them at will

To take this approach, create a new file with a .crontab extension, and put your crontab there.

Now, each time you update the file, update the system crontab:

crontab mycron.crontab

Simples!

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