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bmacri committed Dec 7, 2012
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Expand Up @@ -6,17 +6,17 @@ Laziness is considered a virtue in system administration, meaning you should fig
The ``crontab`` command
=======================

The easiest way To see jobs currently running regularly on your system ("cron jobs") is to type:
The easiest way To see jobs currently running regularly on your system ("cron jobs") is to type: ::

``crontab -l``
$ crontab -l

in the shell. This will show all the cron jobs that currently run on the current user's system. For example, if you are logged in as 'jdoe', the current user is 'jdoe', and if there are no cron jobs running, the output will be something like:
in the shell. This will show all the cron jobs that currently run on the current user's system. For example, if you are logged in as 'jdoe', the current user is 'jdoe', and if there are no cron jobs running, the output will be something like: ::

``no crontab for jdoe``
-bash: crontab: no crontab for jdoe

If there are jobs running, there will be a list of lines that looks something like this:
If there are jobs running, there will be a list of lines that looks something like this: ::

``05 12 * * 0 /home/jdoe /home/jdoe/jobs/copy-to-partition``
$ 05 12 * * 0 /home/jdoe /home/jdoe/jobs/copy-to-partition

Let's dissect this a bit, as it will help when you're creating your own crob jobs. What is this output telling you? It is helpful to know that the fields of a cron job are as follows:

Expand All @@ -30,15 +30,15 @@ Note that order matters. Knowing this, we can see that the output of ``crontab

At 12:05 every Monday, every month, regardless of the day of the month, run the command in the /home/jdoe/jobs directory called copy-to-partition.

Let's take another example and create a cron job that checks disk space available every minute, every hour, every day of the month, every month, for every day of the week, and outputs it to a file called disk_space.txt.
Let's take another example and create a cron job that checks disk space available every minute, every hour, every day of the month, every month, for every day of the week, and outputs it to a file called disk_space.txt. ::

``* * * * * df -h > disk_space.txt``
$ * * * * * df -h > disk_space.txt

would get us what we wanted (df -h is the unix command for checking free disk space).

Field values can also be ranges. Let's say you want to edit this job to run the same command (df -h), but instead of running every minute, you only want the job to run it in the first 5 minutes of every hour, every day of the month, every month, for every day of the week.
Field values can also be ranges. Let's say you want to edit this job to run the same command (df -h), but instead of running every minute, you only want the job to run it in the first 5 minutes of every hour, every day of the month, every month, for every day of the week. ::

``crontab -e``
$ crontab -e

will open up your default shell editor, where you will see a list of your cron jobs. Editing the one we just wrote to:

Expand All @@ -48,7 +48,7 @@ will get you what you want.

Lastly, if you want to remove the command, again type "crontab -e", and then delete the line with that job in it from the file in your editor.

To remove all cron jobs, type:
To remove all cron jobs, type: ::

``crontab -r``
$ crontab -r

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