Laziness is considered a virtue in system administration, meaning you should figure out ways to avoid having to perform the same task regularly. Crontab allows you to run routine jobs on a user's system automatically at regular intervals rather than running such jobs manually. Even knowing just the basics of crontab is a huge win, as this tool increases productivity, saves time and is not prone to human error, i.e. forgetting to run a job.
The easiest way to see jobs running regularly on the current user's system ("cron jobs") is to type:
$ `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 :
-bash: crontab: no crontab for jdoe
If there are jobs scheduled to run, there will be a list of lines that looks something like this:
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 cron jobs. What is this output telling you? It is helpful to know that the fields of a cron job are as follows:
MINUTE HOUR DAYOFMONTH MONTH DAYOFWEEK COMMAND
and that the acceptable values for each field are:
0-59 0-23 1-31 0-11 0-6 `filepath/command`
Note that order matters. Knowing this, we can see that the output of
crontab -l really means:
At 12:05 every Monday, every month, regardless of the day of the month, run the command copy-to-partition in the /home/jdoe/jobs directory.
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
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.
$ `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:
0-5 * * * * `df -h` > disk_space.txt
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:
$ `crontab -r`