Cron/Crontab Basics:

**debtboy** · 10-20-2009, 03:48 PM

What is Cron?
What is a Cron Job?
What is Crontab?

I'm going to go over just the basics:
I think the name cron comes from chronos (Greek for time),
but it could be an acronym for (Command Run ON)

Cron or Crond is a daemon, which runs in the background and executes
jobs (cron jobs), such as programs, scripts or individual commands
automatically at predetermined times or intervals.

I'll be using Fedora 11 for this demonstration as it was much cleaner
then my normal system, which already has many cron jobs scheduled.

Cronie a fork of vixie-cron (the standard), is what's installed by default
on Fedora and works just fine.

First we want to determine if the cron daemon is running.
There are many ways, but I use a simple process command
piped into grep.

As you can see, it's running.

Now lets look at the cron files and directories in /etc

If we simply want to run a script on a hourly, daily, weekly or monthly
basis, we just drop an executable script into the appropriate directory
(cron.hourly, cron.daily, cron.weekly or cron.monthly) and were done,
but I'd like to have a little more control over my jobs than that.

So lets start by configuring user access. You noticed a cron.deny file
in the image above and that's exactly what it's for, denying access to
crontab files.
What are crontab files?
Crontab files are how you configure your cron jobs, I'll get into more
detail later. Access to crontab is controlled by 2 files:
cron.allow and cron.deny
Cron.deny is installed by default, I would have to create cron.allow
if I wanted it. If cron.allow exists, then only the users named in that
file will have access to crontab and if cron.deny exists, then any user
named in the file will not have access to crontab.
I have a cron.deny file:

Now I'm going to add/append my user name (debtboy) to the
cron.deny file. You see I've become root to accomplish this.

Let's use the more command to see that I've been added to the file.

Now I'm attempting to access crontab with no luck, because of the
cron.deny file.

As root, I use vi to remove debtboy from the file and again use
more to confirm.

In this next section, I'm going to create a personal crontab file
and schedule a job or two.
Personal crontab files on Fedora and many other systems are stored
in the /var/spool/cron directory as the name of the user.
You can see from the image that no crontab files currently exist.

Now you can see, I'm (debtboy) creating a file using crontab -e.
Crontab has a few options -u (user) -l (list) -r (remove) -e (edit)
see man pages for more info.

and from this image, you can see, I've created one.

Here is the simple cron job I created, redirect standard ouput
of the date command into a file named date.txt

Now lets go over the crontab configuration /etc/crontab

SHELL, PATH, MAILTO and HOME are self explanatory, so lets
look at the first 5 parameters, separated by a space.

Just like the image shows, each parameter is a time scheduling option
which also accepts "/" inputs which are basically intervals. For example

Code:

*  02  *  *  *   means every day at 2 O'Clock 
*  */02 *  *  * means every 2 hours

Here are a few examples:

Code:

01   *    *    *    *  (The first minute of every hour)
*/05 *    *    *    *  (every 5 minutes)
20   05   *    *    *  (at 5:20 AM)
*    *    29   *    *  (29th of every month)
*    *    *    *   1-5 (Monday thru Friday)
*    *    /02  *    *  (every 2 days)
01   22   12   1    *  (January 12th @ 10:01 PM)

As you can see from the image below...
I've added a second cron job to my file.
of course I use vi

I again use more to verify

Here I quickly look at the 2 outputs I'm already getting
because the cron jobs started as soon as I updated/saved
the file.

Now I better stop those jobs before date.txt eats up my hard drive.

These were just a few of the basics, I hope it makes
cron a little more understandable.

NOTE:
By setting the time for a cron job to run, we assume that
the unix/linux machine will be running 24/7, but Linux has
become a popular desktop OS, so in turn we have to assume
that machines will be turned off and on randomly.
How do we account for this... anacron a cron supplement.
Look into it.