While cron and anacron facilities are great for recurring tasks, how can a user schedule simple tasks that only need to be run “one time” during the middle of the night or on a future date? The AT command is the answer. AT is one of those commands that a lot of people don’t know about but it can be very useful.

AT’s commandline syntax makes scheduling those “one time” tasks quick and easy. Let’s get started with some examples.

Example 1. – Create a new file on the desktop.

Open up a Termial or Console Window. At the command prompt type “at 2:00pm” and hit enter.

[sam@www Desktop]$ at 2:00pm

AT opens a prompt and waits for input. Since we want to create a new file, we’ll use the “touch” command along with the path where we want to create the new file on the desktop and then hit enter.
at> touch /home/sam/Desktop/newfile.txt

AT waits for more input. Press Ctrl-D on the keyboard to closes the prompt and display the job number and date it will execute.

at> <EOT>
job 1 at 2009-02-17 14:00
[sam@www Desktop]

If everything went right, you’ll see a newfile.txt on the desktop at 2:00pm. Pretty simple huh? To see a listing of scheduled jobs use the command “atq”.

[sam@www Desktop]$ atq
1 2009-02-17 14:00 a sam
[sam@www Desktop]$

If you decide to remove a job it’s simple as well using the command “atrm” plus job number.

[sam@www Desktop]$ atrm 1


Real World Example – Update a webpage at midnight.

Let’s say you build websites and a client asks you to update their website for a new promotion they’re having. The only catch is that they want page updated at exactly midnight. Believe me, this sort of request does and will happen. Not a problem with our new found AT abilities!

We create the new index.html page with the promotion info and save it somewhere on the server. I’m storing it in /home/sam/Desktop/index.html for this tutorial.

Open a terminal window and enter “at” along with the time specification. AT also allows for certain keywords to describe times such as noon, teatime, midnight, etc. Be sure to read the man page for more info. I’ll use the midnight keyword here.

[sam@www Desktop]$ at midnight

At at’s prompt we use the “cp” command to copy index.html to the website overwriting the current index page.

at> cp  /home/sam/Desktop/index.html   /var/www/html/testwebsite.com/index.html

Press Ctrl-D on the keyboard to closes the prompt and have the page updated at midnight. The client is happy, were getting our zzz’s, and at does the dirty work.

at> <EOT>
job 2 at 2009-02-18 00:00
[sam@www Desktop]

Conclusion
That’s about all there is to it. AT is simple, effective, and that’s the way it should be.

CentOS also know as “Community Enterprise Operating System” is an Enterprise-class Linux Distribution derived from sources freely provided to the public by a prominent North American Enterprise Linux vendor. CentOS conforms fully with the upstream vendors redistribution policy and aims to be 100% binary compatible.

You might be wondering what this “Enterprise Class, Binary Compatible” jargon means and why it’s important. Basically it means the server and software packages have been well tested and are ready to be used in a business environment, data center, or other venue where stability is a major concern vs. cutting edge features. Binary compatible in this instance means binaries or programs that are compatible between CentOS and a prominent North American Enterprise Linux vendor, Red Hat Linux. Red Hat binaries should run nicely on CentOS. In essence, we”ll be working with a version of  Red Hat Enterprise Linux with logos and branding stripped away.

Some of the advantages are:

• Stability of an Enteprise Level Operating System widely used in Datacenters and Web Hosting Operations
• CentOS is free!
• Third Party Packages created for Red Hat usually work nicely with CentOS  (Binary Compatibility)
• Long Term Support and Security Updates
• Good Documentation

Disadvantages:

Having said all that, CentOS isn’t right for everyone. If you plan on mainly listening to mp3′s, watching movies, browsing the web, or using it as more of a multimedia system, you might be disappointed with CentOS. Although CentOS can do all of those things, it takes some work getting it to that point.  CentOS is geared more towards being a robust and stable server system and that’s what it does best.

Prerequisites:

Basic computer knowledge about downloading software, burning dvd’s is required.

Downloading and Verifying CentOS

If you haven’t already downloaded CentOS, grab a copy from http://mirror.centos.org/centos/5/i386/ . In this tutorial i’ll be installing the 32bit version from dvd. There are also installation CD’s but there are 6 of them so i prefer to use dvd. To speed up the download and conserve bandwdith on the mirrors,  i’m using bittorrent to download the dvd installer file. The file is pretty big so depending on your internet connection speed, prepare to wait for a long download. In the same directory there should be a file named md5sum.txt. Open or save this file because we’ll use it in the next step.

Once the download is complete it’s a good idea to verify the integrity of the iso file using an md5 hash. In simple terms, an md5 hash can be thought of as the fingerprint of a file. Here is a more indepth explanation of md5.  To verify the CentOS iso you’ve just downloaded, you’ll need some type of md5 software. Windows users can download and install digestIT. Once you’ve installed digetsIT, locate CentOS-5.2-i386-bin-DVD.iso, right click on it and choose “digestIT 2004 -> Calculate MD5 Hash”.

Under linux, open a terminal window, change to the directory containing the iso file, and run the command “md5sum CentOS-5.2-i386-bin-DVD.iso“. Mac OS X is similar to linux, just change the command to “md5 CentOS-5.2-i386-bin-DVD.iso“. Locate the line for CentOS-5.2-i386-bin-DVD.iso in md5sum.txt and compare it with the output of digestIT or md5sum. The number should be the same. If it’s not, double check the readings and download CentOS from a different mirror. If all goes well, use your favorite dvd burning software and burn the CentOS iso to dvd.

Starting the Installation

In this tutorial i’m starting with a blank hard drive. It’s possible to install linux along side windows or another OS but to keep things simple i won’t go that route in this tutorial.

On to the next page…