Broadly speaking, there are two types of software that run on a machine: interactive software that the user launches and controls and background software that runs quietly doing what it needs to do. The interactive software is what you think you're using most often – it's the web browsers and word processors and all the other bits and pieces you launch when you want to do something. However, if you take a look at the processes on your computer (just type ps ax in a terminal window), you'll see that most of the software running isn't anything you launched, but the stuff that runs in the background quietly getting on with what it's doing. For the most part, you don't have to think about this – the software just works, and you get to focus on the interactive software and use the computer in the way you want; however, every once in a while, you have to delve a little deeper and see how this background software is running.

Because this software runs in the background, you should never see an error message, a progress bar, or any of the other user interface clues that tell you how interactive software is performing. Instead, it sends all the output to logfiles. These are text files that usually live in the /var/log directory (see the "Init Systems" box). These logfiles are designed to be readable by both humans and machines, so they follow a fixed text format, but one that's sufficiently verbose to understand. The main log on most Linux boxes is the system log, which is /var/log/syslog on Debian and Ubuntu-based machines and /var/log/messages on many others. This is a text file, so you could open it in a text editor. However, it can be very large, so it's usually best to just view the end of it. You can do this from a terminal by typing:

tail /var/log/syslog

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