Running Linux, 5th Edition

Programs That Serve You

We include this section because you should start to be interested in what's running on your system behind your back.

Many modern computer activities are too complex for the system simply to look at a file or some other static resource. Sometimes these activities need to interact with another running process.

For instance, take FTP, which you may have used to download some Linux-related documents or software. When you FTP to another system, another program has to be running on that system to accept your connection and interpret your commands. So there's a program running on that system called ftpd. The d in the name stands for daemon, which is a quaint Unix term for a server that runs in the background all the time. Most daemons handle network activities.

You've probably heard of the buzzword client/server enough to make you sick, but here it is in action—it has been in action for decades on Unix.

Daemons start up when the system is booted. To see how they get started, look in the /etc/inittab and /etc/xinetd.conf files, as well as distribution-specific configuration files. We won't go into their formats here. But each line in these files lists a program that runs when the system starts. You can find the distribution-specific files either by checking the documentation that came with your system or by looking for pathnames that occur frequently in /etc/inittab. Those normally indicate the directory tree where your distribution stores its system startup files. ...