Processes

We mentioned before that GNU/Linux is a multitasking system. It can do many tasks at once. Each of these tasks is called a process. The best way to get a sense of this is to type top at the shell prompt. You'll get a list of processes, sorted according to how much of the computer's processing time they're using. The order will continuously change before your eyes. At the top of the display, there's some information about the system: how many users are logged in, how many total processes there are, how much memory you have and how much you're using.

In the far left column, you'll see the user owning each process. The far right column shows which command invoked the process. You'll probably notice that top itself, invoked by you, is near the top of the list (because anytime top checks on CPU usage, it will be active and using CPU to do the check).

Note that in all the commands ending in ``d'' - such as kflushd and inetd - the ``d'' stands for daemon.

Daemon originally meant Disks And Extensions MONitor. A daemon is a non-interactive process, that is, it's run by the system and users never have to worry about it. Daemons provide services like Internet connectivity, printing, or e-mail.

Now press u and give top your username when it asks. The u command asks to see only those processes belonging to you; it allows you to ignore all the daemons and whatever other people are doing. You might notice bash, the name of your shell. You'll pretty much always be running bash.

Note that column two of the top display shows you the PID, or Process IDentification number. Each process is assigned a unique PID. You can use the PID to control individual processes (more on that later). Another useful trick is to press ? to get a list of top commands.

You may wonder about the difference between a ``process'' and a ``program.'' In practice, people use the terms interchangeably. Technically, the program is the set of instructions written by a programmer and kept on disk. The process is the working instantiation of the program kept in memory by Linux. But it's not that important to keep the terms straight.

Much of your interaction with a computer involves controlling processes. You'll want to start them, stop them, and see what they're up to. Your primary tool for this is the shell.