Linux CLI for Beginners, or, Fear Not the Linux Command Line!

Monday Dec 15th 2008 by Staff

Linux forums, mailing lists, and even how-to articles are full of fearful comments against the Linux command line. Akkana Peck gives an introduction to the Linux command line, starting with a lesson on the fastest way to find files.

Most recent converts to Linux spend most of their time in the GUI -- the graphical desktop (whether Gnome, or KDE, or XFCE, or some other interface) that's made to look and act somewhat like Windows and Mac.

But if you spend all your time in the GUI, you're missing out. The Linux command-line gives you a lot of power -- it lets you do tasks that are difficult or impossible with the GUI, and for tasks that you do a lot, such as launching the same applications everyday, it's often faster. When you read about using the command line, which is often abbreviated to CLI for "command-line interface", they usually mean typing commands into a terminal. This is a term leftover from the olden days of interfacing with mainframes via dumb terminals which had no processing power of their own; they were pretty much just monitors and keyboards. When we refer to a Linux terminal it's a software application, and if you want to get technical it's a terminal emulator.

So the first step is finding a terminal on your Linux system, and I haven't seen a Linux distribution yet that didn't include several by default. On KDE look in your start menus for Konsole, and on Gnome look for Terminal or Gnome-Terminal. There are dozens of different terminals: xterm, aterm, rxvt, eterm, and many more. Apparently Linux geeks love terminals.

Wading through all those menus to launch our terminal is tedious, so our first step in learning mighty command-line powers is a fast keyboard shortcut for opening a "run command" dialog. This way we can quickly type in the name of an application to start it. Press the Alt and F2 keys at the same time, and on KDE you see something like Figure 1. Gnome calls it "run application" and it looks similar, with one very nice addition-- an alphabetical graphical menu to browse. On KDE type in konsole and press Run, and in Gnome type terminal or gnome-terminal and press Run. Lo and behold, your shiny terminal opens before you, just like Figure 2 shows.

What you see displayed in the terminal is the command prompt, in this example carla@xena:~$. This tells you the name of the user, the name of their computer, ~ means the current directory is Carla's home directory, and $ means ordinary, unprivileged user. When you see # that means all-powerful root user.

Now that you have a terminal open you can start entering commands. First, let's take a look at a really useful Linux command-line utility called locate. It's a very fast way of finding files on your system.

For instance, suppose somebody sent me a funny picture a while back of a squirrel with tiger stripes. I know I saved it, but I can't remember where! But it probably had "squirrel" or "tiger" or maybe both somewhere in the name.

From the desktop I can do simple filename searches, but it's a bit slow and, as you'll see, it's not as flexible. So let's try it from the command line. Type:

$ locate tiger

Don't type the $, that's just to show you that this command can be run as an ordinary user. locate tiger gets quite a few matches (Figure 3). The list is short enough that I could read through them all, but why not get the computer to do that work for me?

So what if I try:

$ locate squirrel

Eek! I'm not going to tell you how many files that found. Suffice to say that I take way too many pictures of squirrels, and finding one specific file in that mess looks way too hard.

The locate command depends on a database of every filename on your system, and most Linux distributions automatically update this database daily. It doesn't hurt to update it manually just to make sure, using the updatedb command. You need root powers to do this, which on Ubuntu you get with the sudo command:

$ sudo updatedb

It can take a few minutes, so be patient.

Pipes to the rescue

Fortunately, the Linux command line offers a way to combine these two searches. It's called a pipeline, because it uses the ASCII vertical bar character sometimes called a "pipe", which is the uppercase of the back-slash key: |.

Most basic Linux commands are set up so that you can "pipe" the output of one program into the input of another. The simplest version of a pipeline is one that uses the program "less" to display a file in screen-sized chunks. less shows you one page at a time; hitting the spacebar will take you to the next page, while q will quit. Try it for yourself in your terminal window. In case you don't have as many squirrel pictures as I do, try searching for files containing "cat":

$ locate cat | less

I bet you didn't know you had so many cat files on your system! Remember, q will get you out of less and back to the command prompt.

Okay, how do we use a pipeline to help find the tiger-squirrel picture? With the grep command, probably the single most useful command Linux offers.

"grep" is kind of a nasty name -- there's some disagreement on what it stands for, including "Globally search for Regular Expressions and Print" and "General Regular Expression Parser". But it's an incredibly useful program that prints just the lines matching a particular pattern.

For instance, to combine the squirrel search and the tiger search, you just pipe one search through grep, passing grep the second pattern:

$ locate squirrel | grep tiger

locate squirrel found all those squirrel images, then grep searched through all of them to find any that also had "tiger" in the name. Now I know exactly where the file is.

Counting lines

Pipelines aren't limited to just two commands, though. For instance, remember that long list of files with cat in the name? I know there's a program called "cat"; suppose I want to know how many of those other files with "cat" in the name are programs.

Programs are usually located in /usr/bin, but sometimes they're in /bin or some other place. But they usually have "bin" in the name somewhere. So if I take all the files that have both "cat" and "bin" in the name:

$ locate cat | grep bin
I can then find out how many lines there were, by using the useful wc (word count) program:
$ locate cat | grep bin | wc -l

wc -l says to count the number of lines (that's ell for "lines", not a one) -- if you omit the -l part, it prints lines, words and characters.

On my system I have 34 programs with cat in the name. How many do you have?

Akkana Peck is a freelance programmer whose credits include a tour as a Mozilla developer. She's also the author of Beginning GIMP: From Novice to Professional.

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