Linux Pocket Guide, 2nd Edition

The Shell

In order to run commands on a Linux system, you’ll need somewhere to type them. That “somewhere” is called the shell, which is Linux’s command-line user interface: you type a command and press Enter, and the shell runs whatever program (or programs) you’ve requested. (See Running a Shell to learn how to open a shell window.)

For example, to see who’s logged in, you could execute this command in a shell:

$ who
silver       :0    Sep 23 20:44
byrnes    pts/0    Sep 15 13:51
barrett   pts/1    Sep 22 21:15
silver    pts/2    Sep 22 21:18

(The dollar sign is the shell prompt, which means the shell is ready to run a command.) A single command can also invoke several programs at the same time, and even connect programs together so they interact. Here’s a command that redirects the output of the who program to become the input of the wc program, which counts lines of text in a file; the result is the number of lines in the output of who:

$ who | wc -l
4

telling you how many users are logged in.^[5] The vertical bar, called a pipe, makes the connection between who and wc.

A shell is actually a program itself, and Linux has several. We focus on bash (the Bourne-Again Shell), located in /bin/bash, which is usually the default in Linux distros.

^[5] Actually, how many interactive shells those users are running. If a user has two shells running, like the user silver in our example, he’ll have two lines of output from who.