bash Cookbook, 2nd Edition

Chapter 4. Executing Commands

The main purpose of bash (or of any shell) is to allow you to interact with the computer’s operating system so that you can accomplish whatever you need to do. Usually that involves launching programs, so the shell takes the commands you type, determines from that input what programs need to be run, and launches them for you.

Let’s take a look at the basic mechanism for launching jobs and explore some of the features bash offers for launching programs in the foreground or the background, sequentially or in parallel, indicating whether they succeeded, and more.

4.1 Running Any Executable

Problem

You need to run a command on a Linux or Unix system.

Solution

Use bash and type the name of the command at the prompt:

$ someprog

Discussion

This seems rather simple, and in a way it is, but a lot goes on behind the scenes that you never see. What’s important to understand about bash is that its basic operation is to load and execute programs. All the rest is just window dressing to get ready to run programs. Sure, there are shell variables and control statements for looping and if/then/else branching, and there are ways to control input and output, but they are all icing on the cake of program execution.

So where does it get the program to run?

bash uses a shell variable called $PATH to locate your executable. The $PATH variable is a list of directories. The directories are separated by colons (:). bash searches in each of those directories for a file ...