Book Image

Bash Quick Start Guide

By : Tom Ryder
Book Image

Bash Quick Start Guide

By: Tom Ryder

Overview of this book

Bash and shell script programming is central to using Linux, but it has many peculiar properties that are hard to understand and unfamiliar to many programmers, with a lot of misleading and even risky information online. Bash Quick Start Guide tackles these problems head on, and shows you the best practices of shell script programming. This book teaches effective shell script programming with Bash, and is ideal for people who may have used its command line but never really learned it in depth. This book will show you how even simple programming constructs in the shell can speed up and automate any kind of daily command-line work. For people who need to use the command line regularly in their daily work, this book provides practical advice for using the command-line shell beyond merely typing or copy-pasting commands into the shell. Readers will learn techniques suitable for automating processes and controlling processes, on both servers and workstations, whether for single command lines or long and complex scripts. The book even includes information on configuring your own shell environment to suit your workflow, and provides a running start for interpreting Bash scripts written by others.
Table of Contents (10 chapters)

Exit values

We can tell it was the rmdir command in the previous section that failed, because rmdir is the first word of the error message output. We can test the command in isolation, and look at the value of the special $? parameter with echo, to see its exit status:

$ rmdir ~/nonexistent
rmdir: failed to remove '/home/bashuser/nonexistent': No such file or directory
$ echo $?
1

The rmdir program returned an exit value of 1, because it could not delete a directory that didn't exist. If we create a directory first, and then remove it, both commands succeed, and the value of $? for both steps is 0:

$ mkdir ~/existent
$ echo $?
0
$ rmdir ~/existent
$ echo $?
0

Examining the exit values for the true and false built-in Bash commands is instructive; true always succeeds, and false always fails:

$ true ; echo $?
0
$ false ; echo $?
1

Bash will also raise an exit status...