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)

Using the if keyword

A shell script is fundamentally a sequence of commands. In such a script, we very often want to run a command only if a testable condition is true. In earlier chapters, we already saw one way to do this, with the && control operator:

$ grep -q bash /etc/shells && printf 'Bash is a system shell\n'

The preceding command line separates two commands with &&. The second command, which prints a string to standard output, only runs if the first command, grep -q bash /etc/shells, finds the bash string in the /etc/shells file and hence exits with status zero (success).

Using the if keyword, we can make this conditional approach more flexible, and somewhat easier to read:

$ if grep -q bash /etc/shells ; then printf 'Bash is a system shell\n' ; fi

The preceding command line could also be written like this in script form: