Book Image

Linux Command Line and Shell Scripting Bible - Third Edition

By : Richard Blum, Christine Bresnahan
Book Image

Linux Command Line and Shell Scripting Bible - Third Edition

By: Richard Blum, Christine Bresnahan

Overview of this book

The Linux command line enables you to type specific shell commands directly into the system to manipulate files and query system resources. Command line statements can be combined into short programs called shell scripts, a practice increasing in popularity due to its usefulness in automation. Linux is a robust system with tremendous potential, and Linux Command Line and Shell Scripting Bible opens the door to new possibilities. Linux Command Line and Shell Scripting Bible is your essential Linux guide. It contains new functional examples that are fully updated to align with the latest Linux features. Beginning with command line fundamentals, the book moves into shell scripting and shows you the practical application of commands in automating frequently performed functions. This book is a complete guide providing detailed instruction and expert advice working within this aspect of Linux. Whether used as a tutorial or as a quick reference, this book contains information that every Linux user should know.
Table of Contents (34 chapters)
2
Part I: The Linux Command Line
13
Part II: Shell Scripting Basics
20
Part III: Advanced Shell Scripting
28
Part IV: Creating Practical Scripts
32
End User License Agreement

Exploring the if-then-else Statement

In the if-then statement, you have only one option for whether a command is successful. If the command returns a non-zero exit status code, the bash shell just moves on to the next command in the script. In this situation, it would be nice to be able to execute an alternate set of commands. That's exactly what the if-then-else statement is for.

The if-then-else statement provides another group of commands in the statement:

if command
then
   commands
else
   commands
fi

When the command in the if statement line returns with a zero exit status code, the commands listed in the then section are executed, just as in a normal if-then statement. When the command in the if statement line returns a non-zero exit status code, the bash shell executes the commands in the else section.

Now you can copy and modify the test script to include an else section:

$ cp test3.sh test4.sh
$
$ nano test4.sh
$
$ cat test4.sh
#!/bin/bash
# testing the else section
#
testuser...