Book Image

Mastering Linux Shell Scripting

By : Andrew Mallett
Book Image

Mastering Linux Shell Scripting

By: Andrew Mallett

Overview of this book

Shell scripting is a quick method to prototype a complex application or a problem by automating tasks when working on Linux-based systems. Using both simple one-line commands and command sequences complex problems can be solved with ease, from text processing to backing up sysadmin tools. In this book, you’ll discover everything you need to know to master shell scripting and make informed choices about the elements you employ. Get to grips with the fundamentals of creating and running a script in normal mode, and in debug mode. Learn about various conditional statements' code snippets, and realize the power of repetition and loops in your shell script. Implement functions and edit files using the Stream Editor, script in Perl, program in Python – as well as complete coverage of other scripting languages to ensure you can choose the best tool for your project.
Table of Contents (21 chapters)
Mastering Linux Shell Scripting
About the Author
About the Reviewer

For loops

All our looping controls can be simple and we will begin by looking at for loops. The word for is a keyword in bash and in working it is similar to if. We can use the command type to verify this, as shown in the following example:

$ type for
for is a shell keyword

As a reserved shell keyword, we can use a for loop both in scripts and directly at the command line. In this way, we can utilize loops within and without the scripts optimizing the use of the command line. A simple for loop is shown in the following example code:

# for u in bob joe ; do
useradd $u
echo '$u:Password1' | chpasswd
passwd -e $u

Within a for loop, we read from the list on the right to populate the variable parameter on the left, in this case we will read from the list containing bob and joe into the parameter variable u. Each item from the list is inserted into the variable, one item at a time. In this way, as long as there are items to be processed in the list, the loop will execute until the list is exhausted...