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

Advanced test using [[

The use of the double brackets [[ condition ]] allows us to do more advanced condition testing but is not compatible with the Bourne Shell. The double brackets were first introduced as a defined keyword in the korn shell and are also available in bash and zsh. Unlike the single bracket, this is not a command but a keyword. The use of the type command can confirm this:

$ type [[


The fact that [[ is not a command is significant where whitespace is concerned. As a keyword, [[ parses its arguments before bash expands them. As such, a single parameter will always be represented as a single argument. Even though it goes against best practice, [[ can alleviate some of the issues associated with whitespace within parameter values. Reconsidering the condition we tested earlier, we can omit the quotes when using [[ ,as shown in the following example:

$ echo "The File Contents">"my file"
$ FILE="my file"
$ [[ -f $FILE && -r $FILE ]] && cat "$FILE"