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.

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Mastering Linux Shell Scripting
Andrew Mallett
Dec, 2015 | 198 pages
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Table of Contents (21 chapters)
Mastering Linux Shell Scripting
Mastering Linux Shell Scripting
Credits
Credits
About the Author
About the Author
About the Reviewer
About the Reviewer
www.PacktPub.com
www.PacktPub.com
Preface
Preface
Free Chapter
1
What and Why of Scripting with Bash
What and Why of Scripting with Bash
Bash vulnerabilities
The bash command hierarchy
Preparing text editors for scripting
Creating and executing scripts
Debugging your scripts
Summary
2
Creating Interactive Scripts
Creating Interactive Scripts
Using echo with options
Basic script using read
Script comments
Enhancing scripts with read prompts
Limiting the number of entered characters
Controlling the visibility of entered text
Enhancing learning with simple scripts
Summary
3
Conditions Attached
Conditions Attached
Simple decision paths using command-line lists
Verifying user input with lists
Using the test shell builtin
Creating conditional statements using if
Extending if with else
More conditions with elif
Using case statements
Script – building a front-end with grep
Summary
4
Creating Code Snippets
Creating Code Snippets
Abbreviations
Using code snippets
Summary
5
Alternative Syntax
Alternative Syntax
Recapping test
Providing parameter defaults
When in doubt – Quote!
Advanced test using [[
Arithmetic operations using ((
Summary
6
Iterating with Loops
Iterating with Loops
For loops
Controlling the loop
Reading input from files
Creating operator menus
Summary
7
Creating Building Blocks with Functions
Creating Building Blocks with Functions
Introducing functions
Passing parameters to functions
Returning values from functions
Using functions in menus
Summary
8
Introducing sed
Introducing sed
Using grep to display text
Using regular expressions
Understanding the basics of sed
Summary
9
Automating Apache Virtual Hosts
Automating Apache Virtual Hosts
Apache name-based Virtual Hosts
Automating Virtual Host creation
Summary
10
Awk Fundamentals
Awk Fundamentals
The history behind awk
Displaying and filtering content from files
Formatting output
Further filtering to display users by UID
Awk control files
Summary
11
Summarizing Logs with Awk
Summarizing Logs with Awk
The HTTPD log file format
Displaying data from web logs
Displaying the highest ranking IP address
Displaying the browser data
Working with e-mail logs
Summary
12
A Better lastlog with Awk
A Better lastlog with Awk
Using awk ranges to exclude data
Conditions based on the number of fields
Manipulating the awk record separator to report on XML data
Summary
13
Using Perl as a Bash Scripting Alternative
Using Perl as a Bash Scripting Alternative
What is Perl?
Hello World
Perl arrays
Conditional statements in Perl
Using functions within Perl
Summary
14
Using Python as a Bash Scripting Alternative
Using Python as a Bash Scripting Alternative
What is Python?
Saying Hello World the Python way
Pythonic arguments
Significant whitespace
Reading user input
Summary
Index
Index

Script comments

We should always introduce commenting scripts early in the piece. A script comment is prefaced with a # symbol. Anything after the # symbol is a comment and is not evaluated by the script. The shebang, #!/bin/bash, is primarily a comment and, as such, is not evaluated by the script. The shell running the script reads the shebang so it knows which command interpreter to hand the script over to. A comment may be at the start of a line or partly into the line. Shell scripting does not have the notion of multi-line comments.

If you are not already familiar with comments, then they are added to the script to tell all about who wrote the script, when it was written and last updated, and what the script does. It is the metadata of the script.

The following is an example of comments in scripts:

#!/bin/bash
# Welcome script to display a message to users on login
# Author: @theurbanpenguin
# Date: 1/1/1971

It is a good practice to comment and add comments that explain what the code is...

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