Linux Shell Scripting Essentials

Sinny Kumari

• Publisher: Packt Publishing
• Year: 2015
• ISBN:
  9781785284441
• Edition: 1
• Topic: Computer Science
• Subtopic: Operating Systems
• Index: Computer Science

Learn shell scripting to solve complex shell-related problems and to efficiently automate your day-to-day tasks

About This Book

• Familiarize yourself with the terminal by learning about powerful shell features
• Automate tasks by writing shell scripts for repetitive work
• Packed with easy-to-follow, hands-on examples to help you write any type of shell script with confidence

Who This Book Is For

This book is aimed at administrators and those who have a basic knowledge of shell scripting and who want to learn how to get the most out of writing shell scripts.

What You Will Learn

• Write effective shell scripts easily
• Perform search operations and manipulate large text data with a single shell command
• Modularize reusable shell scripts by creating shell libraries
• Redirect input, output, and errors of a command or script execution to other streams
• Debug code with different shell debugging techniques to make your scripts bug-free
• Manage processes, along with the environment variables needed to execute them properly
• Execute and embed other languages in your scripts
• Manage creation, deletion, and search operations in files

In Detail

Shell scripting is a quick method to prototype complex applications or problems. Shell scripts are a collection of commands to automate tasks, usually those for which the user has a repeated need, when working on Linux-based systems. Using simple commands or a combination of them in a shell can solve complex problems easily.

This book starts with the basics, including essential commands that can be executed on Linux systems to perform tasks within a few nanoseconds. You'll learn to use outputs from commands and transform them to show the data you require. Discover how to write shell scripts easily, execute script files, debug, and handle errors.

Next, you'll explore environment variables in shell programming and learn how to customize them and add a new environment. Finally, the book walks you through processes and how these interact with your shell scripts, along with how to use scripts to automate tasks and how to embed other languages and execute them.

Style and approach

This book is a pragmatic guide to writing efficient shell programs, complete with hands-on examples and tips.

Linux Shell Scripting Essentials

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Table of Contents

Linux Shell Scripting Essentials

Credits

About the Author

About the Reviewers

www.PacktPub.com

Support files, eBooks, discount offers, and more

Why subscribe?

Free access for Packt account holders

Preface

What this book covers

What you need for this book

Who this book is for

Conventions

Reader feedback

Customer support

Downloading the example code

Errata

Piracy

Questions

1. The Beginning of the Scripting Journey

Hello World in shell

Interacting with shell

Let's make it scripted

Define variables of choice

Nomenclature

Assigning a value

Accessing a value

Constant variables

Reading variables from a user input

Builtin shell variables

Operators

The assignment operator

Arithmetic operators

Logical operators

Comparison operators

Shell expansions

~ (Tilde)

* (Asterisk)

? (Question mark)

[ ] (Square brackets)

{ } (Curly brackets)

Construct commands using eval

Make bash behave using set

Exit on the first failure

Enabling/disabling symbolic link's resolution path

Setting/unsetting variables

Summary

2. Getting Hands-on with I/O, Redirection Pipes, and Filters

Standard I/O and error streams

File descriptors

Redirecting the standard I/O and error streams

Redirecting standard output

Redirecting standard input

Redirecting standard errors

Multiple redirection

Pipe and pipelines – connecting commands

Pipe

Pipeline

Regular expressions

Regular expression metacharacters

Character ranges and classes

Character ranges

Matching dates in mm-dd-yyyy format

Matching a valid month

Matching a valid day

Matching the valid year in a date

Combining valid months, days, and years regex to form valid dates

Regex for a valid shell variable

Filtering an output using grep

Syntax

Looking for a pattern in a file

Looking for a pattern in multiple files

A few more grep usages

Searching in a binary file

Searching in a directory

Excluding files/directories from a search

Display a filename with a matching pattern

Matching an exact word

Editing output using sed

String substitution using s

Multiple substitutions

Duplicating a stream using tee

Writing an output to stdout and appending to a file

Sending an output to multiple commands

Sorting and finding unique text

Sorting an input text

Sorting a single file

Redirecting output to sort

Filtering unique elements

Unique elements in a file

Character-based translation using tr

Deleting input characters

Squeezing to a single occurrence

Inverting a character set to be translated

Filtering based on lines—head and tail

Printing lines using head

Printing the first few lines

Printing the first few bytes

Printing lines using tail

Checking log entries

Finding any line in a file

The Cut-based selection

Cutting across columns

Text selection in files

Summary

3. Effective Script Writing

Exiting from scripts and exit codes

Exit codes

Exit codes with a special meaning

Script with exit codes

Testing expressions with a test

File checks

Arithmetic checks

String checks

Expression checks

Using conditional statements with if and else

Simple if and else

The if, elif, and else statements

Nested if

Indexed arrays and associative arrays

Indexed arrays

Array declaration and value assignment

Operations on arrays

The associative array

The declaration and value assignment

Operations on arrays

Looping around with for

Simple iteration

Iterating over a command output

Specifying a range to the for loop

Small and sweet for loop

The select, while, and until loops

Loop using select

The while loop

The until loop

Switch to my choice

Passing stdout as a parameter using xargs

Basic operations with xargs

Using xargs to find a file with the maximum size

Archiving files with a given pattern

Using funct...