Book Image

Learning Linux Shell Scripting - Second Edition

By : Ganesh Sanjiv Naik

Book Image

Learning Linux Shell Scripting - Second Edition

By: Ganesh Sanjiv Naik

Overview of this book

Linux is the most powerful and universally adopted OS. Shell is a program that gives the user direct interaction with the operating system. Scripts are collections of commands that are stored in a file. The shell reads this file and acts on commands as if they were typed on the keyboard. Learning Linux Shell Scripting covers Bash, GNU Bourne Again Shell, preparing you to work in the exciting world of Linux shell scripting. CentOS is a popular rpm-based stable and secured Linux distribution. Therefore, we have used CentOS distribution instead of Ubuntu distribution. Linux Shell Scripting is independent of Linux distributions, but we have covered both types of distros. We start with an introduction to the Shell environment and basic commands used. Next, we explore process management in Linux OS, real-world essentials such as debugging and perform Shell arithmetic fluently. You'll then take a step ahead and learn new and advanced topics in Shell scripting, such as decision making, starting up a system, and customizing a Linux environment. You will also learn about grep, stream editor, and AWK, which are very powerful text filters and editors. Finally, you'll get to grips with taking backup, using other language scripts in Shell Scripts as well as automating database administration tasks for MySQL and Oracle. By the end of this book, you will be able to confidently use your own shell scripts in the real world.

Preface

Who this book is for

What this book covers

To get the most out of this book

Free Chapter

Getting Started and Working with Shell Scripting

Getting Started and Working with Shell Scripting

Comparison of shells

Tasks done by the shell

Working in the shell

Learning basic Linux commands

Our first script – Hello World

Compiler and interpreter – differences in process

When not to use scripts

Various directories

Working more effectively with Shell – basic commands

Working with permissions

Drilling Deep into Process Management, Job Control, and Automation

Drilling Deep into Process Management, Job Control, and Automation

Introducing process basics

Monitoring processes using ps

Process management

Process monitoring tools – top, iostat, and vmstat

Understanding "at"

Understanding crontab

Using Text Processing and Filters in Your Scripts

Using Text Processing and Filters in Your Scripts

Text filtering tools

Pattern matching with the vi editor

Pattern searching using grep

Working with Commands

Working with Commands

Learning shell interpretation of commands

Command substitution

Command separators

Logical operators

Exploring Expressions and Variables

Exploring Expressions and Variables

Understanding variables

Working with environment variables

Working with read-only variables

Working with command-line arguments (special variables, set and shift, getopt)

Understanding getopts

Understanding default parameters

Working with arrays

Neat Tricks with Shell Scripting

Neat Tricks with Shell Scripting

Interactive shell scripts – reading user input

The here document and the << operator

The here string and the <<< operator

Performing Arithmetic Operations in Shell Scripts

Performing Arithmetic Operations in Shell Scripts

Using a declare command for arithmetic

Using the let command for arithmetic

Using the expr command for arithmetic

Binary, octal, and hex arithmetic operations

Floating-point arithmetic

Automating Decision-Making in Scripts

Automating Decision-Making in Scripts

Checking the exit status of commands

Understanding the test command

Conditional constructs – if else

Implementing simple menus with select

Automating Repetitive Tasks

Automating Repetitive Tasks

Looping with the for command

Exiting from the current loop iteration with the continue command

Exiting from a loop with a break

Working with the do – while loop

Piping the output of a loop to a Linux command

Running loops in the background

The IFS and loops

Working with Functions

Working with Functions

Understanding functions

Passing arguments or parameters to functions

Sharing the data with many functions

Declaring local variables in functions

Returning information from functions

Running functions in the background

Creating a library of functions

Using Advanced Functionality in Scripts

Using Advanced Functionality in Scripts

Understanding signals and traps

Using the trap command

Ignoring signals

Using traps inside a function

Running scripts or processes even if the user logs out

Creating dialog boxes with the dialog utility

System Startup and Customizing a Linux System

System Startup and Customizing a Linux System

System startup, inittab, and run levels

User initialization scripts

Pattern Matching and Regular Expressions with sed and awk

Pattern Matching and Regular Expressions with sed and awk

The basics of regular expressions

sed – non-interactive stream editor

Taking Backup and Embedding Other Languages in Shell Scripts

Taking Backup and Embedding Other Languages in Shell Scripts

Backup of files from command line

Embedding other language codes or scripts in Bash shell scripts

Database Administration Using Shell Scripts

Database Administration Using Shell Scripts

Introduction to database administration

Working with a MySQL Database

Working with Oracle Database

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When not to use scripts

Shell scripts have certain advantages over compiler-based programs, such as C or C++ language. However, shell scripting has certain limitations as well.

The following are the advantages:

Scripts are easy to write
Scripts are quick to start and easy for debugging
They save time in development
Tasks of administration are automated
No additional setup or tools are required for developing or testing
shell scripts

The following are the limitations of shell scripts:

Every line in shell script creates a new process in the operating system. When we execute the compiled program, such as a C program, it runs as a single process for the complete program.
Since every command creates a new process, shell scripts are slow compared to compiled programs.
Shell scripts are not suitable if heavy math operations are involved.
There are problems with cross-platform portability.

We cannot use shell scripts in the following situations:

Where extensive file operations are required
Where we need data structures, such as linked lists or trees
Where we need to generate or manipulate graphics or GUIs
Where we need direct access to system hardware
Where we need a port or socket I/O
Where we need to use libraries or interface with legacy code
Where proprietary, closed source applications are used (shell scripts put the source code right out in the open for the entire world to see)