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Learning Linux Shell Scripting - Second Edition

By : Ganesh Sanjiv Naik

3 (6)

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Learning Linux Shell Scripting

3 (6)

By: Ganesh Sanjiv Naik

Buy this Book

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

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Free Chapter

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

Summary

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

Summary

Using Text Processing and Filters in Your Scripts

Text filtering tools

IO redirection

Pattern matching with the vi editor

Pattern searching using grep

Summary

Working with Commands

Learning shell interpretation of commands

Command substitution

Command separators

Logical operators

Pipes

Summary

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

Summary

Neat Tricks with Shell Scripting

Interactive shell scripts – reading user input

The here document and the << operator

The here string and the <<< operator

File handling

Debugging

Summary

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

Summary

Automating Decision-Making in Scripts

Checking the exit status of commands

Understanding the test command

Conditional constructs – if else

Switching case

Implementing simple menus with select

Summary

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

Using until

Piping the output of a loop to a Linux command

Running loops in the background

The IFS and loops

Summary

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

Summary

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

Summary

System Startup and Customizing a Linux System

System startup, inittab, and run levels

User initialization scripts

Summary

Pattern Matching and Regular Expressions with sed and awk

The basics of regular expressions

sed – non-interactive stream editor

Using awk

Summary

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

Summary

Database Administration Using Shell Scripts

Introduction to database administration

Working with a MySQL Database

Working with Oracle Database

Summary

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Running scripts or processes even if the user logs out

Sometimes, we may need our script to run even after we log out, such as when making a backup and similar activities. In this case, even if we log out, the system is powered on and running. In such situations, we can use the nohup command. The nohup command prevents the process from terminating by using the SIGHUP signal.

The nohup command makes our script run without attaching it to a Terminal. Therefore, if we use the echo command to print text on the Terminal it will not be printed in a Terminal, since the script is not attached to a Terminal. In such cases, we need to redirect the output to the file, or nohup will automatically redirect the output to a nohup.out file.

Therefore, if we need to run a process, even if we log out, we need to use the nohup command as follows: