8. Automating Decision Making in Scripts | Learning Linux Shell Scripting

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Learning Linux Shell Scripting

By : Ganesh Sanjiv Naik

4.8 (6)

Learning Linux Shell Scripting

4.8 (6)

By: Ganesh Sanjiv Naik

Overview of this book

Linux is the one of the most powerful and universally adopted OSes. 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 can read this file and act on the commands as if they were typed on the keyboard. Shell scripting is used to automate day-to-day administration, and for testing or product development tasks. This book covers Bash, GNU Bourne Again SHell, preparing you to work in the exciting world of Linux shell scripting. We start with an introduction to the Shell environment and explain basic commands used in Shell. Next we move on to check, kill, and control the execution of processes in Linux OS. Further, we teach you about the filter tools available in Linux and explain standard output and standard errors devices. Then we will ensure you understand Shell’s interpretation of commands and get a firmer grasp so you use them in practice. Next, you’ll experience some real-world essentials such as debugging and perform Shell arithmetic fluently. Then you’ll take a step ahead and learn new and advanced topics in Shell scripting, such as starting up a system and customizing a Linux system. Finally, you’ll get to understand the capabilities of scripting and learn about Grep, Stream Editor, and Awk.

Preface

Preface

What this book covers

What you need for this book

Who this book is for

Conventions

Reader feedback

Customer support

Free Chapter

1. Getting Started and Working with Shell Scripting

1. Getting Started and Working with Shell Scripting

Comparison of shells

Tasks done by shell

Working in shell

Learning basic Linux commands

Our first script – Hello World

Compiler and interpreter – difference in process

When not to use scripts

Various directories

Working more effectively with shell – basic commands

Working with permissions

Summary

2. Drilling Deep into Process Management, Job Control, and Automation

2. 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

3. Using Text Processing and Filters in Your Scripts

3. Using Text Processing and Filters in Your Scripts

Text filtering tools

IO redirection

Pattern matching with the vi editor

Pattern searching using grep

Summary

4. Working with Commands

4. Working with Commands

Learning shell interpretation of commands

Command substitution

Command separators

Logical operators

Pipes

Summary

5. Exploring Expressions and Variables

5. 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

6. Neat Tricks with Shell Scripting

6. 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

7. Performing Arithmetic Operations in Shell Scripts

7. Performing Arithmetic Operations in Shell Scripts

Using a command declare for arithmetic

Using the let command for arithmetic

Using the expr command for arithmetic

Binary, octal, and hex arithmetic operations

A floating-point arithmetic

Summary

8. Automating Decision Making in Scripts

8. 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

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

9. Working with Functions

9. Working with Functions

Understanding functions

Passing arguments or parameters to functions

Sharing the data by many functions

Declaring local variables in functions

Returning information from functions

Running functions in the background

Creating a library of functions

Summary

10. Using Advanced Functionality in Scripts

10. Using Advanced Functionality in Scripts

Understanding signals and traps

Using the trap command

Ignoring signals

Using traps in function

Running scripts or processes even if the user logs out

Creating dialog boxes with the dialog utility

Summary

11. System Startup and Customizing a Linux System

11. System Startup and Customizing a Linux System

System startup, inittab, and run levels

User initialization scripts

Summary

12. Pattern Matching and Regular Expressions with sed and awk

12. Pattern Matching and Regular Expressions with sed and awk

The basics of regular expressions

sed – noninteractive stream editor

Using awk

Summary

Index

Index

The IFS and loops

The shell has one environment variable, which is called the Internal Field Separator (IFS). This variable indicates how the words are separated on the command line. The IFS variable is, normally or by default, a white space (' '). The IFS variable is used as a word separator (token) for the for command. In many documents, IFS can be any one of the white space, ':', '|', ': ' or any other desired character. This will be useful while using commands such as read, set, for, and so on. If we are going to change the default IFS, then it is a good practice to store the original IFS in a variable.

Later on, when we have done our required tasks, then we can assign the original character back to IFS.

In the following script for_16.sh, we are using ":" as the IFS character:

#/bin/bash
cities=Delhi:Chennai:Bangaluru:Kolkata
old_ifs="$IFS"           # Saving original value of IFS
IFS=":"
for place in $cities
do
      echo  The name of city is $place
done

Let's test the program:

$ chmod...

CONTINUE READING

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Programming languages

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