Book Image

Learning Linux Shell Scripting

By : Ganesh Sanjiv Naik
Book Image

Learning Linux Shell Scripting

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.

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Learning Linux Shell Scripting
Ganesh Sanjiv Naik
Dec, 2015 | 306 pages
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Table of Contents (20 chapters)
Learning Linux Shell Scripting
Learning Linux Shell Scripting
Credits
Credits
About the Author
About the Author
Acknowledgments
Acknowledgments
About the Reviewers
About the Reviewers
www.PacktPub.com
www.PacktPub.com
Preface
Preface
Free Chapter
1
Getting Started and Working with Shell Scripting
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
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
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
Working with Commands
Learning shell interpretation of commands
Command substitution
Command separators
Logical operators
Pipes
Summary
5
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
Summary
6
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
File handling
Debugging
Summary
7
Performing Arithmetic Operations in Shell Scripts
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
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
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
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
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
Pattern Matching and Regular Expressions with sed and awk
The basics of regular expressions
sed – noninteractive stream editor
Using awk
Summary
Index
Index

The here document and the << operator

It is a special type of block of text or code. It is also a special form of I/O redirection. It can be used to feed the command list to an interactive program.

The syntax of the usage of the here document or the << operator is as follows:

command << HERE
text1 …..
text 2….

HERE

This tells the shell that the command should receive the data from a current source, such as the here document, until the pattern is received. In this case, the pattern is HERE. We have used the delimiter as HERE. We can use any other word as the delimiter, such as quite or finish. All the text reads up to a pattern; or the HERE text is used as an input for command. The text or file received by the command is called as the Here document:

$ cat << QUIT
> first input line
> ...
> last input line
> QUIT

The block of text inserted after and before QUIT will be treated as a file. This content will be given as input to the command cat. We will also...

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