3. Effective Script Writing | Linux Shell Scripting Essentials

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Linux Shell Scripting Essentials

By : Sinny Kumari

4.5 (2)

Linux Shell Scripting Essentials

4.5 (2)

By: Sinny Kumari

Overview of this book

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.

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. The Beginning of the Scripting Journey

1. The Beginning of the Scripting Journey

Hello World in shell

Define variables of choice

Builtin shell variables

Operators

Shell expansions

Construct commands using eval

Make bash behave using set

Summary

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

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

Standard I/O and error streams

Redirecting the standard I/O and error streams

Pipe and pipelines – connecting commands

Regular expressions

Filtering an output using grep

Editing output using sed

Duplicating a stream using tee

Sorting and finding unique text

Character-based translation using tr

Filtering based on lines—head and tail

The Cut-based selection

Summary

3. Effective Script Writing

3. Effective Script Writing

Exiting from scripts and exit codes

Testing expressions with a test

Using conditional statements with if and else

Indexed arrays and associative arrays

Looping around with for

The select, while, and until loops

Switch to my choice

Passing stdout as a parameter using xargs

Using functions and positional parameters

Alias

pushd and popd

Summary

4. Modularizing and Debugging

4. Modularizing and Debugging

Modularizing your scripts

Passing command line parameters to script

Debugging your scripts

Command completion

Summary

5. Customizing the Environment

5. Customizing the Environment

Knowing the default environment

Modifying a shell environment

Using bash startup files

Knowing your history

Task management

Summary

6. Working with Files

6. Working with Files

Performing basic file operations

Moving and copying files

Comparing files

Finding files

Links to a file

Special files

Temporary files

Permission and ownership

Getting the list of open files

Configuration files

Summary

7. Welcome to the Processes

7. Welcome to the Processes

Process management

Listing and monitoring processes

Process substitution

Process scheduling priorities

Signals

Traps

Inter-process communication

Summary

8. Scheduling Tasks and Embedding Languages in Scripts

8. Scheduling Tasks and Embedding Languages in Scripts

Running tasks at a specific time

Cron jobs

Managing the crontab entry

systemd

Embedding languages

Summary

Index

Index

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Looping around with for

The for loop can be used to iterate over the items in a list or till the condition is true.

The syntax of using the for loop in bash is as follows:

for item in [list]
do
   #Tasks
done

Another way of writing the for loop is the way C does, as follows:

for (( expr1; expr2; expr3 ))
  # Tasks
done

Here, expr1 is initialization, expr2 is condition, and expr3 is increment.

Simple iteration

The following shell script explains how we can use the for loop to print the values of a list:

#!/bin/bash
# Filename: for_loop.sh
# Description: Basic for loop in bash

declare -a names=(Foo Bar Tom Jerry)
echo "Content of names array is:"
for name in ${names[@]}
do
   echo -n "$name "
done
echo

The output of the script is as follows:

Content of names array is:
Foo Bar Tom Jerry

Iterating over a command output

We know that a lot of commands give multiline output such as ls, cat, grep, and so on. In many cases, it makes sense to loop over each line of output and do further processing on them.

The...

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