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.

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Table of Contents (17 chapters)
Preface
Preface
Who this book is for
What this book covers
To get the most out of this book
Get in touch
Free Chapter
1
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
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 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
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
Summary
9
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
Using until
Piping the output of a loop to a Linux command
Running loops in the background
The IFS and loops
Summary
10
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
Summary
11
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
Summary
12
System Startup and Customizing a Linux System
System Startup and Customizing a Linux System
System startup, inittab, and run levels
User initialization scripts
Summary
13
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
Using awk
Summary
14
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
Summary
15
Database Administration Using Shell Scripts
Database Administration Using Shell Scripts
Introduction to database administration
Working with a MySQL Database
Working with Oracle Database
Summary
16
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Understanding crontab

If we need to run a specific task repetitively, then the solution is to use crontab. The syntax of the command is as follows:

    $ crontab -e

This will open a new editor. The following diagram is the syntax to add tasks. The fields to use for repeating tasks at a particular time are explained here:

Finally, to save the jobs, use the following:

    Press Esc then type :wq

The preceding operations will save the job and quit crontab.

The following are a few examples of the crontab command:

  • Use the following command to run a script every hour at the fifth minute, every day:
    5 * * * *      $HOME/bin/daily.job >> $HOME/tmp/out  2>&1
  • Use the following command to run 5 minutes after midnight every day:
    5 0 * * *      $HOME/bin/daily.job >> $HOME/tmp/out  2>&1
  • Use the following command to run at 2.15 p.m. on the first of...
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