book

Learning Linux Shell Scripting

Name: Learning Linux Shell Scripting
Author: Ganesh Sanjiv Naik
ISBN: 9781785286216

by Ganesh Sanjiv Naik

December 2015

Beginner

306 pages

5h 2m

English

Packt Publishing

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Learning Linux Shell Scripting
Table of Contents
Learning Linux Shell Scripting
Credits
About the Author
Acknowledgments
About the Reviewers
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Preface
What this book covers
What you need for this book

Who this book is for
Conventions
Reader feedback
Customer support
Downloading the color images of this bookErrataPiracyQuestions
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
Changing file permissionsCommand chmodTechnique one – the symbolic methodTechnique two – the numeric methodSetting umaskSetuidSetgidSticky bit
Summary
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
Text filtering toolsHead and tailThe diff commandThe cut commandThe paste commandThe join commandThe uniq commandThe comm commandThe tr command
IO redirection
File descriptorsRedirectionBrace expansion
Pattern matching with the vi editor
Pattern searching using grep
Summary
4. Working with Commands
Learning shell interpretation of commandsChecking and disabling shell internal commandsThe exit status
Command substitution
Command separators
Command1; command2Command grouping
Logical operators
Command1 & command2Command1 && command2Command1 || command2
Pipes
Summary
5. Exploring Expressions and Variables
Understanding variables
Working with environment variables
The local variable and its scopeExporting variables
Working with read-only variables
Working with command line arguments (special variables, set and shift, getopt)
Understanding setUnderstanding shiftResetting positional parameters
Understanding getopts
Understanding default parameters
Working with arrays
Creating an array and initializing itAccessing array values
Summary
6. Neat Tricks with Shell Scripting
Interactive Shell scripts – reading user inputSummarizing the read command with options
The here document and the << operator
The here operator with the sort commandThe here operator with the wc commandThe utility ed and here operatorA script for sending messages to all logged-in usersUsing the << here operator for FTP usage and data transferTurning off variable substitution
The here string and the <<< operator
File handling
Introducing file handlingUsing exec to assign file descriptor (fd) to fileUnderstanding the opening, writing, and closing of a fileUnderstanding reading from a fileUnderstanding reading and writing to a fileUsing command read on file descriptor (fd)Reading from one file and writing to another fileDisplaying the file descriptor information from the /proc folderFile handling – reading line by lineExecuting the command and storing the results in a fileSummarizing usage of the exec command
Debugging
Debugging mode – disabling the shell (option -n)Debugging mode – displaying commands (option -v)Debugging mode – the tracing execution (option -x)Summarizing the debugging options for the Bash shellUsing the set commandSummary of debugging options for set commandThe vi editor setting for debuggingGood practices for Shell scripts
Summary
7. Performing Arithmetic Operations in Shell Scripts
Using a command declare for arithmeticListing integers
Using the let command for arithmetic
Using the expr command for arithmetic
Using an arithmetic expansion
Binary, octal, and hex arithmetic operations
A floating-point arithmetic
Summary
8. Automating Decision Making in Scripts
Checking the exit status of commands
Understanding the test command
Using the test command with single bracketsUsing the test command with double bracketsString comparison options for the test commandNumerical comparison operators for the test commandFile test options for the test commandFile testing binary operatorsLogical test operators
Conditional constructs – if else
Numerical handling if constructsUsing the command exit and the ? variableString handling with the if constructChecking for null valuesFile handling with the if commandMultiple test commands and if constructsThe if/elif/else commandThe null command
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
Understanding functionsDisplaying functionsRemoving functions
Passing arguments or parameters to functions
Sharing the data by many functions
Declaring local variables in functions
Returning information from functions
Returning a word or string from a function
Running functions in the background
Command source and period (.)
Creating a library of functions
Summary
10. Using Advanced Functionality in Scripts
Understanding signals and traps
Using the trap command
Ignoring signals
Resetting signalsListing traps
Using traps in function
Running scripts or processes even if the user logs out
Creating dialog boxes with the dialog utility
Creating a message box (msgbox)Creating a message box (msgbox) with a titleThe yes/no box (yesno)The input box (inputbox)The textbox (textbox)A password boxThe menu box (menu)The checklist box (checklist)The radiolist box (radiolist)The progress meter box (gauge)
Summary
11. System Startup and Customizing a Linux System
System startup, inittab, and run levelsThe kernel startup and init processUnderstanding run levelsSystem initialization boot scripts
User initialization scripts
Systemwide settings scriptsUser level settings – default files
Summary
12. Pattern Matching and Regular Expressions with sed and awk
The basics of regular expressions
sed – noninteractive stream editor
Understanding sedUnderstanding regular expression usage in sedAddressing in sedHow to modify a file with sedPrinting – the p commandDeleting – the d commandSubstitution – the s commandRange of selected lines: the commaMultiple edits – the e commandReading from files – the r commandWriting to files – the w commandAppending – the a commandInserting – the i commandChanging – the c commandTransform – the y commandQuit – the q commandHolding and getting – the h and g commandsHolding and exchanging – the h and x commandssed scripting
Using awk
The meaning of awkUsing awkInput from filesInput from commandsHow awk worksawk commands from within a fileRecords and fieldsRecordsThe record separatorThe $0 variableThe NR variableFieldsField separatorsThe input field separatorPatterns and actionsPatternsActionsRegular expressionsWriting the awk script fileUsing variables in awkDecision making using an if statementUsing the for loopUsing the while loopUsing the do while loop
Summary
Index

Content preview from Learning Linux Shell Scripting

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 ...

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Learning Linux Shell Scripting

by Ganesh Sanjiv Naik

The IFS and loops

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