book

Learning Linux Shell Scripting

by Ganesh Sanjiv Naik

December 2015

Beginner

306 pages

5h 2m

English

Packt Publishing

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What this book covers

Downloading the color images of this bookErrataPiracyQuestions
Comparison of shells
Changing file permissionsCommand chmodTechnique one – the symbolic methodTechnique two – the numeric methodSetting umaskSetuidSetgidSticky bit
Introducing process basics
Text filtering toolsHead and tailThe diff commandThe cut commandThe paste commandThe join commandThe uniq commandThe comm commandThe tr command
File descriptorsRedirectionBrace expansion
Learning shell interpretation of commandsChecking and disabling shell internal commandsThe exit status
Command1; command2Command grouping
Command1 & command2Command1 && command2Command1 || command2
Understanding variables
The local variable and its scopeExporting variables
Understanding setUnderstanding shiftResetting positional parameters
Creating an array and initializing itAccessing array values
Interactive Shell scripts – reading user inputSummarizing the read command with options
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
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 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
Using a command declare for arithmeticListing integers
Using an arithmetic expansion
Checking the exit status of commands
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
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
Understanding functionsDisplaying functionsRemoving functions
Returning a word or string from a function
Command source and period (.)
Understanding signals and traps
Resetting signalsListing traps
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)
System startup, inittab, and run levelsThe kernel startup and init processUnderstanding run levelsSystem initialization boot scripts
Systemwide settings scriptsUser level settings – default files
The basics of regular expressions
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
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

Content preview from Learning Linux Shell Scripting

Chapter 8. Automating Decision Making in Scripts

In the last chapter, you learned about performing arithmetic operations in various ways such as using declare, let, expr, and arithmetic expressions. You also learned about representing numbers in different bases such as hex, octal, and binary, and using the bc utility for performing floating point or fractional arithmetic.

In real-world scripts, it is not just a sequential execution of commands, we need to check certain conditions or proceed as per certain logic and accordingly the script should continue executing. This is precisely what we do with automation. Automation refers to performing tasks, the sequence of which will change as per changes in the programming environment. A simple example ...