book

Linux Shell Scripting Essentials

Name: Linux Shell Scripting Essentials
Author: Sinny Kumari
ISBN: 9781785284441

by Sinny Kumari

November 2015

Beginner

282 pages

5h 5m

English

Packt Publishing

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Linux Shell Scripting Essentials
Table of Contents
Linux Shell Scripting Essentials
Credits
About the Author
About the Reviewers
www.PacktPub.com
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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 example codeErrataPiracyQuestions
1. The Beginning of the Scripting Journey
Hello World in shellInteracting with shellLet's make it scripted
Define variables of choice
NomenclatureAssigning a valueAccessing a valueConstant variablesReading variables from a user input
Builtin shell variables
Operators
The assignment operatorArithmetic operatorsLogical operatorsComparison operators
Shell expansions
~ (Tilde)* (Asterisk)? (Question mark)[ ] (Square brackets){ } (Curly brackets)
Construct commands using eval
Make bash behave using set
Exit on the first failureEnabling/disabling symbolic link's resolution pathSetting/unsetting variables
Summary
2. Getting Hands-on with I/O, Redirection Pipes, and Filters
Standard I/O and error streamsFile descriptors
Redirecting the standard I/O and error streams
Redirecting standard outputRedirecting standard inputRedirecting standard errorsMultiple redirection
Pipe and pipelines – connecting commands
PipePipeline
Regular expressions
Regular expression metacharactersCharacter ranges and classesCharacter rangesMatching dates in mm-dd-yyyy formatMatching a valid monthMatching a valid dayMatching the valid year in a dateCombining valid months, days, and years regex to form valid datesRegex for a valid shell variable
Filtering an output using grep
SyntaxLooking for a pattern in a fileLooking for a pattern in multiple filesA few more grep usagesSearching in a binary fileSearching in a directoryExcluding files/directories from a searchDisplay a filename with a matching patternMatching an exact word
Editing output using sed
String substitution using sMultiple substitutions
Duplicating a stream using tee
Writing an output to stdout and appending to a fileSending an output to multiple commands
Sorting and finding unique text
Sorting an input textSorting a single fileRedirecting output to sortFiltering unique elementsUnique elements in a file
Character-based translation using tr
Deleting input charactersSqueezing to a single occurrenceInverting a character set to be translated
Filtering based on lines—head and tail
Printing lines using headPrinting the first few linesPrinting the first few bytesPrinting lines using tailChecking log entriesFinding any line in a file
The Cut-based selection
Cutting across columnsText selection in files
Summary
3. Effective Script Writing
Exiting from scripts and exit codesExit codesExit codes with a special meaningScript with exit codes
Testing expressions with a test
File checksArithmetic checksString checksExpression checks
Using conditional statements with if and else
Simple if and elseThe if, elif, and else statementsNested if
Indexed arrays and associative arrays
Indexed arraysArray declaration and value assignmentOperations on arraysThe associative arrayThe declaration and value assignmentOperations on arrays
Looping around with for
Simple iterationIterating over a command outputSpecifying a range to the for loopSmall and sweet for loop
The select, while, and until loops
Loop using selectThe while loopThe until loop
Switch to my choice
Passing stdout as a parameter using xargs
Basic operations with xargsUsing xargs to find a file with the maximum sizeArchiving files with a given pattern
Using functions and positional parameters
Calling a function in bashPassing parameters to functions
Alias
Creating aliasListing all aliasesRemoving an alias
pushd and popd
Summary
4. Modularizing and Debugging
Modularizing your scriptsSource to a script fileSyntaxCreating a shell script libraryLoading a shell script libraryCalling a shell library in bashCalling shell library in another shell script
Passing command line parameters to script
Reading arguments in scriptsShifting command line argumentsProcessing command line options in a script
Debugging your scripts
Debugging using echoDebugging an entire script using -xDebugging sections of a script using the set options
Command completion
Managing bash completion with completeViewing the existing bash completionModifying default bash completion behaviorRemoving bash completion specificationWriting bash completion for your own applicationAn example of bash completionRunning the created bash completion
Summary
5. Customizing the Environment
Knowing the default environmentViewing a shell environmentprintenvenvDifferences between shell and environment variables
Modifying a shell environment
Creating environment variablesModifying environment variablesDeleting environment variables
Using bash startup files
.bashrc.bash_profile.bash_logout
Knowing your history
Shell variables controlling the historyThe history builtin commandModifying the default history behaviorHandy shortcuts for seeing the history[Ctrl + r]Up and down arrow key!!!(search_string)!?(search_string)
Task management
Running tasks in the backgroundSending a running task to the backgroundListing background tasksMoving tasks to the foregroundTerminating tasks
Summary
6. Working with Files
Performing basic file operationsCreating filesDirectory fileRegular fileTouch commandUsing the command line editorsUsing the cat commandRedirecting the command's outputModifying filesViewing filesViewing content using catmore and lessDeleting filesDeleting a regular fileDeleting a directory
Moving and copying files
Moving filesMoving a directory to a new locationRenaming a fileCopying filesCopying files locallyCopying a file to another locationCopying files remotelyCopying files to a remote server
Comparing files
Files comparison using diffExample
Finding files
Searching files according to use caseFinding and deleting a file based on inode number
Links to a file
Soft linkHard linkDifference between hard link and soft link
Special files
The block device fileNamed pipe fileSocket file
Temporary files
Creating a temporary file using mktemp
Permission and ownership
Viewing the ownership and permission of filesChanging permissionChanging the owner and groupChanging a file's ownerChanging group ownership
Getting the list of open files
Knowing the files opened by a specific applicationListing the applications that opened a fileKnowing the files opened by a user
Configuration files
Viewing and modifying configuration files
Summary
7. Welcome to the Processes
Process managementProcess creation and executionProcess terminationUsing the kill commandUsing the killall commandUsing the pkill command
Listing and monitoring processes
Listing processesSyntaxSimple process selectionProcess selection by listOutput format controlListing all processes with detailsListing all processes run by a userProcesses running in the current terminalListing processes by a command nameTree format display of processesMonitoring processes
Process substitution
Diffing the output of two processes
Process scheduling priorities
Changing scheduling prioritiesUsing niceUsing renice
Signals
Available signals
Traps
Inter-process communication
Information on IPC using ipcsListing information provided by IPCsKnowing processes' PID who recently did IPCs
Summary
8. Scheduling Tasks and Embedding Languages in Scripts
Running tasks at a specific timeExecuting scripts using atScheduling commandsScheduling a script fileListing scheduled tasksRemoving scheduled tasks
Cron jobs
Cron daemonCron configurationCrontab entriesSpecial strings in Crontab
Managing the crontab entry
Listing crontab entriesEditing crontab entriesRemoving crontab entries
systemd
systemd unitsManaging servicesStatus of a serviceEnabling and disabling servicesStart and stop a serviceViewing system logsViewing the latest log entriesViewing logs of a particular time interval
Embedding languages
Embedding Python languageEmbedding AWK language
Summary
Index

Content preview from Linux Shell Scripting Essentials

Pipe and pipelines – connecting commands

The outputs of the programs are generally saved in files for further use. Sometimes, temporary files are created in order to use an output of a program as an input to another program. We can avoid creating temporary files and feed the output of a program as an input to another program using bash pipe and pipelines.

Pipe

The pipe denoted by the operator | connects the standard output of a process in the left to the standard input in the right process by inter process communication mechanism. In other words, the | (pipe) connects commands by providing the output of a command as the input to another command.

Consider the following example:

$ cat /proc/cpuinfo | less

Here, the cat command, instead of displaying ...

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Linux Shell Scripting Essentials

by Sinny Kumari

Pipe and pipelines – connecting commands

Pipe

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