book

Learning the bash Shell, Second Edition

by Cameron Newham, Bill Rosenblatt

January 1998

Beginner

336 pages

10h 6m

English

O'Reilly Media, Inc.

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bash Versions

What Is a Shell?
The Bourne Again ShellFeatures of bash
Commands, Arguments, and Options
DirectoriesThe working directoryTilde notationChanging working directoriesFilenames, Wildcards, and Pathname ExpansionBrace Expansion
Standard I/OI/O RedirectionPipelines
Background I/OBackground Jobs and Priorities
QuotingBackslash-EscapingQuoting Quotation MarksContinuing LinesControl Keys
Enabling Command-Line Editing
Basic CommandsWord CommandsLine CommandsMoving Around in the History FileTextual CompletionMiscellaneous Commands
Simple Control Mode CommandsEntering and Changing TextDeletion CommandsMoving Around in the History FileCharacter-Finding CommandsTextual CompletionMiscellaneous Commands
The readline Startup Filereadline variablesKey Bindings Using bind
The .bash_profile, .bash_logout, and .bashrc Files
shopt
Variables and QuotingBuilt-In VariablesEditing mode variablesMail variablesPrompting variablesCommand search pathCommand hashingDirectory search path and variablesMiscellaneous variables
Environment VariablesTerminal typesOther common variablesThe Environment File
Shell Scripts and FunctionsFunctions
Positional ParametersPositional parameters in functionsLocal Variables in FunctionsQuoting with $@ and $*More on Variable Syntax
Syntax of String OperatorsPatterns and Pattern MatchingLength Operator
if/elseExit Status and ReturnReturnCombinations of Exit StatusesCondition TestsString comparisonsFile attribute checkingInteger Conditionals
Command-Line OptionsshiftOptions with Argumentsgetopts
Arithmetic ConditionalsArithmetic Variables and Assignment
I/O RedirectorsHere-documentsFile Descriptors
echoOptions to echoecho escape sequencesreadReading lines from filesI/O redirection and multiple commandsCommand blocksReading user input
Quotingcommand, builtin, and enableeval
Process IDs and Job Numbers
Foreground and BackgroundSuspending a Job
Control-Key SignalskillpsSystem VBSD
Traps and FunctionsProcess ID Variables and Temporary FilesIgnoring SignalsdisownResetting Traps
waitAdvantages and Disadvantages of CoroutinesParallelization
Subshell InheritanceNested Subshells
Basic Debugging AidsSet OptionsFake SignalsEXITDEBUG
Structure of the DebuggerThe driver scriptexecThe PreambleDebugger FunctionsCommandsSteppingBreakpointsBreak conditionsExecution tracingDebugger limitationsA Sample bashdb SessionExercises
Installing bash as the Standard ShellPOSIX ModeCommand-Line Options
umaskulimitTypes of Global Customization
Restricted ShellA System Break-In ScenarioPrivileged Mode
Obtaining bash
DocumentationConfiguring and Building bashTesting bashPotential ProblemsInstalling bash as a Login ShellExamples
Asking QuestionsReporting Bugs
The Bourne Shell
Invocation
Reserved Words
FTP

Content preview from Learning the bash Shell, Second Edition

String Operators

The curly-bracket syntax allows for the shell’s string operators. String operators allow you to manipulate values of variables in various useful ways without having to write full-blown programs or resort to external UNIX utilities. You can do a lot with string-handling operators even if you haven’t yet mastered the programming features we’ll see in later chapters.

In particular, string operators let you do the following:

Ensure that variables exist (i.e., are defined and have non-null values)
Set default values for variables
Catch errors that result from variables not being set
Remove portions of variables’ values that match patterns

Syntax of String Operators

The basic idea behind the syntax of string operators is that special characters that denote operations are inserted between the variable’s name and the right curly bracket. Any argument that the operator may need is inserted to the operator’s right.

The first group of string-handling operators tests for the existence of variables and allows substitutions of default values under certain conditions. These are listed in Table 4.1. ^[51]

Table 4-1. Substitution Operators

Operator	Substitution
$ { varname :- word }	If varname exists and isn’t null, return its value; otherwise return word.
Purpose:	Returning a default value if the variable is undefined.
Example:	${count:-0} evaluates to 0 if count is undefined.
$ { varname := word}	If varname exists and isn’t null, return its value; otherwise set it to word