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<h1 class="settitle" align="center">Bash2py Reference Manual</h1>
<a name="SEC_Contents"></a>
<h2 class="contents-heading">Table of Contents</h2>
<div class="contents">
<ul class="no-bullet">
<li><a name="toc-Introduction-1" href="#Introduction">1 Introduction</a>
<ul class="no-bullet">
<li><a name="toc-What-is-Bash_003f-1" href="#What-is-Bash_003f">1.1 What is Bash2py?</a></li>
<li><a name="toc-What-is-a-shell_003f-1" href="#What-is-a-shell_003f">1.2 What is a shell?</a></li>
</ul></li>
<li><a name="toc-Definitions-1" href="#Definitions">2 Definitions</a></li>
<li><a name="toc-Basic-Shell-Features-1" href="#Basic-Shell-Features">3 Basic Shell Features</a>
<ul class="no-bullet">
<li><a name="toc-Shell-Syntax-1" href="#Shell-Syntax">3.1 Shell Syntax</a>
<ul class="no-bullet">
<li><a name="toc-Shell-Operation-1" href="#Shell-Operation">3.1.1 Shell Operation</a></li>
<li><a name="toc-Quoting-1" href="#Quoting">3.1.2 Quoting</a>
<ul class="no-bullet">
<li><a name="toc-Escape-Character-1" href="#Escape-Character">3.1.2.1 Escape Character</a></li>
<li><a name="toc-Single-Quotes-1" href="#Single-Quotes">3.1.2.2 Single Quotes</a></li>
<li><a name="toc-Double-Quotes-1" href="#Double-Quotes">3.1.2.3 Double Quotes</a></li>
<li><a name="toc-ANSI_002dC-Quoting-1" href="#ANSI_002dC-Quoting">3.1.2.4 ANSI-C Quoting</a></li>
<li><a name="toc-Locale_002dSpecific-Translation" href="#Locale-Translation">3.1.2.5 Locale-Specific Translation</a></li>
</ul></li>
<li><a name="toc-Comments-1" href="#Comments">3.1.3 Comments</a></li>
</ul></li>
<li><a name="toc-Shell-Commands-1" href="#Shell-Commands">3.2 Shell Commands</a>
<ul class="no-bullet">
<li><a name="toc-Simple-Commands-1" href="#Simple-Commands">3.2.1 Simple Commands</a></li>
<li><a name="toc-Pipelines-1" href="#Pipelines">3.2.2 Pipelines</a></li>
<li><a name="toc-Lists-of-Commands" href="#Lists">3.2.3 Lists of Commands</a></li>
<li><a name="toc-Compound-Commands-1" href="#Compound-Commands">3.2.4 Compound Commands</a>
<ul class="no-bullet">
<li><a name="toc-Looping-Constructs-1" href="#Looping-Constructs">3.2.4.1 Looping Constructs</a></li>
<li><a name="toc-Conditional-Constructs-1" href="#Conditional-Constructs">3.2.4.2 Conditional Constructs</a></li>
<li><a name="toc-Grouping-Commands" href="#Command-Grouping">3.2.4.3 Grouping Commands</a></li>
</ul></li>
<li><a name="toc-Coprocesses-1" href="#Coprocesses">3.2.5 Coprocesses</a></li>
<li><a name="toc-GNU-Parallel-1" href="#GNU-Parallel">3.2.6 GNU Parallel</a></li>
</ul></li>
<li><a name="toc-Shell-Functions-1" href="#Shell-Functions">3.3 Shell Functions</a></li>
<li><a name="toc-Shell-Parameters-1" href="#Shell-Parameters">3.4 Shell Parameters</a>
<ul class="no-bullet">
<li><a name="toc-Positional-Parameters-1" href="#Positional-Parameters">3.4.1 Positional Parameters</a></li>
<li><a name="toc-Special-Parameters-1" href="#Special-Parameters">3.4.2 Special Parameters</a></li>
</ul></li>
<li><a name="toc-Shell-Expansions-1" href="#Shell-Expansions">3.5 Shell Expansions</a>
<ul class="no-bullet">
<li><a name="toc-Brace-Expansion-1" href="#Brace-Expansion">3.5.1 Brace Expansion</a></li>
<li><a name="toc-Tilde-Expansion-1" href="#Tilde-Expansion">3.5.2 Tilde Expansion</a></li>
<li><a name="toc-Shell-Parameter-Expansion-1" href="#Shell-Parameter-Expansion">3.5.3 Shell Parameter Expansion</a></li>
<li><a name="toc-Command-Substitution-1" href="#Command-Substitution">3.5.4 Command Substitution</a></li>
<li><a name="toc-Arithmetic-Expansion-1" href="#Arithmetic-Expansion">3.5.5 Arithmetic Expansion</a></li>
<li><a name="toc-Process-Substitution-1" href="#Process-Substitution">3.5.6 Process Substitution</a></li>
<li><a name="toc-Word-Splitting-1" href="#Word-Splitting">3.5.7 Word Splitting</a></li>
<li><a name="toc-Filename-Expansion-1" href="#Filename-Expansion">3.5.8 Filename Expansion</a>
<ul class="no-bullet">
<li><a name="toc-Pattern-Matching-1" href="#Pattern-Matching">3.5.8.1 Pattern Matching</a></li>
</ul></li>
<li><a name="toc-Quote-Removal-1" href="#Quote-Removal">3.5.9 Quote Removal</a></li>
</ul></li>
<li><a name="toc-Redirections-1" href="#Redirections">3.6 Redirections</a>
<ul class="no-bullet">
<li><a name="toc-Redirecting-Input" href="#Redirecting-Input">3.6.1 Redirecting Input</a></li>
<li><a name="toc-Redirecting-Output" href="#Redirecting-Output">3.6.2 Redirecting Output</a></li>
<li><a name="toc-Appending-Redirected-Output" href="#Appending-Redirected-Output">3.6.3 Appending Redirected Output</a></li>
<li><a name="toc-Redirecting-Standard-Output-and-Standard-Error" href="#Redirecting-Standard-Output-and-Standard-Error">3.6.4 Redirecting Standard Output and Standard Error</a></li>
<li><a name="toc-Appending-Standard-Output-and-Standard-Error" href="#Appending-Standard-Output-and-Standard-Error">3.6.5 Appending Standard Output and Standard Error</a></li>
<li><a name="toc-Here-Documents" href="#Here-Documents">3.6.6 Here Documents</a></li>
<li><a name="toc-Here-Strings" href="#Here-Strings">3.6.7 Here Strings</a></li>
<li><a name="toc-Duplicating-File-Descriptors" href="#Duplicating-File-Descriptors">3.6.8 Duplicating File Descriptors</a></li>
<li><a name="toc-Moving-File-Descriptors" href="#Moving-File-Descriptors">3.6.9 Moving File Descriptors</a></li>
<li><a name="toc-Opening-File-Descriptors-for-Reading-and-Writing" href="#Opening-File-Descriptors-for-Reading-and-Writing">3.6.10 Opening File Descriptors for Reading and Writing</a></li>
</ul></li>
<li><a name="toc-Executing-Commands-1" href="#Executing-Commands">3.7 Executing Commands</a>
<ul class="no-bullet">
<li><a name="toc-Simple-Command-Expansion-1" href="#Simple-Command-Expansion">3.7.1 Simple Command Expansion</a></li>
<li><a name="toc-Command-Search-and-Execution-1" href="#Command-Search-and-Execution">3.7.2 Command Search and Execution</a></li>
<li><a name="toc-Command-Execution-Environment-1" href="#Command-Execution-Environment">3.7.3 Command Execution Environment</a></li>
<li><a name="toc-Environment-1" href="#Environment">3.7.4 Environment</a></li>
<li><a name="toc-Exit-Status-1" href="#Exit-Status">3.7.5 Exit Status</a></li>
<li><a name="toc-Signals-1" href="#Signals">3.7.6 Signals</a></li>
</ul></li>
<li><a name="toc-Shell-Scripts-1" href="#Shell-Scripts">3.8 Shell Scripts</a></li>
</ul></li>
<li><a name="toc-Shell-Builtin-Commands-1" href="#Shell-Builtin-Commands">4 Shell Builtin Commands</a>
<ul class="no-bullet">
<li><a name="toc-Bourne-Shell-Builtins-1" href="#Bourne-Shell-Builtins">4.1 Bourne Shell Builtins</a></li>
<li><a name="toc-Bash-Builtin-Commands" href="#Bash-Builtins">4.2 Bash Builtin Commands</a></li>
<li><a name="toc-Modifying-Shell-Behavior-1" href="#Modifying-Shell-Behavior">4.3 Modifying Shell Behavior</a>
<ul class="no-bullet">
<li><a name="toc-The-Set-Builtin-1" href="#The-Set-Builtin">4.3.1 The Set Builtin</a></li>
<li><a name="toc-The-Shopt-Builtin-1" href="#The-Shopt-Builtin">4.3.2 The Shopt Builtin</a></li>
</ul></li>
<li><a name="toc-Special-Builtins-1" href="#Special-Builtins">4.4 Special Builtins</a></li>
</ul></li>
<li><a name="toc-Shell-Variables-1" href="#Shell-Variables">5 Shell Variables</a>
<ul class="no-bullet">
<li><a name="toc-Bourne-Shell-Variables-1" href="#Bourne-Shell-Variables">5.1 Bourne Shell Variables</a></li>
<li><a name="toc-Bash-Variables-1" href="#Bash-Variables">5.2 Bash Variables</a></li>
</ul></li>
<li><a name="toc-Bash-Features-2" href="#Bash-Features">6 Bash Features</a>
<ul class="no-bullet">
<li><a name="toc-Invoking-Bash-1" href="#Invoking-Bash">6.1 Invoking Bash</a></li>
<li><a name="toc-Bash-Startup-Files-1" href="#Bash-Startup-Files">6.2 Bash Startup Files</a></li>
<li><a name="toc-Interactive-Shells-1" href="#Interactive-Shells">6.3 Interactive Shells</a>
<ul class="no-bullet">
<li><a name="toc-What-is-an-Interactive-Shell_003f-1" href="#What-is-an-Interactive-Shell_003f">6.3.1 What is an Interactive Shell?</a></li>
<li><a name="toc-Is-this-Shell-Interactive_003f-1" href="#Is-this-Shell-Interactive_003f">6.3.2 Is this Shell Interactive?</a></li>
<li><a name="toc-Interactive-Shell-Behavior-1" href="#Interactive-Shell-Behavior">6.3.3 Interactive Shell Behavior</a></li>
</ul></li>
<li><a name="toc-Bash-Conditional-Expressions-1" href="#Bash-Conditional-Expressions">6.4 Bash Conditional Expressions</a></li>
<li><a name="toc-Shell-Arithmetic-1" href="#Shell-Arithmetic">6.5 Shell Arithmetic</a></li>
<li><a name="toc-Aliases-1" href="#Aliases">6.6 Aliases</a></li>
<li><a name="toc-Arrays-1" href="#Arrays">6.7 Arrays</a></li>
<li><a name="toc-The-Directory-Stack-1" href="#The-Directory-Stack">6.8 The Directory Stack</a>
<ul class="no-bullet">
<li><a name="toc-Directory-Stack-Builtins-1" href="#Directory-Stack-Builtins">6.8.1 Directory Stack Builtins</a></li>
</ul></li>
<li><a name="toc-Controlling-the-Prompt" href="#Printing-a-Prompt">6.9 Controlling the Prompt</a></li>
<li><a name="toc-The-Restricted-Shell-1" href="#The-Restricted-Shell">6.10 The Restricted Shell</a></li>
<li><a name="toc-Bash-POSIX-Mode-1" href="#Bash-POSIX-Mode">6.11 Bash POSIX Mode</a></li>
</ul></li>
<li><a name="toc-Job-Control-1" href="#Job-Control">7 Job Control</a>
<ul class="no-bullet">
<li><a name="toc-Job-Control-Basics-1" href="#Job-Control-Basics">7.1 Job Control Basics</a></li>
<li><a name="toc-Job-Control-Builtins-1" href="#Job-Control-Builtins">7.2 Job Control Builtins</a></li>
<li><a name="toc-Job-Control-Variables-1" href="#Job-Control-Variables">7.3 Job Control Variables</a></li>
</ul></li>
<li><a name="toc-Command-Line-Editing-1" href="#Command-Line-Editing">8 Command Line Editing</a>
<ul class="no-bullet">
<li><a name="toc-Introduction-to-Line-Editing" href="#Introduction-and-Notation">8.1 Introduction to Line Editing</a></li>
<li><a name="toc-Readline-Interaction-1" href="#Readline-Interaction">8.2 Readline Interaction</a>
<ul class="no-bullet">
<li><a name="toc-Readline-Bare-Essentials-1" href="#Readline-Bare-Essentials">8.2.1 Readline Bare Essentials</a></li>
<li><a name="toc-Readline-Movement-Commands-1" href="#Readline-Movement-Commands">8.2.2 Readline Movement Commands</a></li>
<li><a name="toc-Readline-Killing-Commands-1" href="#Readline-Killing-Commands">8.2.3 Readline Killing Commands</a></li>
<li><a name="toc-Readline-Arguments-1" href="#Readline-Arguments">8.2.4 Readline Arguments</a></li>
<li><a name="toc-Searching-for-Commands-in-the-History" href="#Searching">8.2.5 Searching for Commands in the History</a></li>
</ul></li>
<li><a name="toc-Readline-Init-File-1" href="#Readline-Init-File">8.3 Readline Init File</a>
<ul class="no-bullet">
<li><a name="toc-Readline-Init-File-Syntax-1" href="#Readline-Init-File-Syntax">8.3.1 Readline Init File Syntax</a></li>
<li><a name="toc-Conditional-Init-Constructs-1" href="#Conditional-Init-Constructs">8.3.2 Conditional Init Constructs</a></li>
<li><a name="toc-Sample-Init-File-1" href="#Sample-Init-File">8.3.3 Sample Init File</a></li>
</ul></li>
<li><a name="toc-Bindable-Readline-Commands-1" href="#Bindable-Readline-Commands">8.4 Bindable Readline Commands</a>
<ul class="no-bullet">
<li><a name="toc-Commands-For-Moving-1" href="#Commands-For-Moving">8.4.1 Commands For Moving</a></li>
<li><a name="toc-Commands-For-Manipulating-The-History" href="#Commands-For-History">8.4.2 Commands For Manipulating The History</a></li>
<li><a name="toc-Commands-For-Changing-Text" href="#Commands-For-Text">8.4.3 Commands For Changing Text</a></li>
<li><a name="toc-Killing-And-Yanking" href="#Commands-For-Killing">8.4.4 Killing And Yanking</a></li>
<li><a name="toc-Specifying-Numeric-Arguments" href="#Numeric-Arguments">8.4.5 Specifying Numeric Arguments</a></li>
<li><a name="toc-Letting-Readline-Type-For-You" href="#Commands-For-Completion">8.4.6 Letting Readline Type For You</a></li>
<li><a name="toc-Keyboard-Macros-1" href="#Keyboard-Macros">8.4.7 Keyboard Macros</a></li>
<li><a name="toc-Some-Miscellaneous-Commands" href="#Miscellaneous-Commands">8.4.8 Some Miscellaneous Commands</a></li>
</ul></li>
<li><a name="toc-Readline-vi-Mode-1" href="#Readline-vi-Mode">8.5 Readline vi Mode</a></li>
<li><a name="toc-Programmable-Completion-1" href="#Programmable-Completion">8.6 Programmable Completion</a></li>
<li><a name="toc-Programmable-Completion-Builtins-1" href="#Programmable-Completion-Builtins">8.7 Programmable Completion Builtins</a></li>
</ul></li>
<li><a name="toc-Using-History-Interactively-1" href="#Using-History-Interactively">9 Using History Interactively</a>
<ul class="no-bullet">
<li><a name="toc-Bash-History-Facilities-1" href="#Bash-History-Facilities">9.1 Bash History Facilities</a></li>
<li><a name="toc-Bash-History-Builtins-1" href="#Bash-History-Builtins">9.2 Bash History Builtins</a></li>
<li><a name="toc-History-Expansion" href="#History-Interaction">9.3 History Expansion</a>
<ul class="no-bullet">
<li><a name="toc-Event-Designators-1" href="#Event-Designators">9.3.1 Event Designators</a></li>
<li><a name="toc-Word-Designators-1" href="#Word-Designators">9.3.2 Word Designators</a></li>
<li><a name="toc-Modifiers-1" href="#Modifiers">9.3.3 Modifiers</a></li>
</ul></li>
</ul></li>
<li><a name="toc-Installing-Bash-1" href="#Installing-Bash">10 Installing Bash</a>
<ul class="no-bullet">
<li><a name="toc-Basic-Installation-1" href="#Basic-Installation">10.1 Basic Installation</a></li>
<li><a name="toc-Compilers-and-Options-1" href="#Compilers-and-Options">10.2 Compilers and Options</a></li>
<li><a name="toc-Compiling-For-Multiple-Architectures-1" href="#Compiling-For-Multiple-Architectures">10.3 Compiling For Multiple Architectures</a></li>
<li><a name="toc-Installation-Names-1" href="#Installation-Names">10.4 Installation Names</a></li>
<li><a name="toc-Specifying-the-System-Type-1" href="#Specifying-the-System-Type">10.5 Specifying the System Type</a></li>
<li><a name="toc-Sharing-Defaults-1" href="#Sharing-Defaults">10.6 Sharing Defaults</a></li>
<li><a name="toc-Operation-Controls-1" href="#Operation-Controls">10.7 Operation Controls</a></li>
<li><a name="toc-Optional-Features-1" href="#Optional-Features">10.8 Optional Features</a></li>
</ul></li>
<li><a name="toc-Reporting-Bugs-1" href="#Reporting-Bugs">Appendix A Reporting Bugs</a></li>
<li><a name="toc-Major-Differences-From-The-Bourne-Shell-1" href="#Major-Differences-From-The-Bourne-Shell">Appendix B Major Differences From The Bourne Shell</a>
<ul class="no-bullet">
<li><a name="toc-Implementation-Differences-From-The-SVR4_002e2-Shell" href="#Implementation-Differences-From-The-SVR4_002e2-Shell">B.1 Implementation Differences From The SVR4.2 Shell</a></li>
</ul></li>
<li><a name="toc-GNU-Free-Documentation-License-1" href="#GNU-Free-Documentation-License">Appendix C GNU Free Documentation License</a></li>
<li><a name="toc-Indexes-1" href="#Indexes">Appendix D Indexes</a>
<ul class="no-bullet">
<li><a name="toc-Index-of-Shell-Builtin-Commands" href="#Builtin-Index">D.1 Index of Shell Builtin Commands</a></li>
<li><a name="toc-Index-of-Shell-Reserved-Words" href="#Reserved-Word-Index">D.2 Index of Shell Reserved Words</a></li>
<li><a name="toc-Parameter-and-Variable-Index" href="#Variable-Index">D.3 Parameter and Variable Index</a></li>
<li><a name="toc-Function-Index-1" href="#Function-Index">D.4 Function Index</a></li>
<li><a name="toc-Concept-Index-1" href="#Concept-Index">D.5 Concept Index</a></li>
</ul></li>
</ul>
</div>
<a name="Top"></a>
<div class="header">
<p>
Next: <a href="#Introduction" accesskey="n" rel="next">Introduction</a>, Previous: <a href="dir.html#Top" accesskey="p" rel="previous">(dir)</a>, Up: <a href="dir.html#Top" accesskey="u" rel="up">(dir)</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Bash-Features-1"></a>
<h1 class="top">Bash Features</h1>
<p>This text is a brief description of the features that are present in
the Bash shell (version 4.2, 28 December 2010).
</p>
<p>This is Edition 4.2, last updated 28 December 2010,
of <cite>The GNU Bash Reference Manual</cite>,
for <code>Bash</code>, Version 4.2.
</p>
<p>Bash contains features that appear in other popular shells, and some
features that only appear in Bash. Some of the shells that Bash has
borrowed concepts from are the Bourne Shell (<samp>sh</samp>), the Korn Shell
(<samp>ksh</samp>), and the C-shell (<samp>csh</samp> and its successor,
<samp>tcsh</samp>). The following menu breaks the features up into
categories based upon which one of these other shells inspired the
feature.
</p>
<p>This manual is meant as a brief introduction to features found in
Bash. The Bash manual page should be used as the definitive
reference on shell behavior.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">• <a href="#Introduction" accesskey="1">Introduction</a>:</td><td> </td><td align="left" valign="top">An introduction to the shell.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Definitions" accesskey="2">Definitions</a>:</td><td> </td><td align="left" valign="top">Some definitions used in the rest of this
manual.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Basic-Shell-Features" accesskey="3">Basic Shell Features</a>:</td><td> </td><td align="left" valign="top">The shell "building blocks".
</td></tr>
<tr><td align="left" valign="top">• <a href="#Shell-Builtin-Commands" accesskey="4">Shell Builtin Commands</a>:</td><td> </td><td align="left" valign="top">Commands that are a part of the shell.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Shell-Variables" accesskey="5">Shell Variables</a>:</td><td> </td><td align="left" valign="top">Variables used or set by Bash.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Bash-Features" accesskey="6">Bash Features</a>:</td><td> </td><td align="left" valign="top">Features found only in Bash.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Job-Control" accesskey="7">Job Control</a>:</td><td> </td><td align="left" valign="top">What job control is and how Bash allows you
to use it.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Command-Line-Editing" accesskey="8">Command Line Editing</a>:</td><td> </td><td align="left" valign="top">Chapter describing the command line
editing features.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Using-History-Interactively" accesskey="9">Using History Interactively</a>:</td><td> </td><td align="left" valign="top">Command History Expansion
</td></tr>
<tr><td align="left" valign="top">• <a href="#Installing-Bash">Installing Bash</a>:</td><td> </td><td align="left" valign="top">How to build and install Bash on your system.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Reporting-Bugs">Reporting Bugs</a>:</td><td> </td><td align="left" valign="top">How to report bugs in Bash.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Major-Differences-From-The-Bourne-Shell">Major Differences From The Bourne Shell</a>:</td><td> </td><td align="left" valign="top">A terse list of the differences
between Bash and historical
versions of /bin/sh.
</td></tr>
<tr><td align="left" valign="top">• <a href="#GNU-Free-Documentation-License">GNU Free Documentation License</a>:</td><td> </td><td align="left" valign="top">Copying and sharing this documentation.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Indexes">Indexes</a>:</td><td> </td><td align="left" valign="top">Various indexes for this manual.
</td></tr>
</table>
<hr>
<a name="Introduction"></a>
<div class="header">
<p>
Next: <a href="#Definitions" accesskey="n" rel="next">Definitions</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Introduction-1"></a>
<h2 class="chapter">1 Introduction</h2>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">• <a href="#What-is-Bash_003f" accesskey="1">What is Bash2py?</a>:</td><td> </td><td align="left" valign="top">A short description of Bash2py.
</td></tr>
<tr><td align="left" valign="top">• <a href="#What-is-a-shell_003f" accesskey="2">What is a shell?</a>:</td><td> </td><td align="left" valign="top">A brief introduction to shells.
</td></tr>
</table>
<hr>
<a name="What-is-Bash_003f"></a>
<div class="header">
<p>
Next: <a href="#What-is-a-shell_003f" accesskey="n" rel="next">What is a shell?</a>, Up: <a href="#Introduction" accesskey="u" rel="up">Introduction</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="What-is-Bash_003f-1"></a>
<h3 class="section">1.1 What is Bash2py?</h3>
<p>
<font color=purple>
Bash2py is a rough and ready translator from Bash to Python. Python is
generally considered superior to Bash, and it is therefore desirable to
be able to migrate from legacy Bash code, to Python.
</font>
<p>
<p>Bash is the shell, or command language interpreter,
for the <small>GNU</small> operating system.
The name is an acronym for the ‘<samp>Bourne-Again SHell</samp>’,
a pun on Stephen Bourne, the author of the direct ancestor of
the current Unix shell <code>sh</code>,
which appeared in the Seventh Edition Bell Labs Research version
of Unix.
</p>
<p>Bash is largely compatible with <code>sh</code> and incorporates useful
features from the Korn shell <code>ksh</code> and the C shell <code>csh</code>.
It is intended to be a conformant implementation of the <small>IEEE</small>
<small>POSIX</small> Shell and Tools portion of the <small>IEEE</small> <small>POSIX</small>
specification (<small>IEEE</small> Standard 1003.1).
It offers functional improvements over <code>sh</code> for both interactive and
programming use.
</p>
<p>While the <small>GNU</small> operating system provides other shells, including
a version of <code>csh</code>, Bash is the default shell.
Like other <small>GNU</small> software, Bash is quite portable. It currently runs
on nearly every version of Unix and a few other operating systems -
independently-supported ports exist for <small>MS-DOS</small>, <small>OS/2</small>,
and Windows platforms.
</p>
<hr>
<a name="What-is-a-shell_003f"></a>
<div class="header">
<p>
Previous: <a href="#What-is-Bash_003f" accesskey="p" rel="previous">What is Bash?</a>, Up: <a href="#Introduction" accesskey="u" rel="up">Introduction</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="What-is-a-shell_003f-1"></a>
<h3 class="section">1.2 What is a shell?</h3>
<p>At its base, a shell is simply a macro processor that executes
commands. The term macro processor means functionality where text
and symbols are expanded to create larger expressions.
</p>
<p>A Unix shell is both a command interpreter and a programming
language. As a command interpreter, the shell provides the user
interface to the rich set of <small>GNU</small> utilities. The programming
language features allow these utilities to be combined.
Files containing commands can be created, and become
commands themselves. These new commands have the same status as
system commands in directories such as <samp>/bin</samp>, allowing users
or groups to establish custom environments to automate their common
tasks.
</p>
<p>Shells may be used interactively or non-interactively. In
interactive mode, they accept input typed from the keyboard.
When executing non-interactively, shells execute commands read
from a file.
</p>
<p>A shell allows execution of <small>GNU</small> commands, both synchronously and
asynchronously.
The shell waits for synchronous commands to complete before accepting
more input; asynchronous commands continue to execute in parallel
with the shell while it reads and executes additional commands.
The <em>redirection</em> constructs permit
fine-grained control of the input and output of those commands.
Moreover, the shell allows control over the contents of commands’
environments.
</p>
<p>Shells also provide a small set of built-in
commands (<em>builtins</em>) implementing functionality impossible
or inconvenient to obtain via separate utilities.
For example, <code>cd</code>, <code>break</code>, <code>continue</code>, and
<code>exec</code>) cannot be implemented outside of the shell because
they directly manipulate the shell itself.
The <code>history</code>, <code>getopts</code>, <code>kill</code>, or <code>pwd</code>
builtins, among others, could be implemented in separate utilities,
but they are more convenient to use as builtin commands.
All of the shell builtins are described in
subsequent sections.
</p>
<p>While executing commands is essential, most of the power (and
complexity) of shells is due to their embedded programming
languages. Like any high-level language, the shell provides
variables, flow control constructs, quoting, and functions.
</p>
<p>Shells offer features geared specifically for
interactive use rather than to augment the programming language.
These interactive features include job control, command line
editing, command history and aliases. Each of these features is
described in this manual.
</p>
<hr>
<a name="Definitions"></a>
<div class="header">
<p>
Next: <a href="#Basic-Shell-Features" accesskey="n" rel="next">Basic Shell Features</a>, Previous: <a href="#Introduction" accesskey="p" rel="previous">Introduction</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Definitions-1"></a>
<h2 class="chapter">2 Definitions</h2>
<p>These definitions are used throughout the remainder of this manual.
</p>
<dl compact="compact">
<dt><code>POSIX</code></dt>
<dd><a name="index-POSIX"></a>
<p>A family of open system standards based on Unix. Bash
is primarily concerned with the Shell and Utilities portion of the
<small>POSIX</small> 1003.1 standard.
</p>
</dd>
<dt><code>blank</code></dt>
<dd><p>A space or tab character.
</p>
</dd>
<dt><code>builtin</code></dt>
<dd><a name="index-builtin-1"></a>
<p>A command that is implemented internally by the shell itself, rather
than by an executable program somewhere in the file system.
</p>
</dd>
<dt><code>control operator</code></dt>
<dd><a name="index-control-operator"></a>
<p>A <code>token</code> that performs a control function. It is a <code>newline</code>
or one of the following:
‘<samp>||</samp>’, ‘<samp>&&</samp>’, ‘<samp>&</samp>’, ‘<samp>;</samp>’, ‘<samp>;;</samp>’,
‘<samp>|</samp>’, ‘<samp>|&</samp>’, ‘<samp>(</samp>’, or ‘<samp>)</samp>’.
</p>
</dd>
<dt><code>exit status</code></dt>
<dd><a name="index-exit-status"></a>
<p>The value returned by a command to its caller. The value is restricted
to eight bits, so the maximum value is 255.
</p>
</dd>
<dt><code>field</code></dt>
<dd><a name="index-field"></a>
<p>A unit of text that is the result of one of the shell expansions. After
expansion, when executing a command, the resulting fields are used as
the command name and arguments.
</p>
</dd>
<dt><code>filename</code></dt>
<dd><a name="index-filename"></a>
<p>A string of characters used to identify a file.
</p>
</dd>
<dt><code>job</code></dt>
<dd><a name="index-job"></a>
<p>A set of processes comprising a pipeline, and any processes descended
from it, that are all in the same process group.
</p>
</dd>
<dt><code>job control</code></dt>
<dd><a name="index-job-control"></a>
<p>A mechanism by which users can selectively stop (suspend) and restart
(resume) execution of processes.
</p>
</dd>
<dt><code>metacharacter</code></dt>
<dd><a name="index-metacharacter"></a>
<p>A character that, when unquoted, separates words. A metacharacter is
a <code>blank</code> or one of the following characters:
‘<samp>|</samp>’, ‘<samp>&</samp>’, ‘<samp>;</samp>’, ‘<samp>(</samp>’, ‘<samp>)</samp>’, ‘<samp><</samp>’, or
‘<samp>></samp>’.
</p>
</dd>
<dt><code>name</code></dt>
<dd><a name="index-name"></a>
<a name="index-identifier"></a>
<p>A <code>word</code> consisting solely of letters, numbers, and underscores,
and beginning with a letter or underscore. <code>Name</code>s are used as
shell variable and function names.
Also referred to as an <code>identifier</code>.
</p>
</dd>
<dt><code>operator</code></dt>
<dd><a name="index-operator_002c-shell"></a>
<p>A <code>control operator</code> or a <code>redirection operator</code>.
See <a href="#Redirections">Redirections</a>, for a list of redirection operators.
Operators contain at least one unquoted <code>metacharacter</code>.
</p>
</dd>
<dt><code>process group</code></dt>
<dd><a name="index-process-group"></a>
<p>A collection of related processes each having the same process
group <small>ID</small>.
</p>
</dd>
<dt><code>process group ID</code></dt>
<dd><a name="index-process-group-ID"></a>
<p>A unique identifier that represents a <code>process group</code>
during its lifetime.
</p>
</dd>
<dt><code>reserved word</code></dt>
<dd><a name="index-reserved-word"></a>
<p>A <code>word</code> that has a special meaning to the shell. Most reserved
words introduce shell flow control constructs, such as <code>for</code> and
<code>while</code>.
</p>
</dd>
<dt><code>return status</code></dt>
<dd><a name="index-return-status"></a>
<p>A synonym for <code>exit status</code>.
</p>
</dd>
<dt><code>signal</code></dt>
<dd><a name="index-signal"></a>
<p>A mechanism by which a process may be notified by the kernel
of an event occurring in the system.
</p>
</dd>
<dt><code>special builtin</code></dt>
<dd><a name="index-special-builtin"></a>
<p>A shell builtin command that has been classified as special by the
<small>POSIX</small> standard.
</p>
</dd>
<dt><code>token</code></dt>
<dd><a name="index-token"></a>
<p>A sequence of characters considered a single unit by the shell.
It is either a <code>word</code> or an <code>operator</code>.
</p>
</dd>
<dt><code>word</code></dt>
<dd><a name="index-word"></a>
<p>A sequence of characters treated as a unit by the shell.
Words may not include unquoted <code>metacharacters</code>.
</p></dd>
</dl>
<hr>
<a name="Basic-Shell-Features"></a>
<div class="header">
<p>
Next: <a href="#Shell-Builtin-Commands" accesskey="n" rel="next">Shell Builtin Commands</a>, Previous: <a href="#Definitions" accesskey="p" rel="previous">Definitions</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Basic-Shell-Features-1"></a>
<h2 class="chapter">3 Basic Shell Features</h2>
<a name="index-Bourne-shell"></a>
<p>Bash is an acronym for ‘<samp>Bourne-Again SHell</samp>’.
The Bourne shell is
the traditional Unix shell originally written by Stephen Bourne.
All of the Bourne shell builtin commands are available in Bash,
The rules for evaluation and quoting are taken from the <small>POSIX</small>
specification for the ‘standard’ Unix shell.
</p>
<p>This chapter briefly summarizes the shell’s ‘building blocks’:
commands, control structures, shell functions, shell <i>parameters</i>,
shell expansions,
<i>redirections</i>, which are a way to direct input and output from
and to named files, and how the shell executes commands.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">• <a href="#Shell-Syntax" accesskey="1">Shell Syntax</a>:</td><td> </td><td align="left" valign="top">What your input means to the shell.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Shell-Commands" accesskey="2">Shell Commands</a>:</td><td> </td><td align="left" valign="top">The types of commands you can use.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Shell-Functions" accesskey="3">Shell Functions</a>:</td><td> </td><td align="left" valign="top">Grouping commands by name.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Shell-Parameters" accesskey="4">Shell Parameters</a>:</td><td> </td><td align="left" valign="top">How the shell stores values.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Shell-Expansions" accesskey="5">Shell Expansions</a>:</td><td> </td><td align="left" valign="top">How Bash expands parameters and the various
expansions available.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Redirections" accesskey="6">Redirections</a>:</td><td> </td><td align="left" valign="top">A way to control where input and output go.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Executing-Commands" accesskey="7">Executing Commands</a>:</td><td> </td><td align="left" valign="top">What happens when you run a command.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Shell-Scripts" accesskey="8">Shell Scripts</a>:</td><td> </td><td align="left" valign="top">Executing files of shell commands.
</td></tr>
</table>
<hr>
<a name="Shell-Syntax"></a>
<div class="header">
<p>
Next: <a href="#Shell-Commands" accesskey="n" rel="next">Shell Commands</a>, Up: <a href="#Basic-Shell-Features" accesskey="u" rel="up">Basic Shell Features</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Shell-Syntax-1"></a>
<h3 class="section">3.1 Shell Syntax</h3>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">• <a href="#Shell-Operation" accesskey="1">Shell Operation</a>:</td><td> </td><td align="left" valign="top">The basic operation of the shell.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Quoting" accesskey="2">Quoting</a>:</td><td> </td><td align="left" valign="top">How to remove the special meaning from characters.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Comments" accesskey="3">Comments</a>:</td><td> </td><td align="left" valign="top">How to specify comments.
</td></tr>
</table>
<p>When the shell reads input, it proceeds through a
sequence of operations. If the input indicates the beginning of a
comment, the shell ignores the comment symbol (‘<samp>#</samp>’), and the rest
of that line.
</p>
<p>Otherwise, roughly speaking, the shell reads its input and
divides the input into words and operators, employing the quoting rules
to select which meanings to assign various words and characters.
</p>
<p>The shell then parses these tokens into commands and other constructs,
removes the special meaning of certain words or characters, expands
others, redirects input and output as needed, executes the specified
command, waits for the command’s exit status, and makes that exit status
available for further inspection or processing.
</p>
<hr>
<a name="Shell-Operation"></a>
<div class="header">
<p>
Next: <a href="#Quoting" accesskey="n" rel="next">Quoting</a>, Up: <a href="#Shell-Syntax" accesskey="u" rel="up">Shell Syntax</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Shell-Operation-1"></a>
<h4 class="subsection">3.1.1 Shell Operation</h4>
<p>The following is a brief description of the shell’s operation when it
reads and executes a command. Basically, the shell does the
following:
</p>
<ol>
<li> Reads its input from a file (see <a href="#Shell-Scripts">Shell Scripts</a>), from a string
supplied as an argument to the <samp>-c</samp> invocation option
(see <a href="#Invoking-Bash">Invoking Bash</a>), or from the user’s terminal.
</li><li> Breaks the input into words and operators, obeying the quoting rules
described in <a href="#Quoting">Quoting</a>. These tokens are separated by
<code>metacharacters</code>. Alias expansion is performed by this step
(see <a href="#Aliases">Aliases</a>).
</li><li> Parses the tokens into simple and compound commands
(see <a href="#Shell-Commands">Shell Commands</a>).
</li><li> Performs the various shell expansions (see <a href="#Shell-Expansions">Shell Expansions</a>), breaking
the expanded tokens into lists of filenames (see <a href="#Filename-Expansion">Filename Expansion</a>)
and commands and arguments.
</li><li> Performs any necessary redirections (see <a href="#Redirections">Redirections</a>) and removes
the redirection operators and their operands from the argument list.
</li><li> Executes the command (see <a href="#Executing-Commands">Executing Commands</a>).
</li><li> Optionally waits for the command to complete and collects its exit
status (see <a href="#Exit-Status">Exit Status</a>).
</li></ol>
<hr>
<a name="Quoting"></a>
<div class="header">
<p>
Next: <a href="#Comments" accesskey="n" rel="next">Comments</a>, Previous: <a href="#Shell-Operation" accesskey="p" rel="previous">Shell Operation</a>, Up: <a href="#Shell-Syntax" accesskey="u" rel="up">Shell Syntax</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Quoting-1"></a>
<h4 class="subsection">3.1.2 Quoting</h4>
<a name="index-quoting"></a>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">• <a href="#Escape-Character" accesskey="1">Escape Character</a>:</td><td> </td><td align="left" valign="top">How to remove the special meaning from a single
character.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Single-Quotes" accesskey="2">Single Quotes</a>:</td><td> </td><td align="left" valign="top">How to inhibit all interpretation of a sequence
of characters.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Double-Quotes" accesskey="3">Double Quotes</a>:</td><td> </td><td align="left" valign="top">How to suppress most of the interpretation of a
sequence of characters.
</td></tr>
<tr><td align="left" valign="top">• <a href="#ANSI_002dC-Quoting" accesskey="4">ANSI-C Quoting</a>:</td><td> </td><td align="left" valign="top">How to expand ANSI-C sequences in quoted strings.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Locale-Translation" accesskey="5">Locale Translation</a>:</td><td> </td><td align="left" valign="top">How to translate strings into different languages.
</td></tr>
</table>
<p>Quoting is used to remove the special meaning of certain
characters or words to the shell. Quoting can be used to
disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent
parameter expansion.
</p>
<p>Each of the shell metacharacters (see <a href="#Definitions">Definitions</a>)
has special meaning to the shell and must be quoted if it is to
represent itself.
When the command history expansion facilities are being used
(see <a href="#History-Interaction">History Interaction</a>), the
<var>history expansion</var> character, usually ‘<samp>!</samp>’, must be quoted
to prevent history expansion. See <a href="#Bash-History-Facilities">Bash History Facilities</a>, for
more details concerning history expansion.
</p>
<p>There are three quoting mechanisms: the
<var>escape character</var>, single quotes, and double quotes.
</p>
<hr>
<a name="Escape-Character"></a>
<div class="header">
<p>
Next: <a href="#Single-Quotes" accesskey="n" rel="next">Single Quotes</a>, Up: <a href="#Quoting" accesskey="u" rel="up">Quoting</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Escape-Character-1"></a>
<h4 class="subsubsection">3.1.2.1 Escape Character</h4>
<font color=green>
<p>A non-quoted backslash ‘<samp>\</samp>’ is the Bash escape character.
It preserves the literal value of the next character that follows,
with the exception of <code>newline</code>. If a <code>\newline</code> pair
appears, and the backslash itself is not quoted, the <code>\newline</code>
is treated as a line continuation (that is, it is removed from
the input stream and effectively ignored).
</font>
</p>
<hr>
<a name="Single-Quotes"></a>
<div class="header">
<p>
Next: <a href="#Double-Quotes" accesskey="n" rel="next">Double Quotes</a>, Previous: <a href="#Escape-Character" accesskey="p" rel="previous">Escape Character</a>, Up: <a href="#Quoting" accesskey="u" rel="up">Quoting</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Single-Quotes-1"></a>
<h4 class="subsubsection">3.1.2.2 Single Quotes</h4>
<font color=green>
<p>Enclosing characters in single quotes (‘<samp>'</samp>’) preserves the literal value
of each character within the quotes. A single quote may not occur
between single quotes, even when preceded by a backslash.
</font>
</p>
<hr>
<a name="Double-Quotes"></a>
<div class="header">
<p>
Next: <a href="#ANSI_002dC-Quoting" accesskey="n" rel="next">ANSI-C Quoting</a>, Previous: <a href="#Single-Quotes" accesskey="p" rel="previous">Single Quotes</a>, Up: <a href="#Quoting" accesskey="u" rel="up">Quoting</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Double-Quotes-1"></a>
<h4 class="subsubsection">3.1.2.3 Double Quotes</h4>
<font color=green>
<p>Enclosing characters in double quotes (‘<samp>"</samp>’) preserves the literal value
of all characters within the quotes, with the exception of
‘<samp>$</samp>’, ‘<samp>`</samp>’, ‘<samp>\</samp>’,
and, <font color=red>when history expansion is enabled, ‘<samp>!</samp>’.</font>
The characters ‘<samp>$</samp>’ and ‘<samp>`</samp>’
retain their special meaning within double quotes (see <a href="#Shell-Expansions">Shell Expansions</a>).
The backslash retains its special meaning only when followed by one of
the following characters:
‘<samp>$</samp>’, ‘<samp>`</samp>’, ‘<samp>"</samp>’, ‘<samp>\</samp>’, or <code>newline</code>.
Within double quotes, backslashes that are followed by one of these
characters are removed. Backslashes preceding characters without a
special meaning are left unmodified.
A double quote may be quoted within double quotes by preceding it with
a backslash.
</font>
<font color=red>
If enabled, history expansion will be performed unless an ‘<samp>!</samp>’
appearing in double quotes is escaped using a backslash.
The backslash preceding the ‘<samp>!</samp>’ is not removed.
</font>
<font color=purple>
N.B. We do not know what is going to be enabled on Python at run time.
</font>
</p>
<p>The special parameters ‘<samp>*</samp>’ and ‘<samp>@</samp>’ have special meaning
when in double quotes (see <a href="#Shell-Parameter-Expansion">Shell Parameter Expansion</a>).
</p>
<hr>
<a name="ANSI_002dC-Quoting"></a>
<div class="header">
<p>
Next: <a href="#Locale-Translation" accesskey="n" rel="next">Locale Translation</a>, Previous: <a href="#Double-Quotes" accesskey="p" rel="previous">Double Quotes</a>, Up: <a href="#Quoting" accesskey="u" rel="up">Quoting</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="ANSI_002dC-Quoting-1"></a>
<h4 class="subsubsection">3.1.2.4 ANSI-C Quoting</h4>
<a name="index-quoting_002c-ANSI"></a>
<font color=green>
<p>Words of the form <code>$'<var>string</var>'</code> are treated specially. The
word expands to <var>string</var>, with backslash-escaped characters replaced
as specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
</p>
<dl compact="compact">
<dt><code>\a</code></dt>
<dd><p>alert (bell)
</p></dd>
<dt><code>\b</code></dt>
<dd><p>backspace
</p></dd>
<dt><code>\e</code></dt>
<dt><code>\E</code></dt>
<dd><p>an escape character (not ANSI C)
</p></dd>
<dt><code>\f</code></dt>
<dd><p>form feed
</p></dd>
<dt><code>\n</code></dt>
<dd><p>newline
</p></dd>
<dt><code>\r</code></dt>
<dd><p>carriage return
</p></dd>
<dt><code>\t</code></dt>
<dd><p>horizontal tab
</p></dd>
<dt><code>\v</code></dt>
<dd><p>vertical tab
</p></dd>
<dt><code>\\</code></dt>
<dd><p>backslash
</p></dd>
<dt><code>\'</code></dt>
<dd><p>single quote
</p></dd>
<dt><code>\"</code></dt>
<dd><p>double quote
</p></dd>
<dt><code>\<var>nnn</var></code></dt>
<dd><p>the eight-bit character whose value is the octal value <var>nnn</var>
(one to three digits)
</p></dd>
<dt><code>\x<var>HH</var></code></dt>
<dd><p>the eight-bit character whose value is the hexadecimal value <var>HH</var>
(one or two hex digits)
</p></dd>
<dt><code>\u<var>HHHH</var></code></dt>
<dd><p>the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
<var>HHHH</var> (one to four hex digits)
<br>
</p></dd>
<dt><code>\U<var>HHHHHHHH</var></code></dt>
<dd><p>the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
<var>HHHHHHHH</var> (one to eight hex digits)
<br>
</p></dd>
<dt><code>\c<var>x</var></code></dt>
<dd><p>a control-<var>x</var> character
</p></dd>
</dl>
<p>The expanded result is single-quoted, as if the dollar sign had not
been present.
</p>
</font>
<hr>
<a name="Locale-Translation"></a>
<div class="header">
<p>
Previous: <a href="#ANSI_002dC-Quoting" accesskey="p" rel="previous">ANSI-C Quoting</a>, Up: <a href="#Quoting" accesskey="u" rel="up">Quoting</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Locale_002dSpecific-Translation"></a>
<h4 class="subsubsection">3.1.2.5 Locale-Specific Translation</h4>
<a name="index-localization"></a>
<a name="index-internationalization"></a>
<a name="index-native-languages"></a>
<a name="index-translation_002c-native-languages"></a>
<font color=red>
<p>A double-quoted string preceded by a dollar sign (‘<samp>$</samp>’) will cause
the string to be translated according to the current locale.
If the current locale is <code>C</code> or <code>POSIX</code>, the dollar sign
is ignored.
If the string is translated and replaced, the replacement is
double-quoted. <font color=purple>The preceding $ will simply be ignored
since we don't know what the run time locale on Python will later be.
</font>
</p>
</font>
<a name="index-LC_005fMESSAGES"></a>
<a name="index-TEXTDOMAIN"></a>
<a name="index-TEXTDOMAINDIR"></a>
<p>Some systems use the message catalog selected by the <code>LC_MESSAGES</code>
shell variable. Others create the name of the message catalog from the
value of the <code>TEXTDOMAIN</code> shell variable, possibly adding a
suffix of ‘<samp>.mo</samp>’. If you use the <code>TEXTDOMAIN</code> variable, you
may need to set the <code>TEXTDOMAINDIR</code> variable to the location of
the message catalog files. Still others use both variables in this
fashion:
<code>TEXTDOMAINDIR</code>/<code>LC_MESSAGES</code>/LC_MESSAGES/<code>TEXTDOMAIN</code>.mo.
</p>
<hr>
<a name="Comments"></a>
<div class="header">
<p>
Previous: <a href="#Quoting" accesskey="p" rel="previous">Quoting</a>, Up: <a href="#Shell-Syntax" accesskey="u" rel="up">Shell Syntax</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Comments-1"></a>
<h4 class="subsection">3.1.3 Comments</h4>
<a name="index-comments_002c-shell"></a>
<p>In a non-interactive shell, or an interactive shell in which the
<code>interactive_comments</code> option to the <code>shopt</code>
builtin is enabled (see <a href="#The-Shopt-Builtin">The Shopt Builtin</a>),
<font color=green>a word beginning with ‘<samp>#</samp>’
causes that word and all remaining characters on that line to
be ignored</font>.
<font color=purple>Comments are translated before the command that follows
them.</font>
An interactive shell without the <code>interactive_comments</code>
option enabled does not allow comments. The <code>interactive_comments</code>
option is on by default in interactive shells.
See <a href="#Interactive-Shells">Interactive Shells</a>, for a description of what makes
a shell interactive.
</p>
<hr>
<a name="Shell-Commands"></a>
<div class="header">
<p>
Next: <a href="#Shell-Functions" accesskey="n" rel="next">Shell Functions</a>, Previous: <a href="#Shell-Syntax" accesskey="p" rel="previous">Shell Syntax</a>, Up: <a href="#Basic-Shell-Features" accesskey="u" rel="up">Basic Shell Features</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Shell-Commands-1"></a>
<h3 class="section">3.2 Shell Commands</h3>
<a name="index-commands_002c-shell"></a>
<p>A simple shell command such as <code>echo a b c</code> consists of the command
itself followed by arguments, separated by spaces.
</p>
<p>More complex shell commands are composed of simple commands arranged together
in a variety of ways: in a pipeline in which the output of one command
becomes the input of a second, in a loop or conditional construct, or in
some other grouping.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">• <a href="#Simple-Commands" accesskey="1">Simple Commands</a>:</td><td> </td><td align="left" valign="top">The most common type of command.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Pipelines" accesskey="2">Pipelines</a>:</td><td> </td><td align="left" valign="top">Connecting the input and output of several
commands.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Lists" accesskey="3">Lists</a>:</td><td> </td><td align="left" valign="top">How to execute commands sequentially.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Compound-Commands" accesskey="4">Compound Commands</a>:</td><td> </td><td align="left" valign="top">Shell commands for control flow.
</td></tr>
<tr><td align="left" valign="top">• <a href="#Coprocesses" accesskey="5">Coprocesses</a>:</td><td> </td><td align="left" valign="top">Two-way communication between commands.
</td></tr>
<tr><td align="left" valign="top">• <a href="#GNU-Parallel" accesskey="6">GNU Parallel</a>:</td><td> </td><td align="left" valign="top">Running commands in parallel.
</td></tr>
</table>
<hr>
<a name="Simple-Commands"></a>
<div class="header">
<p>
Next: <a href="#Pipelines" accesskey="n" rel="next">Pipelines</a>, Up: <a href="#Shell-Commands" accesskey="u" rel="up">Shell Commands</a> [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Indexes" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Simple-Commands-1"></a>
<h4 class="subsection">3.2.1 Simple Commands</h4>