bashref.html

Linux 下bash shell 参考

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, history and aliases.  Each of these features is
described in this manual.
</P><P>

<A NAME="Definitions"></A>
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<H1> 2. Definitions </H1>
<!--docid::SEC4::-->
These definitions are used throughout the remainder of this manual.
<P>

<DL COMPACT>

<DT><CODE>POSIX</CODE>
<DD><A NAME="IDX1"></A>
A family of open system standards based on Unix.  Bash
is concerned with POSIX 1003.2, the Shell and Tools Standard.
<P>

<DT><CODE>blank</CODE>
<DD>A space or tab character.
<P>

<DT><CODE>builtin</CODE>
<DD><A NAME="IDX2"></A>
A command that is implemented internally by the shell itself, rather
than by an executable program somewhere in the file system.
<P>

<DT><CODE>control operator</CODE>
<DD><A NAME="IDX3"></A>
A <CODE>word</CODE> that performs a control function.  It is a <CODE>newline</CODE>
or one of the following:
<SAMP>`||'</SAMP>, <SAMP>`&#38;&#38;'</SAMP>, <SAMP>`&#38;'</SAMP>, <SAMP>`;'</SAMP>, <SAMP>`;;'</SAMP>,
<SAMP>`|'</SAMP>, <SAMP>`('</SAMP>, or <SAMP>`)'</SAMP>.
<P>

<DT><CODE>exit status</CODE>
<DD><A NAME="IDX4"></A>
The value returned by a command to its caller.  The value is restricted
to eight bits, so the maximum value is 255.
<P>

<DT><CODE>field</CODE>
<DD><A NAME="IDX5"></A>
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>

<DT><CODE>filename</CODE>
<DD><A NAME="IDX6"></A>
A string of characters used to identify a file.
<P>

<DT><CODE>job</CODE>
<DD><A NAME="IDX7"></A>
A set of processes comprising a pipeline, and any processes descended
from it, that are all in the same process group.
<P>

<DT><CODE>job control</CODE>
<DD><A NAME="IDX8"></A>
A mechanism by which users can selectively stop (suspend) and restart
(resume) execution of processes.
<P>

<DT><CODE>metacharacter</CODE>
<DD><A NAME="IDX9"></A>
A character that, when unquoted, separates words.  A metacharacter is
a <CODE>blank</CODE> or one of the following characters:
<SAMP>`|'</SAMP>, <SAMP>`&#38;'</SAMP>, <SAMP>`;'</SAMP>, <SAMP>`('</SAMP>, <SAMP>`)'</SAMP>, <SAMP>`&#60;'</SAMP>, or
<SAMP>`&#62;'</SAMP>.
<P>

<DT><CODE>name</CODE>
<DD><A NAME="IDX10"></A>
<A NAME="IDX11"></A>
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>

<DT><CODE>operator</CODE>
<DD><A NAME="IDX12"></A>
A <CODE>control operator</CODE> or a <CODE>redirection operator</CODE>.
See section <A HREF="bashref.html#SEC37">3.6 Redirections</A>, for a list of redirection operators.
<P>

<DT><CODE>process group</CODE>
<DD><A NAME="IDX13"></A>
A collection of related processes each having the same process
group ID.
<P>

<DT><CODE>process group ID</CODE>
<DD><A NAME="IDX14"></A>
A unique identifer that represents a <CODE>process group</CODE>
during its lifetime.
<P>

<DT><CODE>reserved word</CODE>
<DD><A NAME="IDX15"></A>
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>

<DT><CODE>return status</CODE>
<DD><A NAME="IDX16"></A>
A synonym for <CODE>exit status</CODE>.
<P>

<DT><CODE>signal</CODE>
<DD><A NAME="IDX17"></A>
A mechanism by which a process may be notified by the kernel
of an event occurring in the system.
<P>

<DT><CODE>special builtin</CODE>
<DD><A NAME="IDX18"></A>
A shell builtin command that has been classified as special by the
POSIX 1003.2 standard.
<P>

<DT><CODE>token</CODE>
<DD><A NAME="IDX19"></A>
A sequence of characters considered a single unit by the shell.  It is
either a <CODE>word</CODE> or an <CODE>operator</CODE>.
<P>

<DT><CODE>word</CODE>
<DD><A NAME="IDX20"></A>
A <CODE>token</CODE> that is not an <CODE>operator</CODE>.
</DL>
<P>

<A NAME="Basic Shell Features"></A>
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<H1> 3. Basic Shell Features </H1>
<!--docid::SEC5::-->
<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,
and the rules for evaluation and quoting are taken from the POSIX
1003.2 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><P>

<BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> 
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC6">3.1 Shell Syntax</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">What your input means to the shell.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC15">3.2 Shell Commands</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">The types of commands you can use.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC22">3.3 Shell Functions</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Grouping commands by name.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC23">3.4 Shell Parameters</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Special shell variables.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC26">3.5 Shell Expansions</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">How Bash expands variables and the various
				expansions available.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC37">3.6 Redirections</A></TD><TD>&nbsp;&nbsp;</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="bashref.html#SEC47">3.7 Executing Commands</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">What happens when you run a command.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC54">3.8 Shell Scripts</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Executing files of shell commands.</TD></TR>
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<P>

<A NAME="Shell Syntax"></A>
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<H2> 3.1 Shell Syntax </H2>
<!--docid::SEC6::-->
<BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> 
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC7">3.1.1 Shell Operation</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">The basic operation of the shell.</TD></TR>
</TABLE>

<br>
<TABLE BORDER=0 CELLSPACING=0>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC8">3.1.2 Quoting</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">How to remove the special meaning from characters.</TD></TR>
</TABLE>

<br>
<TABLE BORDER=0 CELLSPACING=0>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC14">3.1.3 Comments</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">How to specify comments.</TD></TR>
</TABLE></BLOCKQUOTE>
<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.
                                
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><P>

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<H3> 3.1.1 Shell Operation </H3>
<!--docid::SEC7::-->
<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: