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Linux 下bash shell 参考

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<H3> 3.1.3 Comments </H3>
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<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 section <A HREF="bashref.html#SEC57">4.2 Bash Builtin Commands</A>),
a word beginning with <SAMP>`#'</SAMP>
causes that word and all remaining characters on that line to
be ignored.  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 section <A HREF="bashref.html#SEC73">6.3 Interactive Shells</A>, for a description of what makes
a shell interactive.
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<A NAME="Shell Commands"></A>
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<H2> 3.2 Shell Commands </H2>
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<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><P>

<BLOCKQUOTE><TABLE BORDER=0 CELLSPACING=0> 
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC16">3.2.1 Simple Commands</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">The most common type of command.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC17">3.2.2 Pipelines</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Connecting the input and output of several
				commands.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC18">3.2.3 Lists of Commands</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">How to execute commands sequentially.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC19">3.2.4 Looping Constructs</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Shell commands for iterative action.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC20">3.2.5 Conditional Constructs</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Shell commands for conditional execution.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bashref.html#SEC21">3.2.6 Grouping Commands</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Ways to group commands.</TD></TR>
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<A NAME="Simple Commands"></A>
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<H3> 3.2.1 Simple Commands </H3>
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<P>

A simple command is the kind of command encountered most often.
It's just a sequence of words separated by <CODE>blank</CODE>s, terminated
by one of the shell's control operators (see section <A HREF="bashref.html#SEC4">2. Definitions</A>).  The
first word generally specifies a command to be executed, with the
rest of the words being that command's arguments.
</P><P>

The return status (see section <A HREF="bashref.html#SEC52">3.7.5 Exit Status</A>) of a simple command is
its exit status as provided
by the POSIX 1003.1 <CODE>waitpid</CODE> function, or 128+<VAR>n</VAR> if
the command was terminated by signal <VAR>n</VAR>.
</P><P>

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<H3> 3.2.2 Pipelines </H3>
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<P>

A <CODE>pipeline</CODE> is a sequence of simple commands separated by
<SAMP>`|'</SAMP>.
</P><P>

<A NAME="IDX24"></A>
<A NAME="IDX25"></A>
<A NAME="IDX26"></A>
The format for a pipeline is
<TABLE><tr><td>&nbsp;</td><td class=example><pre>[<CODE>time</CODE> [<CODE>-p</CODE>]] [<CODE>!</CODE>] <VAR>command1</VAR> [<CODE>|</CODE> <VAR>command2</VAR> <small>...</small>]
</pre></td></tr></table></P><P>

The output of each command in the pipeline is connected via a pipe
to the input of the next command.
That is, each command reads the previous command's output.
</P><P>

The reserved word <CODE>time</CODE> causes timing statistics
to be printed for the pipeline once it finishes.
The statistics currently consist of elapsed (wall-clock) time and
user and system time consumed by the command's execution.
The <SAMP>`-p'</SAMP> option changes the output format to that specified
by POSIX.
The <CODE>TIMEFORMAT</CODE> variable may be set to a format string that
specifies how the timing information should be displayed.
See section <A HREF="bashref.html#SEC62">5.2 Bash Variables</A>, for a description of the available formats.
The use of <CODE>time</CODE> as a reserved word permits the timing of
shell builtins, shell functions, and pipelines.  An external
<CODE>time</CODE> command cannot time these easily.
</P><P>

If the pipeline is not executed asynchronously (see section <A HREF="bashref.html#SEC18">3.2.3 Lists of Commands</A>), the
shell waits for all commands in the pipeline to complete.
</P><P>

Each command in a pipeline is executed in its own subshell
(see section <A HREF="bashref.html#SEC50">3.7.3 Command Execution Environment</A>).  The exit
status of a pipeline is the exit status of the last command in the
pipeline.  If the reserved word <SAMP>`!'</SAMP> precedes the pipeline, the
exit status is the logical negation of the exit status of the last command.
</P><P>

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<H3> 3.2.3 Lists of Commands </H3>
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<P>

A <CODE>list</CODE> is a sequence of one or more pipelines separated by one
of the operators <SAMP>`;'</SAMP>, <SAMP>`&#38;'</SAMP>, <SAMP>`&#38;&#38;'</SAMP>, or <SAMP>`||'</SAMP>,
and optionally terminated by one of <SAMP>`;'</SAMP>, <SAMP>`&#38;'</SAMP>, or a
<CODE>newline</CODE>.
</P><P>

Of these list operators, <SAMP>`&#38;&#38;'</SAMP> and <SAMP>`||'</SAMP>
have equal precedence, followed by <SAMP>`;'</SAMP> and <SAMP>`&#38;'</SAMP>,
which have equal precedence.
</P><P>

A sequence of one or more newlines may appear in a <CODE>list</CODE>
to delimit commands, equivalent to a semicolon.
</P><P>

If a command is terminated by the control operator <SAMP>`&#38;'</SAMP>,
the shell executes the command asynchronously in a subshell.
This is known as executing the command in the <VAR>background</VAR>.
The shell does not wait for the command to finish, and the return
status is 0 (true).
When job control is not active (see section <A HREF="bashref.html#SEC86">7. Job Control</A>),
the standard input for asynchronous commands, in the absence of any
explicit redirections, is redirected from <CODE>/dev/null</CODE>.
</P><P>

Commands separated by a <SAMP>`;'</SAMP> are executed sequentially; the shell
waits for each command to terminate in turn.  The return status is the
exit status of the last command executed.
</P><P>

The control operators <SAMP>`&#38;&#38;'</SAMP> and <SAMP>`||'</SAMP>
denote AND lists and OR lists, respectively.
An AND list has the form
<TABLE><tr><td>&nbsp;</td><td class=example><pre><VAR>command1</VAR> &#38;&#38; <VAR>command2</VAR>
</pre></td></tr></table></P><P>

<VAR>command2</VAR> is executed if, and only if, <VAR>command1</VAR>
returns an exit status of zero.
</P><P>

An OR list has the form
<TABLE><tr><td>&nbsp;</td><td class=example><pre><VAR>command1</VAR> || <VAR>command2</VAR>
</pre></td></tr></table></P><P>

<VAR>command2</VAR> is executed if, and only if, <VAR>command1</VAR>
returns a non-zero exit status.
</P><P>

The return status of
AND and OR lists is the exit status of the last command
executed in the list.
</P><P>

<A NAME="Looping Constructs"></A>
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<H3> 3.2.4 Looping Constructs </H3>
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Bash supports the following looping constructs.
</P><P>

Note that wherever a <SAMP>`;'</SAMP> appears in the description of a
command's syntax, it may be replaced with one or more newlines.
</P><P>

<DL COMPACT>
<DT><CODE>until</CODE>
<DD><A NAME="IDX27"></A>
<A NAME="IDX28"></A>
<A NAME="IDX29"></A>
The syntax of the <CODE>until</CODE> command is:
<TABLE><tr><td>&nbsp;</td><td class=example><pre>until <VAR>test-commands</VAR>; do <VAR>consequent-commands</VAR>; done
</pre></td></tr></table>Execute <VAR>consequent-commands</VAR> as long as
<VAR>test-commands</VAR> has an exit status which is not zero.
The return status is the exit status of the last command executed
in <VAR>consequent-commands</VAR>, or zero if none was executed.
<P>

<DT><CODE>while</CODE>
<DD><A NAME="IDX30"></A>
The syntax of the <CODE>while</CODE> command is:
<TABLE><tr><td>&nbsp;</td><td class=example><pre>while <VAR>test-commands</VAR>; do <VAR>consequent-commands</VAR>; done
</pre></td></tr></table><P>