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Linux 下bash shell 参考

Execute <VAR>consequent-commands</VAR> as long as
<VAR>test-commands</VAR> has an exit status of 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><P>

<DT><CODE>for</CODE>
<DD><A NAME="IDX31"></A>
The syntax of the <CODE>for</CODE> command is:
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>for <VAR>name</VAR> [in <VAR>words</VAR> <small>...</small>]; do <VAR>commands</VAR>; done
</pre></td></tr></table>Expand <VAR>words</VAR>, and execute <VAR>commands</VAR> once for each member
in the resultant list, with <VAR>name</VAR> bound to the current member.
If <SAMP>`in <VAR>words</VAR>'</SAMP> is not present, the <CODE>for</CODE> command
executes the <VAR>commands</VAR> once for each positional parameter that is
set, as if <SAMP>`in "$@"'</SAMP> had been specified
(see section <A HREF="bashref.html#SEC25">3.4.2 Special Parameters</A>).
The return status is the exit status of the last command that executes.
If there are no items in the expansion of <VAR>words</VAR>, no commands are
executed, and the return status is zero.
</P><P>

An alternate form of the <CODE>for</CODE> command is also supported:
</P><P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>for (( <VAR>expr1</VAR> ; <VAR>expr2</VAR> ; <VAR>expr3</VAR> )) ; do <VAR>commands</VAR> ; done
</pre></td></tr></table>First, the arithmetic expression <VAR>expr1</VAR> is evaluated according
to the rules described below (see section <A HREF="bashref.html#SEC78">6.5 Shell Arithmetic</A>).
The arithmetic expression <VAR>expr2</VAR> is then evaluated repeatedly
until it evaluates to zero.   
Each time <VAR>expr2</VAR> evaluates to a non-zero value, <VAR>commands</VAR> are
executed and the arithmetic expression <VAR>expr3</VAR> is evaluated.       
If any expression is omitted, it behaves as if it evaluates to 1.
The return value is the exit status of the last command in <VAR>list</VAR>
that is executed, or false if any of the expressions is invalid.
</P><P>

</DL>
<P>

The <CODE>break</CODE> and <CODE>continue</CODE> builtins (see section <A HREF="bashref.html#SEC56">4.1 Bourne Shell Builtins</A>)
may be used to control loop execution.
</P><P>

<A NAME="Conditional Constructs"></A>
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<H3> 3.2.5 Conditional Constructs </H3>
<!--docid::SEC20::-->
<P>

<DL COMPACT>
<DT><CODE>if</CODE>
<DD><A NAME="IDX32"></A>
<A NAME="IDX33"></A>
<A NAME="IDX34"></A>
<A NAME="IDX35"></A>
<A NAME="IDX36"></A>
The syntax of the <CODE>if</CODE> command is:
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>if <VAR>test-commands</VAR>; then
  <VAR>consequent-commands</VAR>;
[elif <VAR>more-test-commands</VAR>; then
  <VAR>more-consequents</VAR>;]
[else <VAR>alternate-consequents</VAR>;]
fi
</pre></td></tr></table></P><P>

The <VAR>test-commands</VAR> list is executed, and if its return status is zero,
the <VAR>consequent-commands</VAR> list is executed.
If <VAR>test-commands</VAR> returns a non-zero status, each <CODE>elif</CODE> list
is executed in turn, and if its exit status is zero,
the corresponding <VAR>more-consequents</VAR> is executed and the   
command completes.
If <SAMP>`else <VAR>alternate-consequents</VAR>'</SAMP> is present, and
the final command in the final <CODE>if</CODE> or <CODE>elif</CODE> clause
has a non-zero exit status, then <VAR>alternate-consequents</VAR> is executed.
The return status is the exit status of the last command executed, or
zero if no condition tested true.
</P><P>

<DT><CODE>case</CODE>
<DD><A NAME="IDX37"></A>
<A NAME="IDX38"></A>
<A NAME="IDX39"></A>
The syntax of the <CODE>case</CODE> command is:
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre><CODE>case <VAR>word</VAR> in [ [(] <VAR>pattern</VAR> [| <VAR>pattern</VAR>]<small>...</small>) <VAR>command-list</VAR> ;;]<small>...</small> esac</CODE>
</pre></td></tr></table></P><P>

<CODE>case</CODE> will selectively execute the <VAR>command-list</VAR> corresponding to
the first <VAR>pattern</VAR> that matches <VAR>word</VAR>.
The <SAMP>`|'</SAMP> is used to separate multiple patterns, and the <SAMP>`)'</SAMP>
operator terminates a pattern list.
A list of patterns and an associated command-list is known
as a <VAR>clause</VAR>.  Each clause must be terminated with <SAMP>`;;'</SAMP>.
The <VAR>word</VAR> undergoes tilde expansion, parameter expansion, command
substitution, arithmetic expansion, and quote removal before matching is
attempted.  Each <VAR>pattern</VAR> undergoes tilde expansion, parameter
expansion, command substitution, and arithmetic expansion.
</P><P>

There may be an arbitrary number of <CODE>case</CODE> clauses, each terminated
by a <SAMP>`;;'</SAMP>.  The first pattern that matches determines the
command-list that is executed.
</P><P>

Here is an example using <CODE>case</CODE> in a script that could be used to
describe one interesting feature of an animal:
</P><P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>echo -n "Enter the name of an animal: "
read ANIMAL
echo -n "The $ANIMAL has "
case $ANIMAL in
  horse | dog | cat) echo -n "four";;
  man | kangaroo ) echo -n "two";;
  *) echo -n "an unknown number of";;
esac
echo " legs."
</pre></td></tr></table></P><P>

The return status is zero if no <VAR>pattern</VAR> is matched.  Otherwise, the
return status is the exit status of the <VAR>command-list</VAR> executed.
</P><P>

<DT><CODE>select</CODE>
<DD><A NAME="IDX40"></A>
<P>

The <CODE>select</CODE> construct allows the easy generation of menus.
It has almost the same syntax as the <CODE>for</CODE> command:
</P><P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>select <VAR>name</VAR> [in <VAR>words</VAR> <small>...</small>]; do <VAR>commands</VAR>; done
</pre></td></tr></table></P><P>

The list of words following <CODE>in</CODE> is expanded, generating a list
of items.  The set of expanded words is printed on the standard
error output stream, each preceded by a number.  If the
<SAMP>`in <VAR>words</VAR>'</SAMP> is omitted, the positional parameters are printed,
as if <SAMP>`in "$@"'</SAMP> had been specifed.
The <CODE>PS3</CODE> prompt is then displayed and a line is read from the
standard input.
If the line consists of a number corresponding to one of the displayed
words, then the value of <VAR>name</VAR> is set to that word.
If the line is empty, the words and prompt are displayed again.
If <CODE>EOF</CODE> is read, the <CODE>select</CODE> command completes.
Any other value read causes <VAR>name</VAR> to be set to null.
The line read is saved in the variable <CODE>REPLY</CODE>.
</P><P>

The <VAR>commands</VAR> are executed after each selection until a
<CODE>break</CODE> command is executed, at which
point the <CODE>select</CODE> command completes.
</P><P>

Here is an example that allows the user to pick a filename from the
current directory, and displays the name and index of the file
selected.
</P><P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>select fname in *;
do
	echo you picked $fname \($REPLY\)
	break;
done
</pre></td></tr></table></P><P>

<DT><CODE>((<small>...</small>))</CODE>
<DD><TABLE><tr><td>&nbsp;</td><td class=example><pre>(( <VAR>expression</VAR> ))
</pre></td></tr></table><P>

The arithmetic <VAR>expression</VAR> is evaluated according to the rules
described below (see section <A HREF="bashref.html#SEC78">6.5 Shell Arithmetic</A>).
If the value of the expression is non-zero, the return status is 0;
otherwise the return status is 1.  This is exactly equivalent to
<TABLE><tr><td>&nbsp;</td><td class=example><pre>let "<VAR>expression</VAR>"
</pre></td></tr></table>See section <A HREF="bashref.html#SEC57">4.2 Bash Builtin Commands</A>, for a full description of the <CODE>let</CODE> builtin.
</P><P>

<DT><CODE>[[<small>...</small>]]</CODE>
<DD><A NAME="IDX41"></A>
<A NAME="IDX42"></A>
<TABLE><tr><td>&nbsp;</td><td class=example><pre>[[ <VAR>expression</VAR> ]]
</pre></td></tr></table><P>

Return a status of 0 or 1 depending on the evaluation of
the conditional expression <VAR>expression</VAR>.
Expressions are composed of the primaries described below in
<A HREF="bashref.html#SEC77">6.4 Bash Conditional Expressions</A>.
Word splitting and filename expansion are not performed on the words
between the <SAMP>`[['</SAMP> and <SAMP>`]]'</SAMP>; tilde expansion, parameter and
variable expansion, arithmetic expansion, command substitution, process
substitution, and quote removal are performed.
</P><P>

When the <SAMP>`=='</SAMP> and <SAMP>`!='</SAMP> operators are used, the string to the
right of the operator is considered a pattern and matched according
to the rules described below in <A HREF="bashref.html#SEC35">3.5.8.1 Pattern Matching</A>.
The return value is 0 if the string matches or does not match
the pattern, respectively, and 1 otherwise.
Any part of the pattern may be quoted to force it to be matched as a
string.
</P><P>

Expressions may be combined using the following operators, listed
in decreasing order of precedence:
</P><P>

<DL COMPACT>
<DT><CODE>( <VAR>expression</VAR> )</CODE>
<DD>Returns the value of <VAR>expression</VAR>.
This may be used to override the normal precedence of operators.
<P>

<DT><CODE>! <VAR>expression</VAR></CODE>
<DD>True if <VAR>expression</VAR> is false.
<P>

<DT><CODE><VAR>expression1</VAR> &#38;&#38; <VAR>expression2</VAR></CODE>
<DD>True if both <VAR>expression1</VAR> and <VAR>expression2</VAR> are true.
<P>

<DT><CODE><VAR>expression1</VAR> || <VAR>expression2</VAR></CODE>
<DD>True if either <VAR>expression1</VAR> or <VAR>expression2</VAR> is true.
</DL>
The <CODE>&#38;&#38;</CODE> and <CODE>||</CODE> operators do not evaluate <VAR>expression2</VAR> if the
value of <VAR>expression1</VAR> is sufficient to determine the return
value of the entire conditional expression.
<P>

</DL>
<P>

<A NAME="Command Grouping"></A>
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<H3> 3.2.6 Grouping Commands </H3>
<!--docid::SEC21::-->
<P>

Bash provides two ways to group a list of commands to be executed
as a unit.  When commands are grouped, redirections may be applied
to the entire command list.  For example, the output of all