commandsub.html

Shall高级编程

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NAME="AEN6482"
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>Command substitution makes it possible to extend the
		toolset available to Bash. It is simply a matter
		of writing a program or script that outputs to
		<TT
CLASS="FILENAME"
>stdout</TT
> (like a well-behaved UNIX
		tool should) and assigning that output to a variable.</P
><P
>		<TABLE
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#include &#60;stdio.h&#62;
   2&nbsp;
   3&nbsp;/*  "Hello, world." C program  */		
   4&nbsp;
   5&nbsp;int main()
   6&nbsp;{
   7&nbsp;  printf( "Hello, world." );
   8&nbsp;  return (0);
   9&nbsp;}</PRE
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><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>gcc -o hello hello.c</B
></TT
>
 	      </PRE
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>
		</P
><P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# hello.sh		
   3&nbsp;
   4&nbsp;greeting=`./hello`
   5&nbsp;echo $greeting</PRE
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> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>sh hello.sh</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>Hello, world.</TT
>
 	        </PRE
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>
	        </P
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><A
NAME="CSPARENS"
></A
>The <B
CLASS="COMMAND"
>$(...)</B
>
		form has superseded backticks for command
		substitution.</P
><P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;output=$(sed -n /"$1"/p $file)   # From "grp.sh"	example.
   2&nbsp;	      
   3&nbsp;# Setting a variable to the contents of a text file.
   4&nbsp;File_contents1=$(cat $file1)      
   5&nbsp;File_contents2=$(&#60;$file2)        # Bash permits this also.</PRE
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></P
><P
>The <B
CLASS="COMMAND"
>$(...)</B
> form of command substitution
	        treats a double backslash in a different way than
		<B
CLASS="COMMAND"
>`...`</B
>.</P
><P
>		
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><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo `echo \\`</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
></TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $(echo \\)</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>\</TT
>
 	      </PRE
></TD
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>
              </P
><P
><A
NAME="CSNEST"
></A
></P
><P
>The <B
CLASS="COMMAND"
>$(...)</B
> form of command
                substitution permits nesting.
		  <A
NAME="AEN6517"
HREF="#FTN.AEN6517"
>[3]</A
>
		
		</P
><P
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;word_count=$( wc -w $(ls -l | awk '{print $9}') )</PRE
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>
              </P
><P
>Or, for something a bit more elaborate . . .</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="AGRAM2"
></A
><P
><B
>Example 11-3. Finding anagrams</B
></P
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CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# agram2.sh
   3&nbsp;# Example of nested command substitution.
   4&nbsp;
   5&nbsp;#  Uses "anagram" utility
   6&nbsp;#+ that is part of the author's "yawl" word list package.
   7&nbsp;#  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
   8&nbsp;#  http://personal.riverusers.com/~thegrendel/yawl-0.3.2.tar.gz
   9&nbsp;
  10&nbsp;E_NOARGS=66
  11&nbsp;E_BADARG=67
  12&nbsp;MINLEN=7
  13&nbsp;
  14&nbsp;if [ -z "$1" ]
  15&nbsp;then
  16&nbsp;  echo "Usage $0 LETTERSET"
  17&nbsp;  exit $E_NOARGS         # Script needs a command-line argument.
  18&nbsp;elif [ ${#1} -lt $MINLEN ]
  19&nbsp;then
  20&nbsp;  echo "Argument must have at least $MINLEN letters."
  21&nbsp;  exit $E_BADARG
  22&nbsp;fi
  23&nbsp;
  24&nbsp;
  25&nbsp;
  26&nbsp;FILTER='.......'         # Must have at least 7 letters.
  27&nbsp;#       1234567
  28&nbsp;Anagrams=( $(echo $(anagram $1 | grep $FILTER) ) )
  29&nbsp;#          $(     $(  nested command sub.    ) )
  30&nbsp;#        (              array assignment         )
  31&nbsp;
  32&nbsp;echo
  33&nbsp;echo "${#Anagrams[*]}  7+ letter anagrams found"
  34&nbsp;echo
  35&nbsp;echo ${Anagrams[0]}      # First anagram.
  36&nbsp;echo ${Anagrams[1]}      # Second anagram.
  37&nbsp;                         # Etc.
  38&nbsp;
  39&nbsp;# echo "${Anagrams[*]}"  # To list all the anagrams in a single line . . .
  40&nbsp;
  41&nbsp;#  Look ahead to the "Arrays" chapter for enlightenment on
  42&nbsp;#+ what's going on here.
  43&nbsp;
  44&nbsp;# See also the agram.sh script for an example of anagram finding.
  45&nbsp;
  46&nbsp;exit $?</PRE
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><P
>Examples of command substitution in shell scripts:
       <OL
TYPE="1"
><LI
><P
><A
HREF="loops.html#BINGREP"
>Example 10-7</A
></P
></LI
><LI
><P
><A
HREF="testbranch.html#CASECMD"
>Example 10-26</A
></P
></LI
><LI
><P
><A
HREF="randomvar.html#SEEDINGRANDOM"
>Example 9-31</A
></P
></LI
><LI
><P
><A
HREF="moreadv.html#EX57"
>Example 15-3</A
></P
></LI
><LI
><P
><A
HREF="textproc.html#LOWERCASE"
>Example 15-21</A
></P
></LI
><LI
><P
><A
HREF="textproc.html#GRP"
>Example 15-17</A
></P
></LI
><LI
><P
><A
HREF="extmisc.html#EX53"
>Example 15-52</A
></P
></LI
><LI
><P
><A
HREF="loops.html#EX24"
>Example 10-13</A
></P
></LI
><LI
><P
><A
HREF="loops.html#SYMLINKS"
>Example 10-10</A
></P
></LI
><LI
><P
><A
HREF="filearchiv.html#STRIPC"
>Example 15-31</A
></P
></LI
><LI
><P
><A
HREF="redircb.html#REDIR4"
>Example 19-8</A
></P
></LI
><LI
><P
><A
HREF="contributed-scripts.html#TREE"
>Example A-17</A
></P
></LI
><LI
><P
><A
HREF="procref1.html#PIDID"
>Example 27-2</A
></P
></LI
><LI
><P
><A
HREF="mathc.html#MONTHLYPMT"
>Example 15-45</A
></P
></LI
><LI
><P
><A
HREF="mathc.html#BASE"
>Example 15-46</A
></P
></LI
><LI
><P
><A
HREF="mathc.html#ALTBC"
>Example 15-47</A
></P
></LI
></OL
>
     </P
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
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CLASS="FOOTNOTES"
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><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN6415"
HREF="commandsub.html#AEN6415"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>For purposes of <I
CLASS="FIRSTTERM"
>command
		substitution</I
>, a <B
CLASS="COMMAND"
>command</B
>
		may be an external system command, an internal scripting
		<A
HREF="internal.html#BUILTINREF"
>builtin</A
>, or even <A
HREF="assortedtips.html#RVT"
>a script function</A
>.</P
></TD
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><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN6421"
HREF="commandsub.html#AEN6421"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>In a more technically correct sense,
		<I
CLASS="FIRSTTERM"
>command substitution</I
> extracts the
		<TT
CLASS="FILENAME"
>stdout</TT
> of a command, then assigns
		it to a variable using the <SPAN
CLASS="TOKEN"
>=</SPAN
>
		operator.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN6517"
HREF="commandsub.html#AEN6517"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>		    In fact, nesting with backticks is also possible,
		    but only by escaping the inner backticks, as John
		    Default points out.
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;word_count=` wc -w \`ls -l | awk '{print $9}'\` `</PRE
></TD
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>
		    </P
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