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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="COMMANDSUB"
></A
>Chapter 12. Command Substitution</H1
><P
>	     <A
NAME="COMMANDSUBREF"
></A
><B
CLASS="COMMAND"
>Command
	     substitution</B
> reassigns the output of a command
		<A
NAME="AEN7205"
HREF="#FTN.AEN7205"
>[1]</A
>
	     or even multiple commands; it literally plugs the command
	     output into another context.

	        <A
NAME="AEN7211"
HREF="#FTN.AEN7211"
>[2]</A
>
	     </P
><P
><A
NAME="BACKQUOTESREF"
></A
>The classic form of command
	      substitution uses <I
CLASS="FIRSTTERM"
>backquotes</I
>
	      (`...`). Commands within backquotes (backticks) generate
	      command-line text.

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;script_name=`basename $0`
   2&nbsp;echo "The name of this script is $script_name."</PRE
></TD
></TR
></TABLE
></P
><DIV
CLASS="FORMALPARA"
><P
><B
>The output of commands can be used as arguments to
	      another command, to set a variable, and even for generating
	      the argument list in a <A
HREF="loops.html#FORLOOPREF1"
>for</A
>
	      loop. </B
></P
></DIV
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;rm `cat filename`   # <SPAN
CLASS="QUOTE"
>"filename"</SPAN
> contains a list of files to delete.
   2&nbsp;#
   3&nbsp;# S. C. points out that "arg list too long" error might result.
   4&nbsp;# Better is              xargs rm -- &#60; filename 
   5&nbsp;# ( -- covers those cases where <SPAN
CLASS="QUOTE"
>"filename"</SPAN
> begins with a <SPAN
CLASS="QUOTE"
>"-"</SPAN
> )
   6&nbsp;
   7&nbsp;textfile_listing=`ls *.txt`
   8&nbsp;# Variable contains names of all *.txt files in current working directory.
   9&nbsp;echo $textfile_listing
  10&nbsp;
  11&nbsp;textfile_listing2=$(ls *.txt)   # The alternative form of command substitution.
  12&nbsp;echo $textfile_listing2
  13&nbsp;# Same result.
  14&nbsp;
  15&nbsp;# A possible problem with putting a list of files into a single string
  16&nbsp;# is that a newline may creep in.
  17&nbsp;#
  18&nbsp;# A safer way to assign a list of files to a parameter is with an array.
  19&nbsp;#      shopt -s nullglob    # If no match, filename expands to nothing.
  20&nbsp;#      textfile_listing=( *.txt )
  21&nbsp;#
  22&nbsp;# Thanks, S.C.</PRE
></TD
></TR
></TABLE
>
            </P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><A
NAME="CSSUBSH"
></A
>Command substitution
              invokes a <A
HREF="subshells.html#SUBSHELLSREF"
>subshell</A
>.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><A
NAME="CSWS"
></A
>Command substitution may
                result in <A
HREF="quoting.html#WSPLITREF"
>word splitting</A
>.
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;COMMAND `echo a b`     # 2 args: a and b
   2&nbsp;
   3&nbsp;COMMAND "`echo a b`"   # 1 arg: "a b"
   4&nbsp;
   5&nbsp;COMMAND `echo`         # no arg
   6&nbsp;
   7&nbsp;COMMAND "`echo`"       # one empty arg
   8&nbsp;
   9&nbsp;
  10&nbsp;# Thanks, S.C.</PRE
></TD
></TR
></TABLE
></P
><P
><A
NAME="CSTRNL"
></A
></P
><P
>Even when there is no word splitting, command
	        substitution can remove trailing newlines.

		<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# cd "`pwd`"  # This should always work.
   2&nbsp;# However...
   3&nbsp;
   4&nbsp;mkdir 'dir with trailing newline
   5&nbsp;'
   6&nbsp;
   7&nbsp;cd 'dir with trailing newline
   8&nbsp;'
   9&nbsp;
  10&nbsp;cd "`pwd`"  # Error message:
  11&nbsp;# bash: cd: /tmp/file with trailing newline: No such file or directory
  12&nbsp;
  13&nbsp;cd "$PWD"   # Works fine.
  14&nbsp;
  15&nbsp;
  16&nbsp;
  17&nbsp;
  18&nbsp;
  19&nbsp;old_tty_setting=$(stty -g)   # Save old terminal setting.
  20&nbsp;echo "Hit a key "
  21&nbsp;stty -icanon -echo           # Disable "canonical" mode for terminal.
  22&nbsp;                             # Also, disable *local* echo.
  23&nbsp;key=$(dd bs=1 count=1 2&#62; /dev/null)   # Using 'dd' to get a keypress.
  24&nbsp;stty "$old_tty_setting"      # Restore old setting. 
  25&nbsp;echo "You hit ${#key} key."  # ${#variable} = number of characters in $variable
  26&nbsp;#
  27&nbsp;# Hit any key except RETURN, and the output is "You hit 1 key."
  28&nbsp;# Hit RETURN, and it's "You hit 0 key."
  29&nbsp;# The newline gets eaten in the command substitution.
  30&nbsp;
  31&nbsp;#Code snippet by Stphane Chazelas.</PRE
></TD
></TR
></TABLE
>
              </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Using <B
CLASS="COMMAND"
>echo</B
> to output an
		<I
CLASS="FIRSTTERM"
>unquoted</I
> variable set with command
		substitution removes trailing newlines characters from
		the output of the reassigned command(s). This can cause
		unpleasant surprises.

		<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;dir_listing=`ls -l`
   2&nbsp;echo $dir_listing     # unquoted
   3&nbsp;
   4&nbsp;# Expecting a nicely ordered directory listing.
   5&nbsp;
   6&nbsp;# However, what you get is:
   7&nbsp;# total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo
   8&nbsp;# bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh
   9&nbsp;
  10&nbsp;# The newlines disappeared.
  11&nbsp;
  12&nbsp;
  13&nbsp;echo "$dir_listing"   # quoted
  14&nbsp;# -rw-rw-r--    1 bozo       30 May 13 17:15 1.txt
  15&nbsp;# -rw-rw-r--    1 bozo       51 May 15 20:57 t2.sh
  16&nbsp;# -rwxr-xr-x    1 bozo      217 Mar  5 21:13 wi.sh</PRE
></TD
></TR
></TABLE
>
              </P
></TD
></TR
></TABLE
></DIV
><P
>Command substitution even permits setting a variable to the
	       contents of a file, using either <A
HREF="io-redirection.html#IOREDIRREF"
>redirection</A
> or the <A
HREF="external.html#CATREF"
>cat</A
> command.</P
><P
>	         <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;variable1=`&#60;file1`      #  Set "variable1" to contents of "file1".
   2&nbsp;variable2=`cat file2`   #  Set "variable2" to contents of "file2".
   3&nbsp;                        #  This, however, forks a new process,
   4&nbsp;                        #+ so the line of code executes slower than the above version.
   5&nbsp;
   6&nbsp;#  Note that the variables may contain embedded whitespace,
   7&nbsp;#+ or even (horrors), control characters.
   8&nbsp;
   9&nbsp;#  It is not necessary to explicitly assign a variable.
  10&nbsp;echo "` &#60;$0`"           # Echoes the script itself to stdout.</PRE
></TD
></TR
></TABLE
>
             </P
><P
>	         <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#  Excerpts from system file, /etc/rc.d/rc.sysinit
   2&nbsp;#+ (on a Red Hat Linux installation)
   3&nbsp;
   4&nbsp;
   5&nbsp;if [ -f /fsckoptions ]; then
   6&nbsp;        fsckoptions=`cat /fsckoptions`
   7&nbsp;...
   8&nbsp;fi
   9&nbsp;#
  10&nbsp;#
  11&nbsp;if [ -e "/proc/ide/${disk[$device]}/media" ] ; then
  12&nbsp;             hdmedia=`cat /proc/ide/${disk[$device]}/media`
  13&nbsp;...
  14&nbsp;fi
  15&nbsp;#
  16&nbsp;#
  17&nbsp;if [ ! -n "`uname -r | grep -- "-"`" ]; then
  18&nbsp;       ktag="`cat /proc/version`"
  19&nbsp;...
  20&nbsp;fi
  21&nbsp;#
  22&nbsp;#
  23&nbsp;if [ $usb = "1" ]; then
  24&nbsp;    sleep 5
  25&nbsp;    mouseoutput=`cat /proc/bus/usb/devices 2&#62;/dev/null|grep -E "^I.*Cls=03.*Prot=02"`
  26&nbsp;    kbdoutput=`cat /proc/bus/usb/devices 2&#62;/dev/null|grep -E "^I.*Cls=03.*Prot=01"`
  27&nbsp;...
  28&nbsp;fi</PRE
></TD
></TR
></TABLE
>
             </P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Do not set a variable to the contents of a
	       <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>long</I
></SPAN
> text file unless you have a very good
	       reason for doing so. Do not set a variable to the contents of a
	       <I
CLASS="FIRSTTERM"
>binary</I
> file, even as a joke.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="STUPSCR"
></A
><P
><B
>Example 12-1. Stupid script tricks</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# stupid-script-tricks.sh: Don't try this at home, folks.
   3&nbsp;# From "Stupid Script Tricks," Volume I.
   4&nbsp;
   5&nbsp;exit 99  ### Comment out this line if you dare.
   6&nbsp;
   7&nbsp;dangerous_variable=`cat /boot/vmlinuz`   # The compressed Linux kernel itself.
   8&nbsp;
   9&nbsp;echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
  10&nbsp;# string-length of $dangerous_variable = 794151
  11&nbsp;# (Newer kernels are bigger.)
  12&nbsp;# Does not give same count as 'wc -c /boot/vmlinuz'.
  13&nbsp;
  14&nbsp;# echo "$dangerous_variable"
  15&nbsp;# Don't try this! It would hang the script.
  16&nbsp;
  17&nbsp;
  18&nbsp;#  The document author is aware of no useful applications for
  19&nbsp;#+ setting a variable to the contents of a binary file.
  20&nbsp;
  21&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Notice that a <I
CLASS="FIRSTTERM"
>buffer overrun</I
>
	       does not occur. This is one instance where an interpreted
	       language, such as Bash, provides more protection from
	       programmer mistakes than a compiled language.</P
></TD
></TR
></TABLE
></DIV
><P
><A
NAME="CSVL"
></A
></P
><P
>Command substitution permits setting a variable to the
		output of a <A
HREF="loops.html#FORLOOPREF1"
>loop</A
>. The
		key to this is grabbing the output of an <A
HREF="internal.html#ECHOREF"
>echo</A
> command within the
		loop.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="CSUBLOOP"
></A
><P
><B
>Example 12-2. Generating a variable from a loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# csubloop.sh: Setting a variable to the output of a loop.
   3&nbsp;
   4&nbsp;variable1=`for i in 1 2 3 4 5
   5&nbsp;do
   6&nbsp;  echo -n "$i"                 #  The 'echo' command is critical
   7&nbsp;done`                          #+ to command substitution here.
   8&nbsp;
   9&nbsp;echo "variable1 = $variable1"  # variable1 = 12345
  10&nbsp;
  11&nbsp;
  12&nbsp;i=0
  13&nbsp;variable2=`while [ "$i" -lt 10 ]
  14&nbsp;do
  15&nbsp;  echo -n "$i"                 # Again, the necessary 'echo'.
  16&nbsp;  let "i += 1"                 # Increment.
  17&nbsp;done`
  18&nbsp;
  19&nbsp;echo "variable2 = $variable2"  # variable2 = 0123456789
  20&nbsp;
  21&nbsp;#  Demonstrates that it's possible to embed a loop
  22&nbsp;#+ inside a variable declaration.
  23&nbsp;
  24&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="CSTOOLSET"
></A
></P
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN7273"
></A
><P
>Command substitution makes it possible to extend the
		toolset available to Bash. It is simply a matter
		of writing a program or script that outputs to
		<TT
CLASS="FILENAME"
>stdout</TT
> (like a well-behaved UNIX
		tool should) and assigning that output to a variable.</P
><P
>		<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#include &#60;stdio.h&#62;
   2&nbsp;
   3&nbsp;/*  "Hello, world." C program  */		
   4&nbsp;
   5&nbsp;int main()
   6&nbsp;{
   7&nbsp;  printf( "Hello, world.\n" );
   8&nbsp;  return (0);
   9&nbsp;}</PRE
></TD
></TR
></TABLE
>
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>gcc -o hello hello.c</B
></TT
>
 	      </PRE
></TD
></TR
></TABLE
>
		</P
><P
>                <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# hello.sh		
   3&nbsp;
   4&nbsp;greeting=`./hello`
   5&nbsp;echo $greeting</PRE
></TD
></TR
></TABLE
>
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>sh hello.sh</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>Hello, world.</TT
>
 	        </PRE
></TD
></TR
></TABLE
>
	        </P
></DIV
></TD
></TR
></TABLE
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><A
NAME="CSPARENS"
></A
>The <B
CLASS="COMMAND"
>$(...)</B
>
		form has superseded backticks for command
		substitution.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;output=$(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;$file2)        # Bash permits this also.</PRE
></TD
></TR
></TABLE
></P
><P
>The <B
CLASS="COMMAND"
>$(...)</B
> form of command substitution
	        treats a double backslash in a different way than
		<B
CLASS="COMMAND"
>`...`</B
>.</P
><P
>		
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><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 $(echo \\)</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>\</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
              </P
><P
><A
NAME="CSNEST"
></A
></P
><P
>The <B
CLASS="COMMAND"
>$(...)</B
> form of command
                substitution permits nesting.
		  <A
NAME="AEN7308"
HREF="#FTN.AEN7308"
>[3]</A
>
		
		</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;word_count=$( wc -w $(echo * | awk '{print $8}') )</PRE
></TD
></TR
></TABLE
>
              </P
><P
>Or, for something a bit more elaborate . . .</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="AGRAM2"
></A
><P
><B
>Example 12-3. Finding anagrams</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
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://bash.deta.in/yawl-0.3.2.tar.gz
   9&nbsp;
  10&nbsp;E_NOARGS=86
  11&nbsp;E_BADARG=87
  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 exercise in anagram finding.
  45&nbsp;
  46&nbsp;exit $?</PRE
></TD
></TR
></TABLE
><HR></DIV
></TD
></TR
></TABLE
></DIV
><P
>Examples of command substitution in shell scripts:
       <OL
TYPE="1"
><LI
><P
><A
HREF="loops.html#BINGREP"
>Example 11-8</A
></P
></LI
><LI
><P
><A
HREF="testbranch.html#CASECMD"
>Example 11-27</A
></P
></LI
><LI
><P
><A
HREF="randomvar.html#SEEDINGRANDOM"
>Example 9-16</A
></P
></LI
><LI
><P
><A
HREF="moreadv.html#EX57"
>Example 16-3</A
></P
></LI
><LI
><P
><A
HREF="textproc.html#LOWERCASE"
>Example 16-22</A
></P
></LI
><LI
><P
><A
HREF="textproc.html#GRP"
>Example 16-17</A
></P
></LI
><LI
><P
><A
HREF="extmisc.html#EX53"
>Example 16-54</A
></P
></LI
><LI
><P
><A
HREF="loops.html#EX24"
>Example 11-14</A
></P
></LI
><LI
><P
><A
HREF="loops.html#SYMLINKS"
>Example 11-11</A
></P
></LI
><LI
><P
><A
HREF="filearchiv.html#STRIPC"
>Example 16-32</A
></P
></LI
><LI
><P
><A
HREF="redircb.html#REDIR4"
>Example 20-8</A
></P
></LI
><LI
><P
><A
HREF="contributed-scripts.html#TREE"
>Example A-16</A
></P
></LI
><LI
><P
><A
HREF="procref1.html#PIDID"
>Example 29-3</A
></P
></LI
><LI
><P
><A
HREF="mathc.html#MONTHLYPMT"
>Example 16-47</A
></P
></LI
><LI
><P
><A
HREF="mathc.html#BASE"
>Example 16-48</A
></P
></LI
><LI
><P
><A
HREF="mathc.html#ALTBC"
>Example 16-49</A
></P
></LI
></OL
>
     </P
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN7205"
HREF="commandsub.html#AEN7205"
>[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
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN7211"
HREF="commandsub.html#AEN7211"
>[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.AEN7308"
HREF="commandsub.html#AEN7308"
>[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.
		      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;word_count=` wc -w \`echo * | awk '{print $8}'\` `</PRE
></TD
></TR
></TABLE
>
		    </P
></TD
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