LDP
/
old-www
1
1
Fork 0
Code Issues Pull Requests Releases Wiki Activity
old-www/LDP/abs/html/commandsub.html

1306 lines
19 KiB
HTML
Raw Permalink Blame History

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML
><HEAD
><TITLE
>Command Substitution</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
REL="HOME"
TITLE="Advanced Bash-Scripting Guide"
HREF="index.html"><LINK
REL="UP"
TITLE="Beyond the Basics"
HREF="part3.html"><LINK
REL="PREVIOUS"
TITLE="Testing and Branching"
HREF="testbranch.html"><LINK
REL="NEXT"
TITLE="Arithmetic Expansion"
HREF="arithexp.html"></HEAD
><BODY
CLASS="CHAPTER"
BGCOLOR="#FFFFFF"
TEXT="#000000"
LINK="#0000FF"
VLINK="#840084"
ALINK="#0000FF"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>Advanced Bash-Scripting Guide: </TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="testbranch.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
></TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="arithexp.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><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"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>
or even multiple commands; it literally plugs the command
output into another context.
<A
NAME="AEN7211"
HREF="#FTN.AEN7211"
><SPAN
CLASS="footnote"
>[2]</SPAN
></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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>script_name=`basename $0`
echo "The name of this script is $script_name."</PRE
></FONT
></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="loops1.html#FORLOOPREF1"
>for</A
>
loop. </B
></P
></DIV
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>rm `cat filename` # <SPAN
CLASS="QUOTE"
>"filename"</SPAN
> contains a list of files to delete.
#
# S. C. points out that "arg list too long" error might result.
# Better is xargs rm -- &#60; filename
# ( -- covers those cases where <SPAN
CLASS="QUOTE"
>"filename"</SPAN
> begins with a <SPAN
CLASS="QUOTE"
>"-"</SPAN
> )
textfile_listing=`ls *.txt`
# Variable contains names of all *.txt files in current working directory.
echo $textfile_listing
textfile_listing2=$(ls *.txt) # The alternative form of command substitution.
echo $textfile_listing2
# Same result.
# A possible problem with putting a list of files into a single string
# is that a newline may creep in.
#
# A safer way to assign a list of files to a parameter is with an array.
# shopt -s nullglob # If no match, filename expands to nothing.
# textfile_listing=( *.txt )
#
# Thanks, S.C.</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><DIV
CLASS="NOTE"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/note.gif"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><A
NAME="CSSUBSH"
></A
>Command substitution
invokes a <A
HREF="subshells.html#SUBSHELLSREF"
>subshell</A
>.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/caution.gif"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><A
NAME="CSWS"
></A
>Command substitution may
result in <A
HREF="quotingvar.html#WSPLITREF"
>word splitting</A
>.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>COMMAND `echo a b` # 2 args: a and b
COMMAND "`echo a b`" # 1 arg: "a b"
COMMAND `echo` # no arg
COMMAND "`echo`" # one empty arg
# Thanks, S.C.</PRE
></FONT
></TD
></TR
></TABLE
></P
><P
><A
NAME="CSTRNL"
></A
></P
><P
>Even when there is no word splitting, command
substitution can remove trailing newlines.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
># cd "`pwd`" # This should always work.
# However...
mkdir 'dir with trailing newline
'
cd 'dir with trailing newline
'
cd "`pwd`" # Error message:
# bash: cd: /tmp/file with trailing newline: No such file or directory
cd "$PWD" # Works fine.
old_tty_setting=$(stty -g) # Save old terminal setting.
echo "Hit a key "
stty -icanon -echo # Disable "canonical" mode for terminal.
# Also, disable *local* echo.
key=$(dd bs=1 count=1 2&#62; /dev/null) # Using 'dd' to get a keypress.
stty "$old_tty_setting" # Restore old setting.
echo "You hit ${#key} key." # ${#variable} = number of characters in $variable
#
# Hit any key except RETURN, and the output is "You hit 1 key."
# Hit RETURN, and it's "You hit 0 key."
# The newline gets eaten in the command substitution.
#Code snippet by St<53>phane Chazelas.</PRE
></FONT
></TD
></TR
></TABLE
>
</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/caution.gif"
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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>dir_listing=`ls -l`
echo $dir_listing # unquoted
# Expecting a nicely ordered directory listing.
# However, what you get is:
# total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo
# bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh
# The newlines disappeared.
echo "$dir_listing" # quoted
# -rw-rw-r-- 1 bozo 30 May 13 17:15 1.txt
# -rw-rw-r-- 1 bozo 51 May 15 20:57 t2.sh
# -rwxr-xr-x 1 bozo 217 Mar 5 21:13 wi.sh</PRE
></FONT
></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="basic.html#CATREF"
>cat</A
> command.</P
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>variable1=`&#60;file1` # Set "variable1" to contents of "file1".
variable2=`cat file2` # Set "variable2" to contents of "file2".
# This, however, forks a new process,
#+ so the line of code executes slower than the above version.
# Note that the variables may contain embedded whitespace,
#+ or even (horrors), control characters.
# It is not necessary to explicitly assign a variable.
echo "` &#60;$0`" # Echoes the script itself to stdout.</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
># Excerpts from system file, /etc/rc.d/rc.sysinit
#+ (on a Red Hat Linux installation)
if [ -f /fsckoptions ]; then
fsckoptions=`cat /fsckoptions`
...
fi
#
#
if [ -e "/proc/ide/${disk[$device]}/media" ] ; then
hdmedia=`cat /proc/ide/${disk[$device]}/media`
...
fi
#
#
if [ ! -n "`uname -r | grep -- "-"`" ]; then
ktag="`cat /proc/version`"
...
fi
#
#
if [ $usb = "1" ]; then
sleep 5
mouseoutput=`cat /proc/bus/usb/devices 2&#62;/dev/null|grep -E "^I.*Cls=03.*Prot=02"`
kbdoutput=`cat /proc/bus/usb/devices 2&#62;/dev/null|grep -E "^I.*Cls=03.*Prot=01"`
...
fi</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/caution.gif"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Do not set a variable to the contents of a
<EM
>long</EM
> 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"
><A
NAME="STUPSCR"
></A
><P
><B
>Example 12-1. Stupid script tricks</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# stupid-script-tricks.sh: Don't try this at home, folks.
# From "Stupid Script Tricks," Volume I.
exit 99 ### Comment out this line if you dare.
dangerous_variable=`cat /boot/vmlinuz` # The compressed Linux kernel itself.
echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
# string-length of $dangerous_variable = 794151
# (Newer kernels are bigger.)
# Does not give same count as 'wc -c /boot/vmlinuz'.
# echo "$dangerous_variable"
# Don't try this! It would hang the script.
# The document author is aware of no useful applications for
#+ setting a variable to the contents of a binary file.
exit 0</PRE
></FONT
></TD
></TR
></TABLE
></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="loops1.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"
><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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# csubloop.sh: Setting a variable to the output of a loop.
variable1=`for i in 1 2 3 4 5
do
echo -n "$i" # The 'echo' command is critical
done` #+ to command substitution here.
echo "variable1 = $variable1" # variable1 = 12345
i=0
variable2=`while [ "$i" -lt 10 ]
do
echo -n "$i" # Again, the necessary 'echo'.
let "i += 1" # Increment.
done`
echo "variable2 = $variable2" # variable2 = 0123456789
# Demonstrates that it's possible to embed a loop
#+ inside a variable declaration.
exit 0</PRE
></FONT
></TD
></TR
></TABLE
></DIV
><P
><A
NAME="CSTOOLSET"
></A
></P
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN7273"
></A
><P
></P
><P
>Command substitution makes it possible to extend the
toolset available to Bash. It is simply a matter
of writing a program or script that outputs to
<TT
CLASS="FILENAME"
>stdout</TT
> (like a well-behaved UNIX
tool should) and assigning that output to a variable.</P
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#include &#60;stdio.h&#62;
/* "Hello, world." C program */
int main()
{
printf( "Hello, world.\n" );
return (0);
}</PRE
></FONT
></TD
></TR
></TABLE
>
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>gcc -o hello hello.c</B
></TT
>
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
> <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# hello.sh
greeting=`./hello`
echo $greeting</PRE
></FONT
></TD
></TR
></TABLE
>
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>sh hello.sh</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>Hello, world.</TT
>
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
></P
></DIV
></TD
></TR
></TABLE
><DIV
CLASS="NOTE"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/note.gif"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><A
NAME="CSPARENS"
></A
>The <B
CLASS="COMMAND"
>$(...)</B
>
form has superseded backticks for command
substitution.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>output=$(sed -n /"$1"/p $file) # From "grp.sh" example.
# Setting a variable to the contents of a text file.
File_contents1=$(cat $file1)
File_contents2=$(&#60;$file2) # Bash permits this also.</PRE
></FONT
></TD
></TR
></TABLE
></P
><P
>The <B
CLASS="COMMAND"
>$(...)</B
> form of command substitution
treats a double backslash in a different way than
<B
CLASS="COMMAND"
>`...`</B
>.</P
><P
>
<TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><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 $(echo \\)</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>\</TT
>
</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
><A
NAME="CSNEST"
></A
></P
><P
>The <B
CLASS="COMMAND"
>$(...)</B
> form of command
substitution permits nesting.
<A
NAME="AEN7308"
HREF="#FTN.AEN7308"
><SPAN
CLASS="footnote"
>[3]</SPAN
></A
>
</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>word_count=$( wc -w $(echo * | awk '{print $8}') )</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
>Or, for something a bit more elaborate . . .</P
><DIV
CLASS="EXAMPLE"
><A
NAME="AGRAM2"
></A
><P
><B
>Example 12-3. Finding anagrams</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# agram2.sh
# Example of nested command substitution.
# Uses "anagram" utility
#+ that is part of the author's "yawl" word list package.
# http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
# http://bash.deta.in/yawl-0.3.2.tar.gz
E_NOARGS=86
E_BADARG=87
MINLEN=7
if [ -z "$1" ]
then
echo "Usage $0 LETTERSET"
exit $E_NOARGS # Script needs a command-line argument.
elif [ ${#1} -lt $MINLEN ]
then
echo "Argument must have at least $MINLEN letters."
exit $E_BADARG
fi
FILTER='.......' # Must have at least 7 letters.
# 1234567
Anagrams=( $(echo $(anagram $1 | grep $FILTER) ) )
# $( $( nested command sub. ) )
# ( array assignment )
echo
echo "${#Anagrams[*]} 7+ letter anagrams found"
echo
echo ${Anagrams[0]} # First anagram.
echo ${Anagrams[1]} # Second anagram.
# Etc.
# echo "${Anagrams[*]}" # To list all the anagrams in a single line . . .
# Look ahead to the Arrays chapter for enlightenment on
#+ what's going on here.
# See also the agram.sh script for an exercise in anagram finding.
exit $?</PRE
></FONT
></TD
></TR
></TABLE
></DIV
></TD
></TR
></TABLE
></DIV
><P
>Examples of command substitution in shell scripts:
<P
></P
><OL
TYPE="1"
><LI
><P
><A
HREF="loops1.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="loops1.html#EX24"
>Example 11-14</A
></P
></LI
><LI
><P
><A
HREF="loops1.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"
><SPAN
CLASS="footnote"
>[1]</SPAN
></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"
><SPAN
CLASS="footnote"
>[2]</SPAN
></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"
><SPAN
CLASS="footnote"
>[3]</SPAN
></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
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>word_count=` wc -w \`echo * | awk '{print $8}'\` `</PRE
></FONT
></TD
></TR
></TABLE
>
</P
></TD
></TR
></TABLE
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="testbranch.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="arithexp.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Testing and Branching</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="part3.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Arithmetic Expansion</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
Reference in New Issue View Git Blame Copy Permalink