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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><H1
><A
NAME="IVR"
></A
>Chapter 28. Indirect References</H1
><P
><A
NAME="IVRREF"
></A
></P
><P
>We have seen that <A
HREF="variables.html#VARSUBN"
>referencing
        a variable</A
>, <TT
CLASS="VARNAME"
>$var</TT
>, fetches its
        <I
CLASS="FIRSTTERM"
>value</I
>. <A
NAME="EVALINDREF"
></A
>But,
        what about the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>value of a value</I
></SPAN
>? What
        about <TT
CLASS="VARNAME"
>$$var</TT
>?</P
><P
>The actual notation is
	<TT
CLASS="REPLACEABLE"
><I
>\$$var</I
></TT
>, usually preceded by
	an <A
HREF="internal.html#EVALREF"
>eval</A
> (and sometimes an
	<A
HREF="internal.html#ECHOREF"
>echo</A
>). This is called an
	<I
CLASS="FIRSTTERM"
>indirect reference</I
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="INDREF"
></A
><P
><B
>Example 28-1. Indirect Variable References</B
></P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# ind-ref.sh: Indirect variable referencing.
   3&nbsp;# Accessing the contents of the contents of a variable.
   4&nbsp;
   5&nbsp;# First, let's fool around a little.
   6&nbsp;
   7&nbsp;var=23
   8&nbsp;
   9&nbsp;echo "\$var   = $var"           # $var   = 23
  10&nbsp;# So far, everything as expected. But ...
  11&nbsp;
  12&nbsp;echo "\$\$var  = $$var"         # $$var  = 4570var
  13&nbsp;#  Not useful ...
  14&nbsp;#  \$\$ expanded to PID of the script
  15&nbsp;#  -- refer to the entry on the $$ variable --
  16&nbsp;#+ and "var" is echoed as plain text.
  17&nbsp;#  (Thank you, Jakob Bohm, for pointing this out.)
  18&nbsp;
  19&nbsp;echo "\\\$\$var = \$$var"       # \$$var = $23
  20&nbsp;#  As expected. The first $ is escaped and pasted on to
  21&nbsp;#+ the value of var ($var = 23 ).
  22&nbsp;#  Meaningful, but still not useful. 
  23&nbsp;
  24&nbsp;# Now, let's start over and do it the right way.
  25&nbsp;
  26&nbsp;# ============================================== #
  27&nbsp;
  28&nbsp;
  29&nbsp;a=letter_of_alphabet   # Variable "a" holds the name of another variable.
  30&nbsp;letter_of_alphabet=z
  31&nbsp;
  32&nbsp;echo
  33&nbsp;
  34&nbsp;# Direct reference.
  35&nbsp;echo "a = $a"          # a = letter_of_alphabet
  36&nbsp;
  37&nbsp;# Indirect reference.
  38&nbsp;  eval a=\$$a
  39&nbsp;# ^^^        Forcing an eval(uation), and ...
  40&nbsp;#        ^   Escaping the first $ ...
  41&nbsp;# ------------------------------------------------------------------------
  42&nbsp;# The 'eval' forces an update of $a, sets it to the updated value of \$$a.
  43&nbsp;# So, we see why 'eval' so often shows up in indirect reference notation.
  44&nbsp;# ------------------------------------------------------------------------
  45&nbsp;  echo "Now a = $a"    # Now a = z
  46&nbsp;
  47&nbsp;echo
  48&nbsp;
  49&nbsp;
  50&nbsp;# Now, let's try changing the second-order reference.
  51&nbsp;
  52&nbsp;t=table_cell_3
  53&nbsp;table_cell_3=24
  54&nbsp;echo "\"table_cell_3\" = $table_cell_3"            # "table_cell_3" = 24
  55&nbsp;echo -n "dereferenced \"t\" = "; eval echo \$$t    # dereferenced "t" = 24
  56&nbsp;# In this simple case, the following also works (why?).
  57&nbsp;#         eval t=\$$t; echo "\"t\" = $t"
  58&nbsp;
  59&nbsp;echo
  60&nbsp;
  61&nbsp;t=table_cell_3
  62&nbsp;NEW_VAL=387
  63&nbsp;table_cell_3=$NEW_VAL
  64&nbsp;echo "Changing value of \"table_cell_3\" to $NEW_VAL."
  65&nbsp;echo "\"table_cell_3\" now $table_cell_3"
  66&nbsp;echo -n "dereferenced \"t\" now "; eval echo \$$t
  67&nbsp;# "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3)
  68&nbsp;
  69&nbsp;
  70&nbsp;echo
  71&nbsp;
  72&nbsp;# (Thanks, Stephane Chazelas, for clearing up the above behavior.)
  73&nbsp;
  74&nbsp;
  75&nbsp;#   A more straightforward method is the ${!t} notation, discussed in the
  76&nbsp;#+ "Bash, version 2" section.
  77&nbsp;#   See also ex78.sh.
  78&nbsp;
  79&nbsp;exit 0</PRE
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><A
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></A
><P
>Indirect referencing in Bash
        is a multi-step process. First, take the name of a variable:
	<TT
CLASS="VARNAME"
>varname</TT
>. Then, reference it:
	<TT
CLASS="VARNAME"
>$varname</TT
>. Then, reference the reference:
	<TT
CLASS="VARNAME"
>$$varname</TT
>. Then, <I
CLASS="FIRSTTERM"
>escape</I
>
	the first <SPAN
CLASS="TOKEN"
>$</SPAN
>: <TT
CLASS="VARNAME"
>\$$varname</TT
>.
	Finally, force a reevaluation of the expression and assign it:
	<B
CLASS="COMMAND"
>eval newvar=\$$varname</B
>.</P
></DIV
></TD
></TR
></TABLE
><P
>Of what practical use is indirect referencing of
	  variables? It gives Bash a little of the functionality
	  of <A
HREF="variables.html#POINTERREF"
>pointers</A
>
	  in <I
CLASS="FIRSTTERM"
>C</I
>, for instance, in <A
HREF="bash2.html#RESISTOR"
>table lookup</A
>.  And, it also has some
	  other very interesting applications. . . .</P
><P
>Nils Radtke shows how to build <SPAN
CLASS="QUOTE"
>"dynamic"</SPAN
>
	  variable names and evaluate their contents. This can be useful
	  when <A
HREF="internal.html#SOURCEREF"
>sourcing</A
> configuration
	  files.</P
><P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;
   4&nbsp;# ---------------------------------------------
   5&nbsp;# This could be "sourced" from a separate file.
   6&nbsp;isdnMyProviderRemoteNet=172.16.0.100
   7&nbsp;isdnYourProviderRemoteNet=10.0.0.10
   8&nbsp;isdnOnlineService="MyProvider"
   9&nbsp;# ---------------------------------------------
  10&nbsp;      
  11&nbsp;
  12&nbsp;remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)")
  13&nbsp;remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)")
  14&nbsp;remoteNet=$(eval "echo \$isdnMyProviderRemoteNet")
  15&nbsp;remoteNet=$(eval "echo $isdnMyProviderRemoteNet")
  16&nbsp;
  17&nbsp;echo "$remoteNet"    # 172.16.0.100
  18&nbsp;
  19&nbsp;# ================================================================
  20&nbsp;
  21&nbsp;#  And, it gets even better.
  22&nbsp;
  23&nbsp;#  Consider the following snippet given a variable named getSparc,
  24&nbsp;#+ but no such variable getIa64:
  25&nbsp;
  26&nbsp;chkMirrorArchs () { 
  27&nbsp;  arch="$1";
  28&nbsp;  if [ "$(eval "echo \${$(echo get$(echo -ne $arch |
  29&nbsp;       sed 's/^\(.\).*/\1/g' | tr 'a-z' 'A-Z'; echo $arch |
  30&nbsp;       sed 's/^.\(.*\)/\1/g')):-false}")" = true ]
  31&nbsp;  then
  32&nbsp;     return 0;
  33&nbsp;  else
  34&nbsp;     return 1;
  35&nbsp;  fi;
  36&nbsp;}
  37&nbsp;
  38&nbsp;getSparc="true"
  39&nbsp;unset getIa64
  40&nbsp;chkMirrorArchs sparc
  41&nbsp;echo $?        # 0
  42&nbsp;               # True
  43&nbsp;
  44&nbsp;chkMirrorArchs Ia64
  45&nbsp;echo $?        # 1
  46&nbsp;               # False
  47&nbsp;
  48&nbsp;# Notes:
  49&nbsp;# -----
  50&nbsp;# Even the to-be-substituted variable name part is built explicitly.
  51&nbsp;# The parameters to the chkMirrorArchs calls are all lower case.
  52&nbsp;# The variable name is composed of two parts: "get" and "Sparc" . . .</PRE
></TD
></TR
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>
        </P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="COLTOTALER2"
></A
><P
><B
>Example 28-2. Passing an indirect reference to <I
CLASS="FIRSTTERM"
>awk</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#  Another version of the "column totaler" script
   4&nbsp;#+ that adds up a specified column (of numbers) in the target file.
   5&nbsp;#  This one uses indirect references.
   6&nbsp;
   7&nbsp;ARGS=2
   8&nbsp;E_WRONGARGS=85
   9&nbsp;
  10&nbsp;if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.
  11&nbsp;then
  12&nbsp;   echo "Usage: `basename $0` filename column-number"
  13&nbsp;   exit $E_WRONGARGS
  14&nbsp;fi
  15&nbsp;
  16&nbsp;filename=$1         # Name of file to operate on.
  17&nbsp;column_number=$2    # Which column to total up.
  18&nbsp;
  19&nbsp;#===== Same as original script, up to this point =====#
  20&nbsp;
  21&nbsp;
  22&nbsp;# A multi-line awk script is invoked by
  23&nbsp;#   awk "
  24&nbsp;#   ...
  25&nbsp;#   ...
  26&nbsp;#   ...
  27&nbsp;#   "
  28&nbsp;
  29&nbsp;
  30&nbsp;# Begin awk script.
  31&nbsp;# -------------------------------------------------
  32&nbsp;awk "
  33&nbsp;
  34&nbsp;{ total += \$${column_number} # Indirect reference
  35&nbsp;}
  36&nbsp;END {
  37&nbsp;     print total
  38&nbsp;     }
  39&nbsp;
  40&nbsp;     " "$filename"
  41&nbsp;# Note that awk doesn't need an eval preceding \$$.
  42&nbsp;# -------------------------------------------------
  43&nbsp;# End awk script.
  44&nbsp;
  45&nbsp;#  Indirect variable reference avoids the hassles
  46&nbsp;#+ of referencing a shell variable within the embedded awk script.
  47&nbsp;#  Thanks, Stephane Chazelas.
  48&nbsp;
  49&nbsp;
  50&nbsp;exit $?</PRE
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><TD
WIDTH="25"
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VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
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><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>This method of indirect referencing is a bit tricky.
	If the second order variable changes its value, then the first
	order variable must be properly dereferenced (as in the above
	example). <A
NAME="IVR2"
></A
>Fortunately, the
	<TT
CLASS="REPLACEABLE"
><I
>${!variable}</I
></TT
> notation introduced
	with <A
HREF="bash2.html#BASH2REF"
>version 2</A
> of Bash
	(see <A
HREF="bash2.html#EX78"
>Example 37-2</A
> and <A
HREF="contributed-scripts.html#HASHEX2"
>Example A-22</A
>) makes
	indirect referencing more intuitive.</P
></TD
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><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN19027"
></A
><P
>Bash does not support pointer arithmetic, and this severely
	 limits the usefulness of indirect referencing. In fact, indirect
	 referencing in a scripting language is, at best, something of
	 an afterthought.</P
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