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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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>Indirect References</TITLE
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><DIV
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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="varsubn.html"
>referencing
a variable</A
>, <TT
CLASS="VARNAME"
>$var</TT
>, fetches its
<I
CLASS="FIRSTTERM"
>value</I
>. <A
NAME="EVALINDREF"
></A
>But,
what about the <EM
>value of a value</EM
>? What
about <TT
CLASS="VARNAME"
>$$var</TT
>?</P
><P
>The actual notation is
<TT
CLASS="REPLACEABLE"
><I
>\$$var</I
></TT
>, usually preceded by
an <A
HREF="internal.html#EVALREF"
>eval</A
> (and sometimes an
<A
HREF="internal.html#ECHOREF"
>echo</A
>). This is called an
<I
CLASS="FIRSTTERM"
>indirect reference</I
>.</P
><DIV
CLASS="EXAMPLE"
><A
NAME="INDREF"
></A
><P
><B
>Example 28-1. Indirect Variable References</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# ind-ref.sh: Indirect variable referencing.
# Accessing the contents of the contents of a variable.
# First, let's fool around a little.
var=23
echo "\$var = $var" # $var = 23
# So far, everything as expected. But ...
echo "\$\$var = $$var" # $$var = 4570var
# Not useful ...
# \$\$ expanded to PID of the script
# -- refer to the entry on the $$ variable --
#+ and "var" is echoed as plain text.
# (Thank you, Jakob Bohm, for pointing this out.)
echo "\\\$\$var = \$$var" # \$$var = $23
# As expected. The first $ is escaped and pasted on to
#+ the value of var ($var = 23 ).
# Meaningful, but still not useful.
# Now, let's start over and do it the right way.
# ============================================== #
a=letter_of_alphabet # Variable "a" holds the name of another variable.
letter_of_alphabet=z
echo
# Direct reference.
echo "a = $a" # a = letter_of_alphabet
# Indirect reference.
eval a=\$$a
# ^^^ Forcing an eval(uation), and ...
# ^ Escaping the first $ ...
# ------------------------------------------------------------------------
# The 'eval' forces an update of $a, sets it to the updated value of \$$a.
# So, we see why 'eval' so often shows up in indirect reference notation.
# ------------------------------------------------------------------------
echo "Now a = $a" # Now a = z
echo
# Now, let's try changing the second-order reference.
t=table_cell_3
table_cell_3=24
echo "\"table_cell_3\" = $table_cell_3" # "table_cell_3" = 24
echo -n "dereferenced \"t\" = "; eval echo \$$t # dereferenced "t" = 24
# In this simple case, the following also works (why?).
# eval t=\$$t; echo "\"t\" = $t"
echo
t=table_cell_3
NEW_VAL=387
table_cell_3=$NEW_VAL
echo "Changing value of \"table_cell_3\" to $NEW_VAL."
echo "\"table_cell_3\" now $table_cell_3"
echo -n "dereferenced \"t\" now "; eval echo \$$t
# "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3)
echo
# (Thanks, Stephane Chazelas, for clearing up the above behavior.)
# A more straightforward method is the ${!t} notation, discussed in the
#+ "Bash, version 2" section.
# See also ex78.sh.
exit 0</PRE
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></A
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><DIV
CLASS="SIDEBAR"
><A
NAME="AEN18998"
></A
><P
></P
><P
>Indirect referencing in Bash
is a multi-step process. First, take the name of a variable:
<TT
CLASS="VARNAME"
>varname</TT
>. Then, reference it:
<TT
CLASS="VARNAME"
>$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
><P
></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="varsubn.html#POINTERREF"
>pointers</A
>
in <I
CLASS="FIRSTTERM"
>C</I
>, for instance, in <A
HREF="bashver2.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
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# ---------------------------------------------
# This could be "sourced" from a separate file.
isdnMyProviderRemoteNet=172.16.0.100
isdnYourProviderRemoteNet=10.0.0.10
isdnOnlineService="MyProvider"
# ---------------------------------------------
remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)")
remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)")
remoteNet=$(eval "echo \$isdnMyProviderRemoteNet")
remoteNet=$(eval "echo $isdnMyProviderRemoteNet")
echo "$remoteNet" # 172.16.0.100
# ================================================================
# And, it gets even better.
# Consider the following snippet given a variable named getSparc,
#+ but no such variable getIa64:
chkMirrorArchs () {
arch="$1";
if [ "$(eval "echo \${$(echo get$(echo -ne $arch |
sed 's/^\(.\).*/\1/g' | tr 'a-z' 'A-Z'; echo $arch |
sed 's/^.\(.*\)/\1/g')):-false}")" = true ]
then
return 0;
else
return 1;
fi;
}
getSparc="true"
unset getIa64
chkMirrorArchs sparc
echo $? # 0
# True
chkMirrorArchs Ia64
echo $? # 1
# False
# Notes:
# -----
# Even the to-be-substituted variable name part is built explicitly.
# The parameters to the chkMirrorArchs calls are all lower case.
# The variable name is composed of two parts: "get" and "Sparc" . . .</PRE
></FONT
></TD
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></TABLE
>
</P
><DIV
CLASS="EXAMPLE"
><A
NAME="COLTOTALER2"
></A
><P
><B
>Example 28-2. Passing an indirect reference to <I
CLASS="FIRSTTERM"
>awk</I
></B
></P
><TABLE
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BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# Another version of the "column totaler" script
#+ that adds up a specified column (of numbers) in the target file.
# This one uses indirect references.
ARGS=2
E_WRONGARGS=85
if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.
then
echo "Usage: `basename $0` filename column-number"
exit $E_WRONGARGS
fi
filename=$1 # Name of file to operate on.
column_number=$2 # Which column to total up.
#===== Same as original script, up to this point =====#
# A multi-line awk script is invoked by
# awk "
# ...
# ...
# ...
# "
# Begin awk script.
# -------------------------------------------------
awk "
{ total += \$${column_number} # Indirect reference
}
END {
print total
}
" "$filename"
# Note that awk doesn't need an eval preceding \$$.
# -------------------------------------------------
# End awk script.
# Indirect variable reference avoids the hassles
#+ of referencing a shell variable within the embedded awk script.
# Thanks, Stephane Chazelas.
exit $?</PRE
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><P
></P
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><TD
WIDTH="25"
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VALIGN="TOP"
><IMG
SRC="../images/caution.gif"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>This method of indirect referencing is a bit tricky.
If the second order variable changes its value, then the first
order variable must be properly dereferenced (as in the above
example). <A
NAME="IVR2"
></A
>Fortunately, the
<TT
CLASS="REPLACEABLE"
><I
>${!variable}</I
></TT
> notation introduced
with <A
HREF="bashver2.html#BASH2REF"
>version 2</A
> of Bash
(see <A
HREF="bashver2.html#EX78"
>Example 37-2</A
> and <A
HREF="contributed-scripts.html#HASHEX2"
>Example A-22</A
>) makes
indirect referencing more intuitive.</P
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><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN19027"
></A
><P
></P
><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
><P
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