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Indirect References to Variables

9.5. Indirect References to Variables

Assume that the value of a variable is the name of a second variable. Is it somehow possible to retrieve the value of this second variable from the first one? For example, if a=letter_of_alphabet and letter_of_alphabet=z, can a reference to a return z? This can indeed be done, and it is called an indirect reference. It uses the unusual eval var1=\$$var2 notation.

Example 9-23. Indirect References

# ind-ref.sh: Indirect variable referencing.
# Accessing the contents of the contents of a variable.

a=letter_of_alphabet   # Variable "a" holds the name of another variable.


# Direct reference.
echo "a = $a"          # a = letter_of_alphabet

# Indirect reference.
eval a=\$$a
echo "Now a = $a"      # Now a = z


# Now, let's try changing the second-order reference.

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 "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)


# (Thanks, Stephane Chazelas, for clearing up the above behavior.)

# Another method is the ${!t} notation, discussed in "Bash, version 2" section.
# See also ex78.sh.

exit 0

Of what practical use is indirect referencing of variables? It gives Bash a little of the functionality of pointers in C, for instance, in table lookup. And, it also has some other very interesting applications. . . .

Nils Radtke shows how to build "dynamic" variable names and evaluate their contents. This can be useful when sourcing configuration files.


# ---------------------------------------------
# This could be "sourced" from a separate file.
# ---------------------------------------------

remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)")
remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)")
remoteNet=$(eval "echo \$isdnMyProviderRemoteNet")
remoteNet=$(eval "echo $isdnMyProviderRemoteNet")

echo "$remoteNet"    #

# ================================================================

#  And, it gets even better.

#  Consider the following snippet given a variable named getSparc,
#+ but no such variable getIa64:

chkMirrorArchs () { 
  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 ]
     return 0;
     return 1;

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" . . .

Example 9-24. Passing an indirect reference to awk


#  Another version of the "column totaler" script
#+ that adds up a specified column (of numbers) in the target file.
#  This one uses indirect references.


if [ $# -ne "$ARGS" ] # Check for proper no. of command line args.
   echo "Usage: `basename $0` filename column-number"
   exit $E_WRONGARGS


#===== 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
     print total

     " "$filename"
# ------------------------------------------------
# End awk script.

#  Indirect variable reference avoids the hassles
#+ of referencing a shell variable within the embedded awk script.
#  Thanks, Stephane Chazelas.

exit 0


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). Fortunately, the ${!variable} notation introduced with version 2 of Bash (see Example 34-2 and Example A-23) makes indirect referencing more intuitive.