9.5. Indirect References

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 9-24. Indirect Variable References

   1 #!/bin/bash
   2 # ind-ref.sh: Indirect variable referencing.
   3 # Accessing the contents of the contents of a variable.
   4 
   5 a=letter_of_alphabet   # Variable "a" holds the name of another variable.
   6 letter_of_alphabet=z
   7 
   8 echo
   9 
  10 # Direct reference.
  11 echo "a = $a"          # a = letter_of_alphabet
  12 
  13 # Indirect reference.
  14 eval a=\$$a
  15 echo "Now a = $a"      # Now a = z
  16 
  17 echo
  18 
  19 
  20 # Now, let's try changing the second-order reference.
  21 
  22 t=table_cell_3
  23 table_cell_3=24
  24 echo "\"table_cell_3\" = $table_cell_3"            # "table_cell_3" = 24
  25 echo -n "dereferenced \"t\" = "; eval echo \$$t    # dereferenced "t" = 24
  26 # In this simple case, the following also works (why?).
  27 #         eval t=\$$t; echo "\"t\" = $t"
  28 
  29 echo
  30 
  31 t=table_cell_3
  32 NEW_VAL=387
  33 table_cell_3=$NEW_VAL
  34 echo "Changing value of \"table_cell_3\" to $NEW_VAL."
  35 echo "\"table_cell_3\" now $table_cell_3"
  36 echo -n "dereferenced \"t\" now "; eval echo \$$t
  37 # "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3)
  38 
  39 echo
  40 
  41 # (Thanks, Stephane Chazelas, for clearing up the above behavior.)
  42 
  43 
  44 # Another method is the ${!t} notation, discussed in "Bash, version 2" section.
  45 # See also ex78.sh.
  46 
  47 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.
   1 #!/bin/bash
   2 
   3 
   4 # ---------------------------------------------
   5 # This could be "sourced" from a separate file.
   6 isdnMyProviderRemoteNet=172.16.0.100
   7 isdnYourProviderRemoteNet=10.0.0.10
   8 isdnOnlineService="MyProvider"
   9 # ---------------------------------------------
  10       
  11 
  12 remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)")
  13 remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)")
  14 remoteNet=$(eval "echo \$isdnMyProviderRemoteNet")
  15 remoteNet=$(eval "echo $isdnMyProviderRemoteNet")
  16 
  17 echo "$remoteNet"    # 172.16.0.100
  18 
  19 # ================================================================
  20 
  21 #  And, it gets even better.
  22 
  23 #  Consider the following snippet given a variable named getSparc,
  24 #+ but no such variable getIa64:
  25 
  26 chkMirrorArchs () { 
  27   arch="$1";
  28   if [ "$(eval "echo \${$(echo get$(echo -ne $arch |
  29        sed 's/^\(.\).*/\1/g' | tr 'a-z' 'A-Z'; echo $arch |
  30        sed 's/^.\(.*\)/\1/g')):-false}")" = true ]
  31   then
  32      return 0;
  33   else
  34      return 1;
  35   fi;
  36 }
  37 
  38 getSparc="true"
  39 unset getIa64
  40 chkMirrorArchs sparc
  41 echo $?        # 0
  42                # True
  43 
  44 chkMirrorArchs Ia64
  45 echo $?        # 1
  46                # False
  47 
  48 # Notes:
  49 # -----
  50 # Even the to-be-substituted variable name part is built explicitly.
  51 # The parameters to the chkMirrorArchs calls are all lower case.
  52 # The variable name is composed of two parts: "get" and "Sparc" . . .


Example 9-25. Passing an indirect reference to awk

   1 #!/bin/bash
   2 
   3 #  Another version of the "column totaler" script
   4 #+ that adds up a specified column (of numbers) in the target file.
   5 #  This one uses indirect references.
   6 
   7 ARGS=2
   8 E_WRONGARGS=65
   9 
  10 if [ $# -ne "$ARGS" ] # Check for proper no. of command line args.
  11 then
  12    echo "Usage: `basename $0` filename column-number"
  13    exit $E_WRONGARGS
  14 fi
  15 
  16 filename=$1
  17 column_number=$2
  18 
  19 #===== Same as original script, up to this point =====#
  20 
  21 
  22 # A multi-line awk script is invoked by   awk ' ..... '
  23 
  24 
  25 # Begin awk script.
  26 # ------------------------------------------------
  27 awk "
  28 
  29 { total += \$${column_number} # indirect reference
  30 }
  31 END {
  32      print total
  33      }
  34 
  35      " "$filename"
  36 # ------------------------------------------------
  37 # End awk script.
  38 
  39 #  Indirect variable reference avoids the hassles
  40 #+ of referencing a shell variable within the embedded awk script.
  41 #  Thanks, Stephane Chazelas.
  42 
  43 
  44 exit 0

Caution

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-24) makes indirect referencing more intuitive.

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