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 9-21. Indirect References

   1 #!/bin/bash
   2 # Indirect variable referencing.
   3 
   4 a=letter_of_alphabet
   5 letter_of_alphabet=z
   6 
   7 echo
   8 
   9 # Direct reference.
  10 echo "a = $a"
  11 
  12 # Indirect reference.
  13 eval a=\$$a
  14 echo "Now a = $a"
  15 
  16 echo
  17 
  18 
  19 # Now, let's try changing the second order reference.
  20 
  21 t=table_cell_3
  22 table_cell_3=24
  23 echo "\"table_cell_3\" = $table_cell_3"
  24 echo -n "dereferenced \"t\" = "; eval echo \$$t
  25 # In this simple case,
  26 #   eval t=\$$t; echo "\"t\" = $t"
  27 # also works (why?).
  28 
  29 echo
  30 
  31 t=table_cell_3
  32 NEW_VAL=387
  33 table_cell_3=$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 echo
  39 
  40 # (Thanks, S.C., for clearing up the above behavior.)
  41 
  42 
  43 # Another method is the ${!t} notation, discussed in "Bash, version 2" section.
  44 # See also example "ex78.sh".
  45 
  46 exit 0


Example 9-22. 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 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 35-2) makes indirect referencing more intuitive.