Chapter 28. Indirect References

#!/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=23echo "$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=zecho# 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 = zecho# Now, let's try changing the second-order reference.t=table_cell_3table_cell_3=24echo ""table_cell_3" = $table_cell_3" # "table_cell_3" = 24echo -n "dereferenced "t" = "; eval echo $$t # dereferenced "t" = 24# In this simple case, the following also works (why?).# eval t=$$t; echo ""t" = $t" echot=table_cell_3NEW_VAL=387table_cell_3=$NEW_VALecho "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

#!/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=2E_WRONGARGS=85if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.then   echo "Usage: `basename $0` filename column-number" exit $E_WRONGARGSfifilename=$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 $?