9.5. Indirect References

We have seen that referencing a variable, $var, fetches its value. But, what about the value of a value? What about $$var?

The actual notation is \$$var, usually preceded by an eval (and sometimes an echo). This is called an indirect reference.

Example 9-24. Indirect Variable 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=23 echo "\$var = $var" # $var = 23 # So far, everything as expected. But ... echo "\$\$var = $$var" # $$var = 4570var # Not meaningful. The contents of a memory location pointed to? # Not useful at this point. 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.) # 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.

#!/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" . . .

Example 9-25. Passing an indirect reference to awk

#!/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=65 if [ $# -ne "$ARGS" ] # Check for proper no. of command line args. then echo "Usage: `basename $0` filename column-number" exit $E_WRONGARGS fi filename=$1 column_number=$2 #===== Same as original script, up to this point =====# # A multi-line awk script is invoked by awk ' ..... ' # Begin awk script. # ------------------------------------------------ awk " { total += \$${column_number} # indirect reference } END { print total } " "$filename" # ------------------------------------------------ # End awk script. # Indirect variable reference avoids the hassles #+ of referencing a shell variable within the embedded awk script. # Thanks, Stephane Chazelas. 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.