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 |
 | 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. |