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 9-23. Indirect 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 9-24. 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 |
 | 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-23) makes indirect referencing more intuitive. |