Chapter 28. Indirect References
We have seen that referencing a variable, $var, fetches its value. But, what about the value of a value? What about $$var?
The actual notation is \$$var, usually preceded by an eval (and sometimes an echo). This is called an indirect reference.
Example 28-1. Indirect Variable References
1 #!/bin/bash
2 # ind-ref.sh: Indirect variable referencing.
3 # Accessing the contents of the contents of a variable.
4
5 # First, let's fool around a little.
6
7 var=23
8
9 echo "\$var = $var" # $var = 23
10 # So far, everything as expected. But ...
11
12 echo "\$\$var = $$var" # $$var = 4570var
13 # Not useful ...
14 # \$\$ expanded to PID of the script
15 # -- refer to the entry on the $$ variable --
16 #+ and "var" is echoed as plain text.
17 # (Thank you, Jakob Bohm, for pointing this out.)
18
19 echo "\\\$\$var = \$$var" # \$$var = $23
20 # As expected. The first $ is escaped and pasted on to
21 #+ the value of var ($var = 23 ).
22 # Meaningful, but still not useful.
23
24 # Now, let's start over and do it the right way.
25
26 # ============================================== #
27
28
29 a=letter_of_alphabet # Variable "a" holds the name of another variable.
30 letter_of_alphabet=z
31
32 echo
33
34 # Direct reference.
35 echo "a = $a" # a = letter_of_alphabet
36
37 # Indirect reference.
38 eval a=\$$a
39 # ^^^ Forcing an eval(uation), and ...
40 # ^ Escaping the first $ ...
41 # ------------------------------------------------------------------------
42 # The 'eval' forces an update of $a, sets it to the updated value of \$$a.
43 # So, we see why 'eval' so often shows up in indirect reference notation.
44 # ------------------------------------------------------------------------
45 echo "Now a = $a" # Now a = z
46
47 echo
48
49
50 # Now, let's try changing the second-order reference.
51
52 t=table_cell_3
53 table_cell_3=24
54 echo "\"table_cell_3\" = $table_cell_3" # "table_cell_3" = 24
55 echo -n "dereferenced \"t\" = "; eval echo \$$t # dereferenced "t" = 24
56 # In this simple case, the following also works (why?).
57 # eval t=\$$t; echo "\"t\" = $t"
58
59 echo
60
61 t=table_cell_3
62 NEW_VAL=387
63 table_cell_3=$NEW_VAL
64 echo "Changing value of \"table_cell_3\" to $NEW_VAL."
65 echo "\"table_cell_3\" now $table_cell_3"
66 echo -n "dereferenced \"t\" now "; eval echo \$$t
67 # "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3)
68
69
70 echo
71
72 # (Thanks, Stephane Chazelas, for clearing up the above behavior.)
73
74
75 # A more straightforward method is the ${!t} notation, discussed in the
76 #+ "Bash, version 2" section.
77 # See also ex78.sh.
78
79 exit 0 |
Of what practical use is indirect referencing of variables? It gives Bash a little of the functionality of pointers in C, for instance, in table lookup. And, it also has some other very interesting applications. . . .
Nils Radtke shows how to build "dynamic" variable names and evaluate their contents. This can be useful when sourcing configuration files.
1 #!/bin/bash
2
3
4 # ---------------------------------------------
5 # This could be "sourced" from a separate file.
6 isdnMyProviderRemoteNet=172.16.0.100
7 isdnYourProviderRemoteNet=10.0.0.10
8 isdnOnlineService="MyProvider"
9 # ---------------------------------------------
10
11
12 remoteNet=$(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 28-2. 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=85
9
10 if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.
11 then
12 echo "Usage: `basename $0` filename column-number"
13 exit $E_WRONGARGS
14 fi
15
16 filename=$1 # Name of file to operate on.
17 column_number=$2 # Which column to total up.
18
19 #===== Same as original script, up to this point =====#
20
21
22 # A multi-line awk script is invoked by
23 # awk "
24 # ...
25 # ...
26 # ...
27 # "
28
29
30 # Begin awk script.
31 # -------------------------------------------------
32 awk "
33
34 { total += \$${column_number} # Indirect reference
35 }
36 END {
37 print total
38 }
39
40 " "$filename"
41 # Note that awk doesn't need an eval preceding \$$.
42 # -------------------------------------------------
43 # End awk script.
44
45 # Indirect variable reference avoids the hassles
46 #+ of referencing a shell variable within the embedded awk script.
47 # Thanks, Stephane Chazelas.
48
49
50 exit $? |
 | 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 37-2 and Example A-22) makes indirect referencing more intuitive. |