9.5. 变量间接引用
假设一个变量的值是第二个变量的名字. 这样要如何才能从第一个变量处重新获得第二个变量的值？例如,a=letter_of_alphabet和letter_of_alphabet=z, 是否能由a引用得到z ? 这确实可以办到,这种技术被称为间接引用.它使用不平常的eval var1=\$$var2 序列.
例子 9-22. 间接引用
1 #!/bin/bash
2 # ind-ref.sh: 间接变量引用.
3 # 访问一个变量内容所指的变量的值.
4
5 a=letter_of_alphabet # 变量"a"保存着另外一个变量的名字.
6 letter_of_alphabet=z
7
8 echo
9
10 # 直接引用.
11 echo "a = $a" # a = letter_of_alphabet
12
13 # 间接引用.
14 eval a=\$$a
15 echo "Now a = $a" # 现在 a = z
16
17 echo
18
19
20 # 现在，让我们试试更改第二次引用的顺序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 # 显示：dereferenced "t" = 24
26 # 在这个简单的情况下，下面的也可以工作吗？(为什么？).
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"带着两个参数："echo"和"\$$t"(相当于 $table_cell_3)
38
39 echo
40
41 # (多谢Stephane Chazelas解释了上面的语句的现象)
42
43
44 # 另外一个办法是使用${!t}符号，这个在"Bash, 版本 2"章节中讨论.
45 # 参考脚本 ex78.sh.
46
47 exit 0 |
变量间接引用的实际用处是什么? 它提供了Bash具有C中一点指针的功能，例如，在表格查找中的用处，另外它也有其他一些有趣的应用. . . .
Nils Radtke展示了如何建立动态变量名和求它们的值。当source配置文件时这个技巧很有用。
1 #!/bin/bash
2
3
4 # ---------------------------------------------
5 # 这个文件可被另外单独的文件用source命令执行.
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 # 可以做的更好.
22
23 # 注意下面的片断给出了变量getSparc,
24 #+ 但没有变量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 # 真
43
44 chkMirrorArchs Ia64
45 echo $? # 1
46 # 假
47
48 # 注意:
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" . . . |
例子 9-23. 传递一个间接引用给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 " "$filename"
36 # ------------------------------------------------
37 # End awk script.
38
39 # Indirect variable reference avoids the hassles
40 #+ of referencing a shell variable within the embedded awk script.
41 # Thanks, Stephane Chazelas.
42
43
44 exit 0 |
 | 间接引用的方法是个小窍门。如果第二个变量更改了它的值，第一个变量必须适当地解除引用(就像上面的例子一样)This method of indirect referencing is a bit tricky. If the second order variable changes its value, then the first order variable must be properly dereferenced (as in the above example). 幸运地是在bash(参考例子。。。)版本2中介绍的${!variable}符号能使间接引用更智能一些。 notation introduced with version 2 of Bash (see Example 34-2) makes indirect referencing more intuitive. |