问个关于read的问题

lastart

bash环境:

1. 
   
2. ~$  echo $dd
   
3. bb
   
4. ~$  echo cc|read dd
   
5. ~$  echo $dd
   
6. bb
   
7. ~$  echo cc|if read dd; then echo $dd; fi
   
8. cc
   
9. ~$  echo $dd
   
10. bb
    

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怎么才能把这个值(cc)传递出来

寂寞烈火

echo cc|read dd
无论dd没有被定义,cc都不会传给dd的,
这样的语句可以参考大多数的shell的手册,都有解释,例如:
http://www.cs.mun.ca/~michael/pdksh/

lastart

不是的, 其实我是想把字符串比如:
a='a b c'
一次性传给三个变量: e f g
使得
e=a
f=b
g=c
即:
echo $e $f $g
输出
a b c
我不明白的是
echo cc|if read dd; then echo $dd; fi
的时候在判断语句内部dd的值是cc,为什么出来的时候就没有了

nika

是不是可以认为read新建了个bash环境?

lastart

不知道,可能是以在 寂寞烈火 提到的地方的BUG REPORT看到这个

1. 
   
2. * pdksh 5.1.3, - (reported by Brad Warkentin & others): if the last command of
   
3.   a pipeline is a shell builtin, it is not executed in the parent shell,
   
4.   so "echo a b | read foo bar" does not set foo and bar in the parent
   
5.   shell (at&t ksh will).
   
6.   [see Mail.7:32,Mail.9:65]
   

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lastart

在Bash的FAQ里找到这个:

1. 
   
2. E4) If I pipe the output of a command into `read variable', why doesn't
   
3.     the output show up in $variable when the read command finishes?
   
4. 
   
5. This has to do with the parent-child relationship between Unix
   
6. processes.  It affects all commands run in pipelines, not just
   
7. simple calls to `read'.  For example, piping a command's output
   
8. into a `while' loop that repeatedly calls `read' will result in
   
9. the same behavior.
   
10. 
    
11. Each element of a pipeline, even a builtin or shell function,
    
12. runs in a separate process, a child of the shell running the
    
13. pipeline.  A subprocess cannot affect its parent's environment.
    
14. When the `read' command sets the variable to the input, that
    
15. variable is set only in the subshell, not the parent shell.  When
    
16. the subshell exits, the value of the variable is lost.
    
17. 
    
18. Many pipelines that end with `read variable' can be converted
    
19. into command substitutions, which will capture the output of
    
20. a specified command.  The output can then be assigned to a
    
21. variable:
    
22. 
    
23.         grep ^gnu /usr/lib/news/active | wc -l | read ngroup
    
24. 
    
25. can be converted into
    
26. 
    
27.         ngroup=$(grep ^gnu /usr/lib/news/active | wc -l)
    
28. 
    
29. This does not, unfortunately, work to split the text among
    
30. multiple variables, as read does when given multiple variable
    
31. arguments.  If you need to do this, you can either use the
    
32. command substitution above to read the output into a variable
    
33. and chop up the variable using the bash pattern removal
    
34. expansion operators or use some variant of the following
    
35. approach.
    
36. 
    
37. Say /usr/local/bin/ipaddr is the following shell script:
    
38. 
    
39. #! /bin/sh
    
40. host `hostname` | awk '/address/ {print $NF}'
    
41. 
    
42. Instead of using
    
43. 
    
44.         /usr/local/bin/ipaddr | read A B C D
    
45. 
    
46. to break the local machine's IP address into separate octets, use
    
47. 
    
48.         OIFS="$IFS"
    
49.         IFS=.
    
50.         set -- $(/usr/local/bin/ipaddr)
    
51.         IFS="$OIFS"
    
52.         A="$1" B="$2" C="$3" D="$4"
    
53. 
    
54. Beware, however, that this will change the shell's positional
    
55. parameters.  If you need them, you should save them before doing
    
56. this.
    
57. 
    
58. This is the general approach -- in most cases you will not need to
    
59. set $IFS to a different value.
    
60. 
    
61. Some other user-supplied alternatives include:
    
62. 
    
63. read A B C D << HERE
    
64.     $(IFS=.; echo $(/usr/local/bin/ipaddr))
    
65. HERE
    
66. 
    
67. and, where process substitution is available,
    
68. 
    
69. read A B C D < <(IFS=.; echo $(/usr/local/bin/ipaddr))
    

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sunxiaobo

exactly! because the varable definition is in the subprocess. you should write a script for it. then run it in the current shell like this $ . script. look at the attachment.