实在忍不住了,一直想不通的问题, backslash在$() 和 ``中的不同

johnny_jiang

Post by yongjian
subshell inherits everything from parent shell but not the other way, unless you use export..

1. a=1;echo `echo $a`

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an easy sample. For using en... I was able to type ch but several weeks ago, I upgraded to dapper, and firefox just crashes every time when I use it. I switched to opera but it doesn't support ch input... have no choice right now but waiting for a stable firefox update.

那么yongjian兄，我可不可以把subshell理解为subprocess呢。
如果我的理解没错的话，subprocess应该只能继承被export过的变量吧？

期待中...

yongjian

1. 
   
2. #!/bin/bash
   
3. 
   
4. a=10
   
5. echo $a
   
6. 
   
7. 
   
8. (
   
9. echo "in subshell"
   
10. echo $a
    
11. )
    

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Here is how I tested it. You know cmds within () creates subshell right...

johnny_jiang

多谢各位一直以来的帮助,小弟参考了Learning the bash Shell - 2nd Edition (o'reilly)，在仔细的看了bash manual，发现了一些信息，我希望对大家有用

The most important things you need to know about subshells are what characteristics they get, or inherit, from their parents. These are as follows:

·         The current directory

·         Environment variables

·         Standard input, output, and error, plus any other open file descriptors

·         Signals that are ignored

Just as important are the things that a subshell does not inherit from its parent:

·         Shell variables, except environment variables and those defined in the environment file (usually .bashrc)

·         Handling of signals that are not ignored

Code inside a nested subshell obeys the above rules of subshell inheritance, except that it knows about variables defined in the surrounding shell.

在bash manual中

$:   Expands to the process ID of the shell. In a ( )　subshell, it expands to the process ID of the current shell, not the subshell.

从以上的信息可以看出nested subshell和脚本引用的subshell是有区别的
1.nested subshell除了可以继承环境变量，它对shell的其他变量也继承了，但是用脚本引用的subshell不会
2.(echo $$)将回显的是current shell的进程号

小弟个人意见，仅供参考，欢迎砸帖，呵呵！

yongjian

ya, if you do like a=1 and then write a script to echo $a, you get nothing. I am thinking one is using child processing, and the other is threading...