沒理解“()將 command group 置于 sub-shell 去執(zhí)行”

shreychen

測試代碼

1. #!/bin/bash
   
2. 
   
3. A=B
   
4. echo "in main shell PID is $,\$A=$A"
   
5. 
   
6. (
   
7.         A=C
   
8.         echo "in () PID is $,\$A=$A"
   
9. )
   
10. 
    
11. echo "in main shell PID is $,\$A=$A"
    
12. 
    
13. {
    
14.         A=D
    
15.         echo "in {} PID is $,\$A=$A"
    
16. }
    
17. 
    
18. echo "in main shell PID is $,\$A=$A"

復(fù)制代碼

執(zhí)行結(jié)果

in main shell PID is 45612,$A=B
in () PID is 45612,$A=C   #我以為()里的代碼會fork一個(gè)新的pid，不知道我是不是理解錯(cuò)了
in main shell PID is 45612,$A=B
in {} PID is 45612,$A=D
in main shell PID is 45612,$A=D

jason680

回復(fù) 1# shreychen

try $BASHPID


$ man bash
       ...
       BASHPID
              Expands to the process ID of the  current  bash  process.   This
              differs  from  $$ under certain circumstances, such as subshells
              that do not require bash to be re-initialized.
       ...

shreychen

回復(fù) 2# jason680

感謝，我自己看錯(cuò)了，是sub-shell，我理解成sub-process了