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I know pipe will create subshell.

I test $BASH_SUBSHELL and $BASHPID , and find $BASH_SUBSHELL doesn't increase in pipe 

echo $BASH_SUBSHELL         # 0
echo $BASHPID               # 8347

echo $BASH_SUBSHELL | cat   # 0
echo $BASHPID | cat         # 9727

(echo $BASH_SUBSHELL)       # 1
(echo $BASHPID)             # 9778
John Gauss
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  • What about `eval 'echo $BASH_SUBSHELL' | cat` or `{ echo $BASH_SUBSHELL } | cat`? – choroba May 21 '13 at 11:43
  • @choroba it works, but why $BASH_SUBSHELL not increase in my example – John Gauss May 21 '13 at 12:10
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    I am not sure. The variable is probably expanded by the parent shell before running the subshell, but I cannot find the exact documentation and I am not going to search the source :-) – choroba May 21 '13 at 12:20
  • Even a command group `{ echo $BASH_SUBSHELL; } | cat` is enough to increment the value, so it looks like the `bash` parser needs some explicit clue in the command itself that a subshell will be spawned. – chepner May 21 '13 at 12:36

1 Answers1

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When bash merely forks, sets up a command and execves it, it doesn't count it as a subshell.

For any kind of control structure, like command groups, if statements and while loops, where the shell actually has to be involved beyond setup, it does.

This is apparent from the bash source, execute_command_internal in execute_cmd.c.

that other guy
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