Re: When does bash spawn a sub-shell with ( list ) ?

[Alex fxmbsw7 Ratchev]

On Thu, Jan 27, 2022, 21:29 Chet Ramey <chet.ramey@case.edu> wrote:

> On 1/27/22 2:45 PM, Masahiro Yamada wrote:
> > Hi,
> >
> > The bash manual [1] says
> > "Placing a list of commands between parentheses causes
> > a subshell environment to be created"
> >
> > In this statement, "a list of commands"
> > means "two commands or more",
> > excluding a single command.
> >
> > Is this correct?
>
> No. Bash will sometimes optimize away a child process creation (in effect,
> turning `( foo )' into `(exec foo)') if it can determine that the command
> has no side effects. You're seeing that here.
>
> > masahiro@grover:/tmp/test$ cat test.c
> > #include <stdio.h>
> > #include <unistd.h>
> >
> > int main(void)
> > {
> >      printf("pid=%d, ppid=%d\n", getpid(), getppid());
> > }
> >
> > masahiro@grover:/tmp/test$ gcc test.c
> > masahiro@grover:/tmp/test$
> > masahiro@grover:/tmp/test$ echo $$
> > 1071740
> > masahiro@grover:/tmp/test$ ./a.out
> > pid=1073015, ppid=1071740
> > masahiro@grover:/tmp/test$ ( ./a.out )
> > pid=1073016, ppid=1071740
>
> The shell optimized away the second fork; the command is still being
> executed in a subshell environment and actions by ./a.out will not
> affect its parent.
>
> > masahiro@grover:/tmp/test$ ( ./a.out; ./a.out )
> > pid=1073020, ppid=1073019
> > pid=1073021, ppid=1073019
>
> The shell couldn't optimize it away (it probably could with the last
> command in the list, but it doesn't try).
>

i suggest u code in that small effective opt


> >
> >
> > The command
> >      ./a.out
> > and
> >      ( ./a.out )
> > printed the same ppid, 1071740.
> > So,  ( ./a.out ) did not fork a new subshell.
>
> It did. Then it ran ./a.out in that subshell without forking again.
> Why do you need the shell to fork twice?
>
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                  ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/
>
>