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I saw this video which explains that when running a command in parentheses it actually runs the command in a subshell, which is a child process of the original shell. Running the following experiment:

// one shell with PID 5344
~$ (find /)

// another shell
~$ ps l
F   UID     PID    PPID PRI  NI    VSZ   RSS WCHAN  STAT TTY        TIME COMMAND
...
0  1000    5344    5333  20   0  10888  5220 do_wai Ss   pts/0      0:00 bash
0  1000    5384    5333  20   0  10888  5140 do_wai Ss   pts/1      0:00 bash
0  1000    7239    5344  20   0  10860  3444 -      R+   pts/0      0:00 find /
4  1000    7240    5384  20   0  11400  3224 -      R+   pts/1      0:00 ps l

We can see that find / is a child of 5344, with no other shell in between. Wheres running:

// one shell with PID 5344
(cd /; find /)

// second shell
~$ ps l
F   UID     PID    PPID PRI  NI    VSZ   RSS WCHAN  STAT TTY        TIME COMMAND
...
0  1000    5344    5333  20   0  10888  5220 do_wai Ss   pts/0      0:00 bash
0  1000    5384    5333  20   0  10888  5140 do_wai Ss   pts/1      0:00 bash
1  1000    7379    5344  20   0  10888  3036 do_wai S+   pts/0      0:00 bash
4  1000    7380    7379  20   0  10864  3536 -      R+   pts/0      0:01 find /
4  1000    7381    5384  20   0  11400  3184 -      R+   pts/1      0:00 ps l

Now we can see that other shell in between. My guess is that it is some optimization of bash: In the first case, it doesn't really have to spawn another shell, so it just doesn't do it. In the second case, since the commands include cd /, which will have affect on the current shell, it has to spawn another process. Is that so?

ctrl-alt-delor
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YoavKlein
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  • Related, if not a dupe: https://unix.stackexchange.com/questions/401020/why-bash-does-not-spawn-a-subshell-for-simple-commands https://unix.stackexchange.com/questions/430050/why-doesnt-spawn-a-new-child-process-when-run-in-background – Kusalananda Jan 06 '21 at 19:38
  • I was going to say what you said in the last paragraph. When a spec say it dose something, it means that it must behave as if it did the something. So yes I believe it is optimisation. However this is not based on any in depth knowledge of bash. It is based on knowledge of other programming languages and compilers. – ctrl-alt-delor Jan 06 '21 at 19:46
  • Note also that running a command in a subshell environment doesn't have to involve child processes, they're just the easy way of implementing subshells. – ilkkachu Jan 06 '21 at 21:05

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