Is there a subshell created when I run `sh -c “command”`, a new shell or none of these?

Question

In bash, when I run the following command:

sh -c \"command\"

is there created a subshell and then the command

Answer 1

Regardless bash

Running this simple line will show you a lot:

$ echo $$;sh -c 'echo $$;ps axfw | grep -B2 $$'
14152
12352
14147 ?        S      0:00 xterm
14152 pts/4    Ss     0:00  \_ bash
12352 pts/4    S+     0:00      \_ sh -c echo $$;ps axfw | grep -B2 $$
12353 pts/4    R+     0:00          \_ ps axfw
12354 pts/4    S+     0:00          \_ grep -B2 12352

This is clearly a child but what's a subshell?

Well, my current running shell pid is 14152

$ echo $$ $BASHPID $BASH_SUBSHELL 
14152 14152 0
$ (echo $$ $BASHPID $BASH_SUBSHELL)
14152 12356 1
$ ( (echo $$ $BASHPID $BASH_SUBSHELL) )
14152 12357 2

Well nice: BASHPID is a dynamic variable which take alway the value of executing pid:

$ echo $BASHPID | sed \$a$BASHPID | sed \$a$BASHPID
12371
12372
12373

Hmmm where is my current working pid?

$ sed "\$a$BASHPID $$" < <(echo "$BASHPID $$"| sed "\$a$BASHPID $$")
12386 14152
12387 14152
14152 14152

Well I've find them!

$ echo $BASHPID $$;(echo $BASHPID $$;ps axfw | grep -B3 $BASHPID)
14152 14152
12469 14152
14152 pts/4    Ss     0:00  \_ bash
12469 pts/4    S+     0:00      \_ bash
12470 pts/4    R+     0:00          \_ ps axfw
12471 pts/4    S+     0:00          \_ grep -B3 12471

Conclusion

When you

$ echo $BASHPID $$ $BASH_SUBSHELL;(echo $BASHPID $$ $BASH_SUBSHELL)
14152 14152 0
12509 14152 1

use parenthesis or pipe (|), you will create subshell which is a forked child of a running shell.

But when you

$ echo $BASHPID $$ $BASH_SUBSHELL;bash -c 'echo $BASHPID $$ $BASH_SUBSHELL'
14152 14152 0
12513 12513 0

You will burn a child, running a shell interpreter, so it's a subshell, but not linked to his parent.

Nota: If the child is not linked to his parent, they use same I/O files descriptors. So if you close your window (pts/4 in my run), you will send a SIGHUP to all process using them.