When does command substitution spawn more subshells than the same commands in isolation?

Question

Yesterday it was suggested to me that using command substitution in bash causes an unnecessary subshell to be spawned. The advice was specific to this use case:

Answer 1

In Bash, a subshell always executes in a new process space. You can verify this fairly trivially in Bash 4, which has the $BASHPID and $$ environment variables:

• $$ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell.
• BASHPID Expands to the process id of the current bash process. This differs from $$ under certain circumstances, such as subshells that do not require bash to be re-initialized

in practice:

$ type echo
echo is a shell builtin
$ echo $$-$BASHPID
4671-4671
$ ( echo $$-$BASHPID )
4671-4929
$ echo $( echo $$-$BASHPID )
4671-4930
$ echo $$-$BASHPID | { read; echo $REPLY:$$-$BASHPID; }
4671-5086:4671-5087
$ var=$(echo $$-$BASHPID ); echo $var
4671-5006

About the only case where the shell can elide an extra subshell is when you pipe to an explicit subshell:

$ echo $$-$BASHPID | ( read; echo $REPLY:$$-$BASHPID; )
4671-5118:4671-5119

Here, the subshell implied by the pipe is explicitly applied, but not duplicated.

This varies from some other shells that try very hard to avoid fork-ing. Therefore, while I feel the argument made in js-shell-parse misleading, it is true that not all shells always fork for all subshells.