Why the process run by the shell does not inherit the shell's Process Group ID?

Question

I have read that when Process A start Process B, then Process B will inherit Process A's Process Group ID.

I tested this by making bash execute the cat program, but I found out that bash and cat had two different Process Group ID, why is that?

Answer 1

The sid (session id) should be consistent.

When bash runs a command, it generally assigns the PGID to the first process. If its a pipeline, all the piped commands have the same PGID as the first process.