Shell exec and pipes

Question

I'm using bash, and as I understand it, exec followed by a command is supposed to replace the shell without creating a new process. For example,

exec echo hello

has the appearance of printing "hello" and then immediately exiting, because after echo is done, the shell process isn't there to return to anymore.

If I put this as part of a pipeline - for instance,

exec echo hello | sed 's/hell/heck/'

or

echo hello | exec sed 's/hell/heck/'

my expectation is that, similarly, the shell would terminate as a result of its process being replaced away. This is not what happens in reality, though - both the commands above print "hecko" and return to the shell normally, just as if the word "exec" wasn't there. Why is this?

Answer 1

There is sentence in bash manual:

Each command in a pipeline is executed as a separate process (i.e., in a subshell).

So in both examples two processes are spawned by the pipeline first and 'exec' is executed inside one of spawned process - without impact on shell executing the pipeline.