shell builtin with redirection

Question

We are working on a shell (school project). We don't understand a behaviour. Why builtins are not acting when redirected ?

like

cd - | command

does not change the directory.

or

export NAME=VALUE | command

does not create the variable.

Thank you.

Answer 1

Links of a pipeline are run in forked subshells.

In bash, you can print the PID of the current process with $BASHPID, so something like:

self(){ echo $BASHPID; } ; self ; self >&2 | self; self

should give you something like:

12849
12851
12852
12849

with the middle two PIDs being different than the first and the last (the mother shell) (in some shells, the first or the last link is run in the mother shell, but not in bash).

Changing the current directory or exporting a value in a subshell won't affect the parent shell in any way whatsoever.