Shell sourced file output piped vs redirected has different effects. - shell

Question

I'm struggling to understand the difference between | and > operators
I've looked in places like:
https://www.gnu.org/software/bash/manual/html_node/Redirections.html
and
Pipe vs redirect into process
But can't make enough sense of the explanations.
Here is my practical example:
test-a.sh:
alias testa='echo "alias testa here"'
echo "testa echo"
echo "testa echo2"
test-b.sh:
alias testb='echo "alias testb here"'
echo "testa echo"
echo "testa echo2"
test-pipes.sh:
function indent() {
input=$(cat)
echo "$input" | perl -p -e 's/(.*)/ \1/'
}
source test-a.sh | indent
testa
source test-b.sh > >(indent)
testb
output:
$ source test-pipes.sh
testa echo
testa echo2
test-pipes.sh:10: command not found: testa
testa echo
testa echo2
alias testb here
Piping doesn't allow the alias to be set in the current process, but the redirection does.
Can someone give a simple explanation?

Answer 1

From the bash man page:
Each command in a pipeline is executed as a separate process (i.e., in a sub‐shell).
Many things child processes do are isolated from the parent. Among the list are: changing the current directory, setting shell variables, setting environment variables, and aliases.
$ alias foo='echo bar' | :
$ foo
foo: command not found
$ foo=bar | :; echo $foo
$ export foo=bar | :; echo $foo
$ cd / | :; $ pwd
/home/jkugelman
Notice how none of the changes took effect. You can see the same thing with explicit subshells:
$ (alias foo='echo bar')
$ foo
foo: command not found
$ (foo=bar); echo $foo
$ (export foo=bar); echo $foo
$ (cd /); pwd
/home/jkugelman
Redirections, on the other hand, do not create subshells. They merely change where the input and output of a command go. The same goes with function calls. Functions are executed in the current shell, no subshell, so they're able to create aliases.