How to redirect an output file descriptor of a subshell to an input file descriptor in the parent shell?

Question

BEWARE, BASHISM AHEAD (there are posix shells that are significantly faster than bash, e.g. ash or dash, that don’t have process substitution).

You can do a handle dance to move original standard output to a new descriptor to make standard output available for piping (from the top of my head):

exec 3>&1 # open 3 to the same output as 1
run_in_subshell() { # just shortcut for the two cases below
    echo "This goes to STDOUT" >&3
    echo "And this goes to THE OTHER FUNCTION"
}

Now you should be able to write:

while read line; do
    process $line
done < <(run_in_subshell)

but the <() construct is a bashism. You can replace it with pipeline

run_in_subshell | while read line; do
    process $line
done

except than the second command also runs in subshell, because all commands in pipeline do.

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