Pipe STDOUT & STDERR separately

Question

I have a function test1 which produces output on both stdout and stderr:

test1() {
    echo "out 1"
    sleep 0.1

    echo "error 1" 1>&2
    sleep 0.1

    echo "out 2"
    sleep 0.1

    echo "error 2" 1>&2
    sleep 0.1
}

I then have two aliases which format stdin and send it to stdout:

alias o='sed -E "s/^/O /"'
alias e='sed -E "s/^/E /"'

How can I pipe the two streams from test1 through o and e and end up with formatted output on stdout and stderr? I'm using Bash on Alpine Linux, so GNU utilities are not an option.

My reasoning goes as follows, but I don't know how to execute it syntactically:

1. Send err to out and out to 3.
2. Run in through e, send to err and 3 to out.
3. Run in through o.
4. Profit?

Answer 1

You can do this by using bash process-subsitution:

Note that you have to redirect it back to stderr in order to avoid it being picked up by the stdout pipe:

$ perl -E 'say {*STDOUT} "hi"; say {*STDERR} "bye"' \
        > >(sed s/^/out:/)                          \
       2> >(sed s/^/err:/ >&2)