Redirect script stdin to file in the background

Question

In order to pass my script's stdin to stdin of a command in an alacritty instance it spawns, I'm trying to redirect it to a file in the background (so that the command can start reading it):

t="$(mktemp --directory)"
in="$t/in"
#mkfifo "$in"
out="$t/out"
#mkfifo "$out"

cat - 3>"$in" >&3 &
alacritty -e /bin/sh -c "cat '$in' | {...} > '$out'" >&2

cat "$out"

As you can see, I tried making it a named pipe, and also using fd3 thinking the problem might be some sort of special casing that backgrounded processes don't get a 'stdin' file descriptor.

In case it's not clear, the reason for trying is that the input may be large enough that I want to start the ... process without waiting for EOF.

I'm probably going about this all wrong - is there a way to 'reassign' 'my' file descriptors to the shell launched by alacritty? That was my first thought, but I couldn't find anything on passing file descriptors from a shell, just C (etc.) pointers.

Answer 1

cat - 3>"$in" >&3 &

If you run that from a script, cat's stdin will be redirected from /dev/null.

If you run it from an interactive shell, cat will be stopped by a SIGTTIN signal.

In any case, it won't write anything to the "$in" pipe.

If that's part of a script, the "solution" (scarequotes because I'm not really clear what you're trying to do) is to "save" the stdin with an exec [fd]<&0 redirection, and then redirect the input of the background process from that [fd]. Compare:

echo yes | sh -c 'cat & sleep 1'

which won't write anything to stdout, with

echo yes | sh -c 'exec 5<&0; cat <&5 & sleep 1'

which will do.