#!/bin/bash

devices="$(v4l2-ctl --list-devices)"

if echo "$devices" | grep -q "^Cam Link"; then
    device="Cam Link"
else
    device="USB Video"
fi

possible_streams="$(v4l2-ctl --list-devices | grep "^$device" -A 20 | \
                    grep "^\s*/dev/" | sed 's/^\s*//')"

[ -z "$possible_streams" ] && exit

while IFS= read -r stream; do
    doas v4l2-ctl --device="$stream" --all | grep -q "^Video input" &&
    echo "$stream" && exit
done <<< "$possible_streams"