getGpuBusIDs.sh

#!/usr/bin/env nix-shell
#! nix-shell -i bash
#! nix-shell -p bash lshw

convert_bus_id() {
    local raw="${1#*@}"  # remove pci@
    IFS=':.'
    read -r domain bus slot func <<< "$raw"
    echo "PCI:$((16#$bus)):$((16#$slot)):$((16#$func))"
}

# Parse blocks using awk and extract fields
lshw -c display 2>/dev/null | awk '
BEGIN { RS="*-display"; }
/bus info:/ {
    # Reset vars
    bus=""; driver=""; product=""

    # Read block line by line
    n = split($0, lines, "\n")
    for (i = 1; i <= n; ++i) {
        line = lines[i]
        if (match(line, /bus info: pci@([0-9a-f:.]+)/, m)) {
            bus = m[1]
        }
        if (match(line, /configuration:.*driver=([^ ]+)/, m)) {
            driver = m[1]
        }
        if (match(line, /product:[ \t]*(.+)/, m)) {
            product = tolower(m[1])
        }
    }

    if (bus != "") {
        printf("BUS:%s DRIVER:%s PRODUCT:%s\n", bus, driver, product)
    }
}' | while IFS= read -r line; do
    bus=$(echo "$line" | sed -n 's/.*BUS:\([^ ]*\).*/\1/p')
    driver=$(echo "$line" | sed -n 's/.*DRIVER:\([^ ]*\).*/\1/p')
    product=$(echo "$line" | sed -n 's/.*PRODUCT:\(.*\)/\1/p')

    pci_formatted=$(convert_bus_id "$bus")

    case "$driver" in
        i915)
            echo "intelBusId = \"$pci_formatted\";"
            ;;
        nvidia|nouveau)
            echo "nvidiaBusId = \"$pci_formatted\";"
            ;;
        amdgpu|radeon)
            echo "amdBusId = \"$pci_formatted\";"
            ;;
        *)
            echo "Unknown display device (product='$product', driver='$driver') with bus $pci_formatted"
            ;;
    esac

done

Should return along the lines of:

nvidiaBusId = "PCI:1:0:0";
intelBusId = "PCI:0:2:0";

Which is perfect for NixOS; specifically when you're doing Prime setup on Laptops.