nixpkgs/nixos/modules/installer/tools/get-version-suffix

getVersion() {
    local dir="$1"
    rev=
    if [ -e "$dir/.git" ]; then
        if [ -z "$(type -P git)" ]; then
            echo "warning: Git not found; cannot figure out revision of $dir" >&2
            return
        fi
        cd "$dir"
        rev=$(git rev-parse --short HEAD)
        if git describe --always --dirty | grep -q dirty; then
            rev+=M
        fi
    fi
}

if nixpkgs=$(nix-instantiate --find-file nixpkgs "$@"); then
    getVersion $nixpkgs
    if [ -n "$rev" ]; then
        echo ".git.$rev"
    fi
fi