.direnvrc

#!/bin/bash
#
#     Cache nix-shell environment
#
#         - watches shell.nix (or default.nix), ~/.direnvrc and .envrc
#         - based on https://github.com/direnv/direnv/wiki/Nix
#
use_nix() {
    local shell_file=$(test -f shell.nix && echo shell.nix || echo default.nix)
    if [[ ! -f "$shell_file" ]]; then return; fi
    local cache_key=$(nix-instantiate "$shell_file" 2> /dev/null | shasum -a 1 | cut -d ' ' -f 1)

    # Use ram as virtualenv storage
    local tmpdir
    case $(uname -s) in
        Linux*) tmpdir=$XDG_RUNTIME_DIR;;
        Darwin*) tmpdir_SDK=$TMPDIR;;
        *) tmpdir=/tmp
    esac
    if test "$tmpdir" = ""; then
        tmpdir="/tmp"
    fi

    local cachedir="${tmpdir}/direnv-nix"
    mkdir -p $cachedir

    local cache="$cachedir/$cache_key"
    if [[ ! -e "$cache" ]] || \
           [[ "~/.direnvrc" -nt "$cache" ]] || \
           [[ ".envrc" -ot "$cache" ]] || \
           [[ ".envrc" -nt "$cache" ]];
    then
        local tmp="$(mktemp "${cache_key}.tmp-XXXXXXXX")"
        trap "rm -r '$tmp' || true" EXIT
        nix-instantiate "$shell_file" --indirect --add-root ~/.direnv-gcroots/"$cache_key"
        nix-shell --show-trace "$@" --run 'direnv dump' > "$tmp" || exit $?
        # Copy the filetime but use POSIX precision for both
        touch -r ".envrc" "$tmp"
        mv "$tmp" "$cache"
        echo "Created cache at $cache"
    else
        echo "Using cache at $cache"
    fi
    direnv_load cat "$cache"
    if [[ $# -eq 0 ]]; then
        watch_file "$shell_file"
        watch_file shell.nix
    fi
}

# GistId: 3cb29d9eea83938622eec9ff3bfb3b69

Same as https://gist.github.com/lionello/3cb29d9eea83938622eec9ff3bfb3b69