Skip to content
Switch branches/tags
Go to file
Cannot retrieve contributors at this time
# Load environment variables from `nix-shell` and export it out.
# Usage: use_nix [-s <nix-expression>] [-w <path>] [-w <path>] ...
# -s nix-expression: The nix expression to use for building the shell environment.
# -w path: watch a file for changes. It can be specified multiple times. The
# shell specified with -s is automatically watched.
# If no nix-expression were given with -s, it will attempt to find and load
# the shell using the following files in order: shell.nix and default.nix.
# Example:
# - use_nix
# - use_nix -s shell.nix -w .nixpkgs-version.json
# The dependencies pulled by nix-shell are added to Nix's garbage collector
# roots, such that the environment remains persistent.
# Nix-shell is invoked only once per environment, and the output is cached for
# better performance. If any of the watched files change, then the environment
# is rebuilt.
# To remove old environments, and allow the GC to collect their dependencies:
# rm -f .direnv
use_nix() {
if ! validate_version; then
echo "This .envrc requires direnv version 2.18.2 or above."
exit 1
# define all local variables
local shell
local files_to_watch=()
local opt OPTARG OPTIND # define vars used by getopts locally
while getopts ":n:s:w:" opt; do
case "${opt}" in
files_to_watch=("${files_to_watch[@]}" "${shell}")
files_to_watch=("${files_to_watch[@]}" "${OPTARG}")
fail "Invalid option: $OPTARG requires an argument"
fail "Invalid option: $OPTARG"
shift $((OPTIND -1))
if [[ -z "${shell}" ]]; then
if [[ -f shell.nix ]]; then
files_to_watch=("${files_to_watch[@]}" shell.nix)
elif [[ -f default.nix ]]; then
files_to_watch=("${files_to_watch[@]}" default.nix)
fail "ERR: no shell was given"
local f
for f in "${files_to_watch[@]}"; do
if ! [[ -f "${f}" ]]; then
fail "cannot watch file ${f} because it does not exist"
# compute the hash of all the files that makes up the development environment
local env_hash="$(hash_contents "${files_to_watch[@]}")"
# define the paths
local dir="$(direnv_layout_dir)"
local wd="${dir}/wd-${env_hash}"
local drv="${wd}/env.drv"
local dump="${wd}/dump.env"
# Generate the environment if we do not have one generated already.
if [[ ! -f "${drv}" ]]; then
mkdir -p "${wd}"
log_status "use nix: deriving new environment"
IN_NIX_SHELL=1 nix-instantiate --add-root "${drv}" --indirect "${shell}" > /dev/null
nix-store -r $(nix-store --query --references "${drv}") --add-root "${wd}/dep" --indirect > /dev/null
if [[ "${?}" -ne 0 ]] || [[ ! -f "${drv}" ]]; then
rm -rf "${wd}"
fail "use nix: was not able to derive the new environment. Please run 'direnv reload' to try again."
log_status "use nix: updating cache"
nix-shell --pure "${drv}" --show-trace --run "$(join_args "$direnv" dump bash)" > "${dump}"
if [[ "${?}" -ne 0 ]] || [[ ! -f "${dump}" ]] || ! grep -q IN_NIX_SHELL "${dump}"; then
rm -rf "${wd}"
fail "use nix: was not able to update the cache of the environment. Please run 'direnv reload' to try again."
# evaluate the dump created by nix-shell earlier, but have to merge the PATH
# with the current PATH
# NOTE: we eval the dump here as opposed to direnv_load it because we don't
# want to persist environment variables coming from the shell at the time of
# the dump. See for context.
local path_backup="${PATH}"
eval $(cat "${dump}")
export PATH="${PATH}:${path_backup}"
# cleanup the environment of variables that are not requried, or are causing issues.
unset shellHook # when shellHook is present, then any nix-shell'd script will execute it!
# watch all the files we were asked to watch for the environment
for f in "${files_to_watch[@]}"; do
watch_file "${f}"
fail() {
log_error "${@}"
exit 1
hash_contents() {
if has md5sum; then
cat "${@}" | md5sum | cut -c -32
elif has md5; then
cat "${@}" | md5 -q
hash_file() {
if has md5sum; then
md5sum "${@}" | cut -c -32
elif has md5; then
md5 -q "${@}"
validate_version() {
local version="$("${direnv}" version)"
local major="$(echo "${version}" | cut -d. -f1)"
local minor="$(echo "${version}" | cut -d. -f2)"
local patch="$(echo "${version}" | cut -d. -f3)"
if [[ "${major}" -gt 2 ]]; then return 0; fi
if [[ "${major}" -eq 2 ]] && [[ "${minor}" -gt 18 ]]; then return 0; fi
if [[ "${major}" -eq 2 ]] && [[ "${minor}" -eq 18 ]] && [[ "${patch}" -ge 2 ]]; then return 0; fi
return 1
use_nix -s shell.nix -w .nixpkgs-version.json