Document how to pass env vars to modern nix commands

Resolves #149.

README.md

@@ -34,7 +34,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v19
+    - uses: cachix/install-nix-action@v20
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - run: nix-build
@@ -52,7 +52,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v19
+    - uses: cachix/install-nix-action@v20
       with:
         github_access_token: ${{ secrets.GITHUB_TOKEN }}
     - run: nix build
@@ -84,7 +84,7 @@ To install Nix from any commit, go to [the corresponding installer_test action](
   run: nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'
 ```
 
-### How can I run NixOS tests?
+### How do I run NixOS tests?
 
 With the following inputs:
 
@@ -96,7 +96,7 @@ With the following inputs:
 
 [Note that there's no hardware acceleration on GitHub Actions.](https://github.com/actions/virtual-environments/issues/183#issuecomment-610723516).
 
-### How can I install packages via nix-env from the specified `nix_path`?
+### How do I install packages via nix-env from the specified `nix_path`?
 
 ```
 nix-env -i mypackage -f '<nixpkgs>'
@@ -120,9 +120,29 @@ Otherwise, you can add any binary cache to nix.conf using
 install-nix-action's own `extra_nix_config` input:
 
 ```yaml
-- uses: cachix/install-nix-action@v19
+- uses: cachix/install-nix-action@v20
   with:
     extra_nix_config: |
       trusted-public-keys = hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
       substituters = https://hydra.iohk.io https://cache.nixos.org/
 ```
+
+### How do I pass environment variables to commands run with `nix develop` or `nix shell`?
+
+Nix runs commands in a restricted environment by default, called `pure mode`.
+In pure mode, environment variables are not passed through to improve the reproducibility of the shell.
+
+You can use the `--keep / -k` flag to keep certain environment variables:
+
+```yaml
+- name: Run a command with nix develop
+  run: nix develop --ignore-environment --keep MY_ENV_VAR --command echo $MY_ENV_VAR
+  env:
+    MY_ENV_VAR: "hello world"
+```
+
+Or you can disable pure mode entirely with the `--impure` flag:
+
+```
+nix develop --impure
+```

install-nix.sh

@@ -1,8 +1,8 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-if type -p nix &>/dev/null ; then
-  echo "Aborting: Nix is already installed at $(type -p nix)"
+if nix_path="$(type -p nix)" ; then
+  echo "Aborting: Nix is already installed at ${nix_path}"
   exit
 fi
 
@@ -15,7 +15,7 @@ trap 'rm -rf "$workdir"' EXIT
 
 # Configure Nix
 add_config() {
-  echo "$1" | tee -a "$workdir/nix.conf" >/dev/null
+  echo "$1" >> "$workdir/nix.conf"
 }
 # Set jobs to number of cores
 add_config "max-jobs = auto"
@@ -28,7 +28,7 @@ elif [[ -n "${GITHUB_TOKEN:-}" ]]; then
   add_config "access-tokens = github.com=$GITHUB_TOKEN"
 fi
 # Append extra nix configuration if provided
-if [[ $INPUT_EXTRA_NIX_CONFIG != "" ]]; then
+if [[ -n "${INPUT_EXTRA_NIX_CONFIG:-}" ]]; then
   add_config "$INPUT_EXTRA_NIX_CONFIG"
 fi
 if [[ ! $INPUT_EXTRA_NIX_CONFIG =~ "experimental-features" ]]; then
@@ -54,10 +54,10 @@ else
   add_config "build-users-group ="
   sudo mkdir -p /etc/nix
   sudo chmod 0755 /etc/nix
-  sudo cp $workdir/nix.conf /etc/nix/nix.conf
+  sudo cp "$workdir/nix.conf" /etc/nix/nix.conf
 fi
 
-if [[ $INPUT_INSTALL_OPTIONS != "" ]]; then
+if [[ -n "${INPUT_INSTALL_OPTIONS:-}" ]]; then
   IFS=' ' read -r -a extra_installer_options <<< "$INPUT_INSTALL_OPTIONS"
   installer_options=("${extra_installer_options[@]}" "${installer_options[@]}")
 fi
@@ -91,7 +91,7 @@ echo "/nix/var/nix/profiles/default/bin" >> "$GITHUB_PATH"
 # new path for nix 2.14
 echo "$HOME/.nix-profile/bin" >> "$GITHUB_PATH"
 
-if [[ $INPUT_NIX_PATH != "" ]]; then
+if [[ -n "${INPUT_NIX_PATH:-}" ]]; then
   echo "NIX_PATH=${INPUT_NIX_PATH}" >> "$GITHUB_ENV"
 fi