Add migration script

default.nix

@@ -1,5 +1,112 @@
 { pkgs ? import <nixpkgs> { } }:
 
-{
+rec {
   nix-on-droid = pkgs.callPackage ./nix-on-droid { };
+
+  basic-environment = pkgs.runCommand
+    "basic-environment"
+    {
+      preferLocalBuild = true;
+      allowSubstitutes = false;
+    }
+    ''
+      echo
+      echo "===================================================="
+      echo
+      echo "You are currently using an old installation of nix-on-droid. The way it manages your device has changed \
+      from installing the basic-environment package to a real module system. For more information see \
+      https://github.com/t184256/nix-on-droid-bootstrap."
+      echo
+      echo "You can either reinstall the app or run"
+      echo "  nix-shell '<nix-on-droid>' -A migration"
+      echo "to migrate your system to the new version."
+      echo
+      echo "Please note, that all files in /etc currently provided by nix-on-droid are getting installed. The current \
+      files will be backuped with a '.bak' file extension."
+      echo "For setups with home-manager (only recognised of HOME_MANAGER_CONFIG is set or ~/.config/nixpkgs/home.nix \
+      is present) there is one manual step necessary *before* running the migration script: Remove basic-environment \
+      package of 'home.packages' list."
+      echo
+      echo "===================================================="
+      echo
+      exit 1
+    '';
+
+  migration = pkgs.runCommand
+    "migration"
+    {
+      preferLocalBuild = true;
+      allowSubstitutes = false;
+      shellHookOnly = true;
+      shellHook = ''
+        set -eu -o pipefail
+
+        echo "Installing nix-on-droid.nix default config file..."
+        ${pkgs.coreutils}/bin/mkdir --parents $HOME/.config/nixpkgs
+        ${pkgs.coreutils}/bin/cp ${./modules/build/initial-build/nix-on-droid.nix.default} $HOME/.config/nixpkgs/nix-on-droid.nix
+
+        if command -v home-manager && [[ -n "$HOME_MANAGER_CONFIG" && -r "$HOME_MANAGER_CONFIG" || -r "$HOME/.config/nixpkgs/home.nix" ]]; then
+          echo "Migrating home-manager installation..."
+          if [[ -r "$HOME/.config/nixpkgs/home.nix" ]]; then
+            ${pkgs.patch}/bin/patch --no-backup-if-mismatch $HOME/.config/nixpkgs/nix-on-droid.nix ${pkgs.writeText "patch" ''
+              @@ -27,15 +27,8 @@
+                 # Read the changelog before changing this value
+                 system.stateVersion = "19.09";
+
+              -  # After installing home-manager channel like
+              -  #   nix-channel --add https://github.com/rycee/home-manager/archive/master.tar.gz home-manager
+              -  #   nix-channel --update
+              -  # you can configure home-manager in here like
+              -  #home-manager.config =
+              -  #  { pkgs, ... }:
+              -  #  {
+              -  #    # insert home-manager config
+              -  #  };
+              +  # Home Manager config file
+              +  home-manager.config = import ./home.nix;
+               }
+
+               # vim: ft=nix
+            ''}
+          else
+            ${pkgs.patch}/bin/patch --no-backup-if-mismatch $HOME/.config/nixpkgs/nix-on-droid.nix ${pkgs.writeText "patch" ''
+              @@ -27,15 +27,8 @@
+                 # Read the changelog before changing this value
+                 system.stateVersion = "19.09";
+
+              -  # After installing home-manager channel like
+              -  #   nix-channel --add https://github.com/rycee/home-manager/archive/master.tar.gz home-manager
+              -  #   nix-channel --update
+              -  # you can configure home-manager in here like
+              -  #home-manager.config =
+              -  #  { pkgs, ... }:
+              -  #  {
+              -  #    # insert home-manager config
+              -  #  };
+              +  # Home Manager config file
+              +  home-manager.config = import (builtins.getEnv "HOME_MANAGER_CONFIG");
+               }
+
+               # vim: ft=nix
+            ''}
+          fi
+        fi
+
+        echo "Decrease priority of basic-environment..."
+        ${pkgs.nix}/bin/nix-env --set-flag priority 100 basic-environment
+
+        echo "Install first nix-on-droid generation..."
+        ${nix-on-droid}/bin/nix-on-droid switch --verbose
+
+        echo "Uninstall basic-environment..."
+        ${pkgs.nix}/bin/nix-env --uninstall basic-environment
+
+        echo "Installation successful! Please restart the app to complete the migration as starting new sessions will fail."
+        exit 0
+      '';
+    }
+    ''
+      echo "This derivation is not buildable, instead run it using nix-shell."
+      exit 1
+    '';
 }