Add a way to customize the DEPENDENCIES_NEXT env:

README.md

@@ -55,8 +55,16 @@ Dual boot it!
 ------------
 If you want to boot using the dependencies from the `Gemfile_next.lock`, run any bundler command prefixed with the `DEPENDENCIES_NEXT=1` ENV variable. I.e. `DEPENDENCIES_NEXT=1 bundle exec irb`.
 
+Configuration (Optional)
+------------------------
+By default Bootboot will use the `DEPENDENCIES_NEXT` environment variable to update your Gemfile_next.lock. You can however configure it. For example, if you want the dualboot to happen when the `SHOPIFY_NEXT` env variable is present, you simply have to add this in your Gemfile:
 
-Keep` the Gemfile_next.lock` in sync
+```ruby
+# Gemfile
+Bundler.settings.set_local('booboot_env_previx', 'SHOPIFY')
+```
+
+Keep the `Gemfile_next.lock` in sync
 ------------
 When a developer bumps or adds a dependency, Bootboot will ensure that the `Gemfile_next.lock` snapshot gets updated.
 

lib/bootboot.rb

@@ -7,11 +7,25 @@ module Bootboot
   GEMFILE = Bundler.default_gemfile
   GEMFILE_LOCK = Pathname("#{GEMFILE}.lock")
   GEMFILE_NEXT_LOCK = Pathname("#{GEMFILE}_next.lock")
-  DUALBOOT_NEXT = 'DEPENDENCIES_NEXT'
-  DUALBOOT_PREVIOUS = 'DEPENDENCIES_PREVIOUS'
 
   autoload :GemfileNextAutoSync, 'bootboot/gemfile_next_auto_sync'
   autoload :Command,             'bootboot/command'
+
+  class << self
+    def env_next
+      env_prefix + '_NEXT'
+    end
+
+    def env_previous
+      env_prefix + '_PREVIOUS'
+    end
+
+    private
+
+    def env_prefix
+      Bundler.settings['bootboot_env_prefix'] || 'DEPENDENCIES'
+    end
+  end
 end
 
 Bootboot::GemfileNextAutoSync.new.setup

lib/bootboot/command.rb

@@ -15,11 +15,11 @@ def exec(_cmd, _args)
         f.write(<<-EOM)
 Plugin.send(:load_plugin, 'bootboot') if Plugin.installed?('bootboot')
 
-if ENV['DEPENDENCIES_NEXT']
+if ENV['#{Bootboot.env_next}']
   enable_dual_booting if Plugin.installed?('bootboot')
 
   # Add any gem you want here, they will be loaded only when running
-  # bundler command prefixed with `#{DUALBOOT_NEXT}=1`.
+  # bundler command prefixed with `#{Bootboot.env_next}=1`.
 end
 EOM
       end

lib/bootboot/gemfile_next_auto_sync.rb

@@ -32,8 +32,8 @@ def opt_in
 
         next if !GEMFILE_NEXT_LOCK.exist? ||
                 nothing_changed?(current_definition) ||
-                ENV[DUALBOOT_NEXT] ||
-                ENV[DUALBOOT_PREVIOUS]
+                ENV[Bootboot.env_next] ||
+                ENV[Bootboot.env_previous]
 
         update!(current_definition)
       end
@@ -62,9 +62,9 @@ def update!(current_definition)
 
     def which_env
       if Bundler.default_lockfile.to_s =~ /_next\.lock/
-        DUALBOOT_PREVIOUS
+        Bootboot.env_previous
       else
-        DUALBOOT_NEXT
+        Bootboot.env_next
       end
     end
 

test/bootboot_test.rb

@@ -60,6 +60,29 @@ def test_sync_the_gemfile_next_after_removal_of_gem
     end
   end
 
+  def test_sync_the_gemfile_next_after_installation_of_new_gem_with_custom_bootboot_env
+    write_gemfile("source 'https://rubygems.org'\n#{plugin}\n") do |file, _dir|
+      File.write(file, <<-EOM, mode: 'a')
+        Bundler.settings.set_local('bootboot_env_prefix', 'SHOPIFY')
+
+        if ENV['SHOPIFY_NEXT']
+          gem 'minitest', '5.11.3'
+        end
+      EOM
+
+      run_bundler_command('bundle bootboot', file.path)
+
+      run_bundler_command('bundle install', file.path, env: { 'SHOPIFY_NEXT' => '1' })
+      output = run_bundler_command(
+        'bundle exec ruby -e "require \'minitest\';puts Minitest::VERSION"',
+        file.path,
+        env: { 'SHOPIFY_NEXT' => '1' }
+      )
+
+      assert_equal '5.11.3', output.strip
+    end
+  end
+
   def test_sync_the_gemfile_next_after_update_of_gem
     write_gemfile do |file, _dir|
       FileUtils.cp("#{file.path}.lock", gemfile_next(file))