diff --git a/Gemfile b/Gemfile
index 6ac5785..4ba4089 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 source 'https://rubygems.org'
 
 group :development do
diff --git a/app/controllers/docker_manager/admin_controller.rb b/app/controllers/docker_manager/admin_controller.rb
index 3ababf7..2532027 100644
--- a/app/controllers/docker_manager/admin_controller.rb
+++ b/app/controllers/docker_manager/admin_controller.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require_dependency 'docker_manager/git_repo'
 require_dependency 'docker_manager/upgrader'
 
diff --git a/app/controllers/docker_manager/application_controller.rb b/app/controllers/docker_manager/application_controller.rb
index ebb9d73..5426cf4 100644
--- a/app/controllers/docker_manager/application_controller.rb
+++ b/app/controllers/docker_manager/application_controller.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module DockerManager
   class ApplicationController < ActionController::Base
     helper DockerManager::ApplicationHelper
diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index c690c05..3615ccc 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module DockerManager::ApplicationHelper
   def discourse_root_url
     Discourse.base_uri
diff --git a/bin/pull_translations.rb b/bin/pull_translations.rb
index d9aa66d..96a5dac 100755
--- a/bin/pull_translations.rb
+++ b/bin/pull_translations.rb
@@ -1,4 +1,5 @@
 #!/usr/bin/env ruby
+# frozen_string_literal: true
 
 # Usage:
 #   bundle install
diff --git a/config/routes.rb b/config/routes.rb
index 615ae98..99b4e6e 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 DockerManager::Engine.routes.draw do
   get "admin/docker", to: redirect("/admin/upgrade"), constraints: AdminConstraint.new
   get "admin/upgrade" => "admin#index", constraints: AdminConstraint.new
diff --git a/lib/docker_manager/git_repo.rb b/lib/docker_manager/git_repo.rb
index 2cf7dda..f44d8ad 100644
--- a/lib/docker_manager/git_repo.rb
+++ b/lib/docker_manager/git_repo.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # like Grit just very very minimal
 class DockerManager::GitRepo
   attr_reader :path, :name, :branch
@@ -96,7 +98,7 @@ class DockerManager::GitRepo
     return unless result.present?
 
     if result =~ /-(\d+)-/
-      result.gsub!(/-(\d+)-.*/, " +#{$1}")
+      result = result.gsub(/-(\d+)-.*/, " +#{$1}")
     end
     result
   end
@@ -106,7 +108,7 @@ class DockerManager::GitRepo
   end
 
   def commit_date(commit)
-    unix_timestamp = run('show -s --format="%ct" ' << commit).to_i
+    unix_timestamp = run(+'show -s --format="%ct" ' << commit).to_i
     Time.at(unix_timestamp).to_datetime
   end
 
diff --git a/lib/docker_manager/upgrader.rb b/lib/docker_manager/upgrader.rb
index 26ee44d..3e75e5c 100644
--- a/lib/docker_manager/upgrader.rb
+++ b/lib/docker_manager/upgrader.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class DockerManager::Upgrader
 
   def initialize(user_id, repos, from_version)
@@ -121,7 +123,7 @@ class DockerManager::Upgrader
 
   def run(cmd)
     log "$ #{cmd}"
-    msg = ""
+    msg = +""
 
     allowed_env = %w{
       PWD
diff --git a/plugin.rb b/plugin.rb
index a955a86..2f8dc36 100644
--- a/plugin.rb
+++ b/plugin.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # name: docker_manager
 # about: Docker manager for Discourse image
 # version: 0.1
@@ -6,7 +8,7 @@
 
 module ::DockerManager
   # should be automatic, but something is weird
-  load File.expand_path(File.dirname(__FILE__)) << '/app/helpers/application_helper.rb'
+  load File.expand_path(File.dirname(__FILE__)) + '/app/helpers/application_helper.rb'
   class Engine < ::Rails::Engine
     engine_name "docker_manager"
     isolate_namespace DockerManager
diff --git a/spec/lib/git_repo_spec.rb b/spec/lib/git_repo_spec.rb
index c8aede6..466d040 100644
--- a/spec/lib/git_repo_spec.rb
+++ b/spec/lib/git_repo_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'rails_helper'
 require 'docker_manager/git_repo'
 
diff --git a/spec/requests/admin_controller_spec.rb b/spec/requests/admin_controller_spec.rb
index 23e8678..dc25dc9 100644
--- a/spec/requests/admin_controller_spec.rb
+++ b/spec/requests/admin_controller_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'rails_helper'
 
 RSpec.describe DockerManager::AdminController do