Add #supported? method for libnotify and notify-send

lib/capistrano/send/libnotify.rb

@@ -1 +1,16 @@
+begin
+  require 'libnotify'
+rescue LoadError
+end
+
+module Capistrano
+  module Send
+    module Libnotify
+      def self.supported?
+        defined?(Libnotify) && Libnotify::API.instance_methods.include?(:notify_init)
+      end
+    end
+  end
+end
+
 load File.expand_path('../../tasks/libnotify.cap', __FILE__)

lib/capistrano/send/notify-send.rb

@@ -1 +1,11 @@
+module Capistrano
+  module Send
+    module NotifySend
+      def self.supported?
+        system('notify-send --version >/dev/null 2>&1') != nil
+      end
+    end
+  end
+end
+
 load File.expand_path('../../tasks/notify-send.cap', __FILE__)

lib/capistrano/tasks/libnotify.cap

@@ -1,13 +1,8 @@
-begin
-  require 'libnotify'
-rescue LoadError
-end
-
 namespace :send do
   namespace :libnotify do
     desc "Display a notification using libnotify"
     task :notify do
-      if defined?(Libnotify)
+      if Capistrano::Send::Libnotify.supported?
         Libnotify.show(
           summary: "Deploy successful on #{fetch(:stage)}!",
           body: revision_log_message,

lib/capistrano/tasks/notify-send.cap

@@ -2,11 +2,13 @@ namespace :send do
   namespace :'notify-send' do
     desc "Display a notification using notify-send"
     task :notify do
-      expire_time = 10000
-      summary = "Deploy successful on #{fetch(:stage)}!"
-      body = revision_log_message
+      if Capistrano::Send::NotifySend.supported?
+        expire_time = 10000
+        summary = "Deploy successful on #{fetch(:stage)}!"
+        body = revision_log_message
 
-      `notify-send --expire-time=#{expire_time} '#{summary}' '#{body}'`
+        `notify-send --expire-time=#{expire_time} '#{summary}' '#{body}'`
+      end
     end
   end
 end