update-license-data: wrote rspec test and added one more parseable arg

Library/Homebrew/dev-cmd/update-license-data.rb

@@ -22,6 +22,11 @@ module Homebrew
       switch "--fail-if-outdated",
              description: "Return a failing status code if current license data's version is different from the upstream. This "\
                           "can be used to notify CI when the SPDX license data is out of date."
+
+      switch "--do-not-replace",
+             description: "Flags out discrepancy between local and upstream versions, but does not replace"
+
+      max_named 0
     end
   end
 
@@ -39,16 +44,16 @@ module Homebrew
     updated_spdx_hash = JSON.parse(updated_spdx_string)
     if curr_spdx_hash["licenseListVersion"] != updated_spdx_hash["licenseListVersion"]
 
-      puts "Current version is #{curr_spdx_hash["licenseListVersion"]} but newest version is #{updated_spdx_hash["licenseListVersion"]}\n"\
-      "Updating existing licences data file..."
-      File.open(SPDX_FOLDER_PATH/FILE_NAME, "wb") do |file|
-          file.write(updated_spdx_string)
-      end
-      if args.fail_if_outdated
-        Homebrew.failed = true
-      end
+      puts "Current version is #{curr_spdx_hash["licenseListVersion"]} but newest version is #{updated_spdx_hash["licenseListVersion"]}"
+      unless args.do_not_replace?
+        puts "Updating existing licences data file..."
+        File.open(SPDX_FOLDER_PATH/FILE_NAME, "wb") do |file|
+            file.write(updated_spdx_string)
+        end
       end
+      Homebrew.failed = !!args.fail_if_outdated
     else
       puts "Current version of license data is updated. No change required"
     end
   end
+  end

Library/Homebrew/test/dev-cmd/update_license_data_spec.rb

@@ -0,0 +1,9 @@
+require 'rspec'
+
+describe 'My behaviour' do
+
+  it 'should do something' do
+
+    true.should == false
+  end
+end