Allow customizing input/output via env variables

The Rails `assets:precompile` task internally calls `bin/rails tailwindcss:build`,
which uses hardcoded input/output file paths. Previously, the only way to
customize these paths was to bypass the Rails integration entirely and use
the Tailwind CLI directly (via `bundle exec tailwindcss`).

This change introduces TAILWINDCSS_INPUT_FILE and TAILWINDCSS_OUTPUT_FILE
environment variables that allow users to override the default paths while
still using the standard `assets:precompile` workflow.

This is useful for projects with non-standard asset structures or when
integrating with custom build pipelines.

Author: thewatts

lib/tailwindcss/commands.rb

@@ -2,15 +2,18 @@
 
 module Tailwindcss
   module Commands
+    INPUT_FILE = "app/assets/tailwind/application.css"
+    OUTPUT_FILE = "app/assets/builds/tailwind.css"
+
     class << self
       def compile_command(debug: false, **kwargs)
         debug = ENV["TAILWINDCSS_DEBUG"].present? if ENV.key?("TAILWINDCSS_DEBUG")
         rails_root = defined?(Rails) ? Rails.root : Pathname.new(Dir.pwd)
 
         command = [
           Tailwindcss::Ruby.executable(**kwargs),
-          "-i", rails_root.join("app/assets/tailwind/application.css").to_s,
-          "-o", rails_root.join("app/assets/builds/tailwind.css").to_s,
+          "-i", rails_root.join(input_file).to_s,
+          "-o", rails_root.join(output_file).to_s,
         ]
 
         command << "--minify" unless (debug || rails_css_compressor?)
@@ -38,6 +41,14 @@ def command_env(verbose:)
       def rails_css_compressor?
         defined?(Rails) && Rails&.application&.config&.assets&.css_compressor.present?
       end
+
+      def input_file
+        ENV.fetch("TAILWINDCSS_INPUT_FILE", INPUT_FILE)
+      end
+
+      def output_file
+        ENV.fetch("TAILWINDCSS_OUTPUT_FILE", OUTPUT_FILE)
+      end
     end
   end
 end

test/lib/tailwindcss/commands_test.rb

@@ -95,6 +95,31 @@ def setup
     end
   end
 
+  test ".compile_command file with & without input/output ENV variables" do
+    begin
+      Rails.stub(:root, File) do # Rails.root won't work in this test suite
+        ENV["TAILWINDCSS_INPUT_FILE"] = nil
+        ENV["TAILWINDCSS_OUTPUT_FILE"] = nil
+
+        command = Tailwindcss::Commands.compile_command.join(" ")
+
+        assert_includes(command, "-i #{Tailwindcss::Commands::INPUT_FILE}")
+        assert_includes(command, "-o #{Tailwindcss::Commands::OUTPUT_FILE}")
+
+        ENV["TAILWINDCSS_INPUT_FILE"] = "my-input-file"
+        ENV["TAILWINDCSS_OUTPUT_FILE"] = "my-output-file"
+
+        command = Tailwindcss::Commands.compile_command.join(" ")
+
+        assert_includes(command, "-i my-input-file")
+        assert_includes(command, "-o my-output-file")
+      end
+    ensure
+      ENV.delete('TAILWINDCSS_INPUT_FILE')
+      ENV.delete('TAILWINDCSS_OUTPUT_FILE')
+    end
+  end
+
   test ".watch_command" do
     Rails.stub(:root, File) do # Rails.root won't work in this test suite
       actual = Tailwindcss::Commands.watch_command