Write [Shakapacker] log messages to stderr instead of stdout

CHANGELOG.md

@@ -14,6 +14,7 @@ Changes since the last non-beta release.
 ### Fixed
 
 - **Fixed NODE_ENV=test causing DefinePlugin warnings**. [PR #870](https://github.com/shakacode/shakapacker/pull/870) by [justin808](https://github.com/justin808). When RAILS_ENV=test, Shakapacker now sets NODE_ENV=development instead of NODE_ENV=test. This prevents webpack/rspack DefinePlugin conflicts since these bundlers only recognize "development" and "production" as valid NODE_ENV values.
+- **Fixed `--json` flag output being corrupted by log messages**. [PR #869](https://github.com/shakacode/shakapacker/pull/869) by [justin808](https://github.com/justin808). `[Shakapacker]` log messages are now written to stderr instead of stdout, allowing `bin/shakapacker --profile --json` to produce valid JSON output that can be piped to tools like `webpack-bundle-analyzer`. Resolves [#868](https://github.com/shakacode/shakapacker/issues/868).
 
 ## [v9.5.0] - January 7, 2026
 

lib/shakapacker/dev_server_runner.rb

@@ -78,10 +78,10 @@ def self.run_with_build_config(argv, build_config)
       # This ensures the bundler override (from --bundler or build config) is respected
       ENV["SHAKAPACKER_ASSETS_BUNDLER"] = build_config[:bundler]
 
-      puts "[Shakapacker] Running dev server for build: #{build_config[:name]}"
-      puts "[Shakapacker] Description: #{build_config[:description]}" if build_config[:description]
-      puts "[Shakapacker] Bundler: #{build_config[:bundler]}"
-      puts "[Shakapacker] Config file: #{build_config[:config_file]}" if build_config[:config_file]
+      $stderr.puts "[Shakapacker] Running dev server for build: #{build_config[:name]}"
+      $stderr.puts "[Shakapacker] Description: #{build_config[:description]}" if build_config[:description]
+      $stderr.puts "[Shakapacker] Bundler: #{build_config[:bundler]}"
+      $stderr.puts "[Shakapacker] Config file: #{build_config[:config_file]}" if build_config[:config_file]
 
       # Pass bundler override so Configuration.assets_bundler reflects the build
       new(argv, build_config, build_config[:bundler]).run

lib/shakapacker/runner.rb

@@ -99,9 +99,9 @@ def self.run(argv)
 
           # If this build uses dev server, delegate to DevServerRunner
           if loader.uses_dev_server?(build_config)
-            $stdout.puts "[Shakapacker] Build '#{build_name}' requires dev server"
-            $stdout.puts "[Shakapacker] Running: bin/shakapacker-dev-server --build #{build_name}"
-            $stdout.puts ""
+            $stderr.puts "[Shakapacker] Build '#{build_name}' requires dev server"
+            $stderr.puts "[Shakapacker] Running: bin/shakapacker-dev-server --build #{build_name}"
+            $stderr.puts ""
             require_relative "dev_server_runner"
             DevServerRunner.run_with_build_config(remaining_argv, build_config)
             return
@@ -179,10 +179,10 @@ def self.run_with_build_config(argv, build_config)
       # This ensures the bundler override (from --bundler or build config) is respected
       ENV["SHAKAPACKER_ASSETS_BUNDLER"] = build_config[:bundler]
 
-      puts "[Shakapacker] Running build: #{build_config[:name]}"
-      puts "[Shakapacker] Description: #{build_config[:description]}" if build_config[:description]
-      puts "[Shakapacker] Bundler: #{build_config[:bundler]}"
-      puts "[Shakapacker] Config file: #{build_config[:config_file]}" if build_config[:config_file]
+      $stderr.puts "[Shakapacker] Running build: #{build_config[:name]}"
+      $stderr.puts "[Shakapacker] Description: #{build_config[:description]}" if build_config[:description]
+      $stderr.puts "[Shakapacker] Bundler: #{build_config[:bundler]}"
+      $stderr.puts "[Shakapacker] Config file: #{build_config[:config_file]}" if build_config[:config_file]
 
       # Create runner with modified argv and bundler from build_config
       # The build_config[:bundler] already has any CLI --bundler override applied
@@ -240,40 +240,40 @@ def package_json
     end
 
     def run
-      puts "[Shakapacker] Preparing environment for assets bundler execution..."
+      $stderr.puts "[Shakapacker] Preparing environment for assets bundler execution..."
       env = Shakapacker::Compiler.env
       env["SHAKAPACKER_CONFIG"] = @shakapacker_config
       env["NODE_OPTIONS"] = ENV["NODE_OPTIONS"] || ""
 
       cmd = build_cmd
-      puts "[Shakapacker] Base command: #{cmd.join(" ")}"
+      $stderr.puts "[Shakapacker] Base command: #{cmd.join(" ")}"
 
       if @argv.delete("--debug-shakapacker")
-        puts "[Shakapacker] Debug mode enabled (--debug-shakapacker)"
+        $stderr.puts "[Shakapacker] Debug mode enabled (--debug-shakapacker)"
         env["NODE_OPTIONS"] = "#{env["NODE_OPTIONS"]} --inspect-brk"
       end
 
       if @argv.delete "--trace-deprecation"
-        puts "[Shakapacker] Trace deprecation enabled (--trace-deprecation)"
+        $stderr.puts "[Shakapacker] Trace deprecation enabled (--trace-deprecation)"
         env["NODE_OPTIONS"] = "#{env["NODE_OPTIONS"]} --trace-deprecation"
       end
 
       if @argv.delete "--no-deprecation"
-        puts "[Shakapacker] Deprecation warnings disabled (--no-deprecation)"
+        $stderr.puts "[Shakapacker] Deprecation warnings disabled (--no-deprecation)"
         env["NODE_OPTIONS"] = "#{env["NODE_OPTIONS"]} --no-deprecation"
       end
 
       # Commands are not compatible with --config option.
       if (@argv & assets_bundler_commands).empty?
-        puts "[Shakapacker] Adding config file: #{@webpack_config}"
+        $stderr.puts "[Shakapacker] Adding config file: #{@webpack_config}"
         cmd += ["--config", @webpack_config]
       else
-        puts "[Shakapacker] Skipping config file (running assets bundler command: #{(@argv & assets_bundler_commands).join(", ")})"
+        $stderr.puts "[Shakapacker] Skipping config file (running assets bundler command: #{(@argv & assets_bundler_commands).join(", ")})"
       end
 
       cmd += @argv
-      puts "[Shakapacker] Final command: #{cmd.join(" ")}"
-      puts "[Shakapacker] Working directory: #{@app_path}"
+      $stderr.puts "[Shakapacker] Final command: #{cmd.join(" ")}"
+      $stderr.puts "[Shakapacker] Working directory: #{@app_path}"
 
       watch_mode = @argv.include?("--watch") || @argv.include?("-w")
       start_time = Time.now unless watch_mode
@@ -288,7 +288,7 @@ def run
         minutes = (elapsed_time / 60).floor
         seconds = (elapsed_time % 60).round(2)
         time_display = minutes > 0 ? "#{minutes}:#{format('%05.2f', seconds)}s" : "#{elapsed_time.round(2)}s"
-        puts "[Shakapacker] Completed #{bundler_name} build in #{time_display} (#{elapsed_time.round(2)}s)"
+        $stderr.puts "[Shakapacker] Completed #{bundler_name} build in #{time_display} (#{elapsed_time.round(2)}s)"
       end
       exit($?.exitstatus || 1) unless $?.success?
     end
@@ -601,10 +601,10 @@ def find_rspack_config_with_fallback
           File.join(@app_path, config_dir, "rspack.config.#{ext}")
         end
 
-        puts "[Shakapacker] Looking for Rspack config in: #{rspack_paths.join(", ")}"
+        $stderr.puts "[Shakapacker] Looking for Rspack config in: #{rspack_paths.join(", ")}"
         rspack_path = rspack_paths.find { |f| File.exist?(f) }
         if rspack_path
-          puts "[Shakapacker] Found Rspack config: #{rspack_path}"
+          $stderr.puts "[Shakapacker] Found Rspack config: #{rspack_path}"
           return rspack_path
         end
 
@@ -613,7 +613,7 @@ def find_rspack_config_with_fallback
           File.join(@app_path, config_dir, "webpack.config.#{ext}")
         end
 
-        puts "[Shakapacker] Rspack config not found, checking for webpack config fallback..."
+        $stderr.puts "[Shakapacker] Rspack config not found, checking for webpack config fallback..."
         webpack_path = webpack_paths.find { |f| File.exist?(f) }
         if webpack_path
           $stderr.puts "⚠️  DEPRECATION WARNING: Using webpack config file for Rspack assets bundler."
@@ -629,7 +629,7 @@ def find_rspack_config_with_fallback
             File.join(@app_path, "config/webpack", "webpack.config.#{ext}")
           end
 
-          puts "[Shakapacker] Checking config/webpack/ for backward compatibility..."
+          $stderr.puts "[Shakapacker] Checking config/webpack/ for backward compatibility..."
           webpack_dir_path = webpack_dir_paths.find { |f| File.exist?(f) }
           if webpack_dir_path
             $stderr.puts "⚠️  DEPRECATION WARNING: Found webpack config in config/webpack/ but assets_bundler is set to 'rspack'."
@@ -656,13 +656,13 @@ def find_webpack_config
         possible_paths = %w[ts js].map do |ext|
           File.join(@app_path, config_dir, "webpack.config.#{ext}")
         end
-        puts "[Shakapacker] Looking for Webpack config in: #{possible_paths.join(", ")}"
+        $stderr.puts "[Shakapacker] Looking for Webpack config in: #{possible_paths.join(", ")}"
         path = possible_paths.find { |f| File.exist?(f) }
         unless path
           print_config_not_found_error("webpack", possible_paths.last, config_dir)
           exit(1)
         end
-        puts "[Shakapacker] Found Webpack config: #{path}"
+        $stderr.puts "[Shakapacker] Found Webpack config: #{path}"
         path
       end
   end

spec/shakapacker/rspack_runner_spec.rb

@@ -143,7 +143,7 @@
           true
         end
 
-        output = capture_stdout { klass.run([]) }
+        output = capture_stderr { klass.run([]) }
 
         # The test app may have webpack config, so bundler name could be either
         # Time format can be either "X.XXs" or "M:SS.SSs" for the display, always "X.XXs" in parentheses
@@ -164,7 +164,7 @@
           true
         end
 
-        output = capture_stdout { klass.run(["--watch"]) }
+        output = capture_stderr { klass.run(["--watch"]) }
 
         expect(output).not_to match(/Completed (webpack|rspack) build/)
       end
@@ -183,7 +183,7 @@
           true
         end
 
-        output = capture_stdout { klass.run(["-w"]) }
+        output = capture_stderr { klass.run(["-w"]) }
 
         expect(output).not_to match(/Completed (webpack|rspack) build/)
       end
@@ -192,13 +192,13 @@
 
   private
 
-    def capture_stdout
-      old_stdout = $stdout
-      $stdout = StringIO.new
+    def capture_stderr
+      old_stderr = $stderr
+      $stderr = StringIO.new
       yield
-      $stdout.string
+      $stderr.string
     ensure
-      $stdout = old_stdout
+      $stderr = old_stderr
     end
 
     def verify_command(cmd, argv: [])

spec/shakapacker/runner_build_config_spec.rb

@@ -48,7 +48,7 @@
         allow(klass).to receive(:new).and_return(instance)
         allow(instance).to receive(:system).and_return(true)
 
-        output = capture_stdout do
+        output = capture_stderr do
           klass.run(["--build", "prod"])
         end
 
@@ -85,7 +85,7 @@
         allow(dev_server_klass).to receive(:new).and_return(dev_server_instance)
         allow(dev_server_instance).to receive(:run).and_return(nil)
 
-        output = capture_stdout do
+        output = capture_stderr do
           klass.run(["--build", "dev-hmr"])
         end
 
@@ -114,7 +114,7 @@
         allow(klass).to receive(:new).and_return(instance)
         allow(instance).to receive(:system).and_return(true)
 
-        output = capture_stdout do
+        output = capture_stderr do
           klass.run(["nonexistent"])
         end
 
@@ -132,7 +132,7 @@
         allow(klass).to receive(:new).and_return(instance)
         allow(instance).to receive(:system).and_return(true)
 
-        output = capture_stdout do
+        output = capture_stderr do
           klass.run([])
         end
 
@@ -198,7 +198,7 @@
         allow(klass).to receive(:new).and_return(instance)
         allow(instance).to receive(:run).and_return(nil)
 
-        output = capture_stdout do
+        output = capture_stderr do
           klass.run(["--build", "dev"])
         end
 
@@ -264,7 +264,7 @@
         allow(dev_server_klass).to receive(:new).and_return(dev_server_instance)
         allow(dev_server_instance).to receive(:run).and_return(nil)
 
-        output = capture_stdout do
+        output = capture_stderr do
           klass.run(["--build", "dev-hmr"])
         end
 
@@ -277,15 +277,6 @@
 
   private
 
-    def capture_stdout
-      old_stdout = $stdout
-      $stdout = StringIO.new
-      yield
-      $stdout.string
-    ensure
-      $stdout = old_stdout
-    end
-
     def capture_stderr
       old_stderr = $stderr
       $stderr = StringIO.new

spec/shakapacker/webpack_runner_spec.rb

@@ -158,7 +158,7 @@
           true
         end
 
-        output = capture_stdout { klass.run([]) }
+        output = capture_stderr { klass.run([]) }
 
         # Time format can be either "X.XXs" or "M:SS.SSs" for the display, always "X.XXs" in parentheses
         expect(output).to match(/\[Shakapacker\] Completed webpack build in (\d+:\d+\.\d+s|\d+\.\d+s) \(\d+\.\d+s\)/)
@@ -178,7 +178,7 @@
           true
         end
 
-        output = capture_stdout { klass.run(["--watch"]) }
+        output = capture_stderr { klass.run(["--watch"]) }
 
         expect(output).not_to match(/Completed webpack build/)
       end
@@ -197,7 +197,7 @@
           true
         end
 
-        output = capture_stdout { klass.run([]) }
+        output = capture_stderr { klass.run([]) }
 
         # Should show format like "3.29s (3.29s)" without minutes
         expect(output).to match(/\[Shakapacker\] Completed webpack build in \d+\.\d+s \(\d+\.\d+s\)/)
@@ -206,15 +206,73 @@
     end
   end
 
+  describe "stdout/stderr separation for JSON output" do
+    it "does not write [Shakapacker] log messages to stdout" do
+      Dir.chdir(test_app_path) do
+        klass = Shakapacker::WebpackRunner
+        instance = klass.new(["--json"])
+
+        allow(klass).to receive(:new).and_return(instance)
+        allow(Shakapacker::Utils::Manager).to receive(:error_unless_package_manager_is_obvious!)
+
+        allow(instance).to receive(:system) do |*args|
+          system("true")
+          true
+        end
+
+        stdout_output, stderr_output = capture_stdout_and_stderr { klass.run(["--json"]) }
+
+        # Stdout should NOT contain [Shakapacker] log messages
+        expect(stdout_output).not_to match(/\[Shakapacker\]/)
+
+        # Stderr SHOULD contain [Shakapacker] log messages
+        expect(stderr_output).to match(/\[Shakapacker\]/)
+      end
+    end
+
+    it "keeps stdout clean for valid JSON output when using --json flag" do
+      Dir.chdir(test_app_path) do
+        klass = Shakapacker::WebpackRunner
+        instance = klass.new(["--profile", "--json"])
+
+        allow(klass).to receive(:new).and_return(instance)
+        allow(Shakapacker::Utils::Manager).to receive(:error_unless_package_manager_is_obvious!)
+
+        allow(instance).to receive(:system) do |*args|
+          system("true")
+          true
+        end
+
+        stdout_output, = capture_stdout_and_stderr { klass.run(["--profile", "--json"]) }
+
+        # Stdout should be empty (no log messages polluting it)
+        # The actual JSON would come from webpack itself, not shakapacker
+        expect(stdout_output).to be_empty
+      end
+    end
+  end
+
   private
 
-    def capture_stdout
+    def capture_stdout_and_stderr
       old_stdout = $stdout
+      old_stderr = $stderr
       $stdout = StringIO.new
+      $stderr = StringIO.new
       yield
-      $stdout.string
+      [$stdout.string, $stderr.string]
     ensure
       $stdout = old_stdout
+      $stderr = old_stderr
+    end
+
+    def capture_stderr
+      old_stderr = $stderr
+      $stderr = StringIO.new
+      yield
+      $stderr.string
+    ensure
+      $stderr = old_stderr
     end
 
     def verify_command(cmd, argv: [])