systemd should also log to stderr

logp/config.go

@@ -71,15 +71,19 @@ const (
 // Beat is supposed to be run within.
 func DefaultConfig(environment Environment) Config {
 	toFiles := true
+	toStderr := false
 
-	// If running in a container environment, don't write to files by default.
-	if environment == ContainerEnvironment {
+	// For container and systemd environments, we don't write to files by default.
+	switch environment {
+	case ContainerEnvironment, SystemdEnvironment:
 		toFiles = false
+		toStderr = true
 	}
 
 	return Config{
-		Level:   defaultLevel,
-		ToFiles: toFiles,
+		Level:    defaultLevel,
+		ToFiles:  toFiles,
+		ToStderr: toStderr,
 		Files: FileConfig{
 			MaxSize:         10 * 1024 * 1024,
 			MaxBackups:      7,

logp/defaults_test.go

@@ -159,15 +159,62 @@ func TestDefaultConfigContainer(t *testing.T) {
 		return
 	}
 
-	// This is running in a separate process. By default the
-	// container environment should be logging to stderr.
+	// This is running in a separate process.
 	cfg := logp.DefaultConfig(logp.ContainerEnvironment)
 	assert.NoError(t, logp.Configure(cfg))
 	logger := logp.L()
 	defer logger.Close()
 	logger.Info("foo")
 }
 
+func TestDefaultConfigSystemd(t *testing.T) {
+	switch runtime.GOOS {
+	case "wasip1", "js", "ios":
+		t.Skipf("cannot exec subprocess on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+
+	if os.Getenv("TEST_DEFAULT_CONFIG_SYSTEMD") != "1" {
+		cmd := exec.Command(os.Args[0], "-test.run=^TestDefaultConfigSystemd$", "-test.v") //nolint:gosec // This is intentionally running a subprocess
+		cmd.Env = append(cmd.Env, "TEST_DEFAULT_CONFIG_SYSTEMD=1")
+
+		var stderr bytes.Buffer
+		cmd.Stderr = &stderr
+
+		err := cmd.Run()
+		data := stderr.Bytes()
+		assert.NoError(t, err, "command failed with error: %s\nstderr: %s", err, data)
+		t.Logf("output:\n%s", data)
+
+		logEntry := struct {
+			LogLevel  string `json:"log.level"`
+			LogOrigin struct {
+				FileName string `json:"file.name"`
+				FileLine int    `json:"file.line"`
+			} `json:"log.origin"`
+			Message string `json:"message"`
+		}{}
+
+		assert.NoError(t, json.Unmarshal(data, &logEntry), "cannot unmarshal log entry from stderr")
+
+		assert.Equal(t, "info", logEntry.LogLevel)
+		assert.Equal(t, "foo", logEntry.Message)
+
+		_, fileName, _, _ := runtime.Caller(0)
+		expectedFileName := filepath.Base(fileName)
+		gotFileName := filepath.Base(logEntry.LogOrigin.FileName)
+		assert.Equal(t, expectedFileName, gotFileName)
+
+		return
+	}
+
+	// This is running in a separate process.
+	cfg := logp.DefaultConfig(logp.SystemdEnvironment)
+	assert.NoError(t, logp.Configure(cfg))
+	logger := logp.L()
+	defer logger.Close()
+	logger.Info("foo")
+}
+
 func TestWith(t *testing.T) {
 	tempDir := t.TempDir()