add --log-stderr: write log output to stderr

command/app.go

@@ -58,6 +58,10 @@ var app = &cli.App{
 			},
 			Usage: "log level: (trace, debug, info, error)",
 		},
+		&cli.BoolFlag{
+			Name:  "log-stderr",
+			Usage: "direct log output to stderr instead of stdout",
+		},
 		&cli.BoolFlag{
 			Name:  "install-completion",
 			Usage: "get completion installation instructions for your shell (only available for bash, pwsh, and zsh)",
@@ -96,10 +100,11 @@ var app = &cli.App{
 		workerCount := c.Int("numworkers")
 		printJSON := c.Bool("json")
 		logLevel := c.String("log")
+		logStderr := c.Bool("log-stderr")
 		isStat := c.Bool("stat")
 		endpointURL := c.String("endpoint-url")
 
-		log.Init(logLevel, printJSON)
+		log.Init(logLevel, printJSON, logStderr)
 		parallel.Init(workerCount)
 
 		if retryCount < 0 {

log/log.go

@@ -18,29 +18,37 @@ var outputCh = make(chan output, 10000)
 var global *Logger
 
 // Init inits global logger.
-func Init(level string, json bool) {
-	global = New(level, json)
+func Init(level string, json bool, stderr bool) {
+	global = New(level, json, stderr)
+}
+
+func LoggerOutFile(logger *Logger) *os.File {
+	if logger.stderr {
+		return os.Stderr
+	}
+
+	return os.Stdout
 }
 
 // Trace prints message in trace mode.
 func Trace(msg Message) {
-	global.printf(LevelTrace, msg, os.Stdout)
+	global.printf(LevelTrace, msg, LoggerOutFile(global))
 }
 
 // Debug prints message in debug mode.
 func Debug(msg Message) {
-	global.printf(LevelDebug, msg, os.Stdout)
+	global.printf(LevelDebug, msg, LoggerOutFile(global))
 }
 
 // Info prints message in info mode.
 func Info(msg Message) {
-	global.printf(LevelInfo, msg, os.Stdout)
+	global.printf(LevelInfo, msg, LoggerOutFile(global))
 }
 
 // Stat prints stat message regardless of the log level with info print formatting.
 // It uses printfHelper instead of printf to ignore the log level condition.
 func Stat(msg Message) {
-	global.printfHelper(LevelInfo, msg, os.Stdout)
+	global.printfHelper(LevelInfo, msg, LoggerOutFile(global))
 }
 
 // Error prints message in error mode.
@@ -61,15 +69,17 @@ type Logger struct {
 	donech chan struct{}
 	json   bool
 	level  LogLevel
+	stderr bool
 }
 
 // New creates new logger.
-func New(level string, json bool) *Logger {
+func New(level string, json bool, stderr bool) *Logger {
 	logLevel := LevelFromString(level)
 	logger := &Logger{
 		donech: make(chan struct{}),
 		json:   json,
 		level:  logLevel,
+		stderr: stderr,
 	}
 	go logger.out()
 	return logger

storage/s3_test.go

@@ -393,7 +393,7 @@ func TestS3ListContextCancelled(t *testing.T) {
 }
 
 func TestS3Retry(t *testing.T) {
-	log.Init("debug", false)
+	log.Init("debug", false, false)
 
 	testcases := []struct {
 		name          string
@@ -576,7 +576,7 @@ func TestS3Retry(t *testing.T) {
 }
 
 func TestS3RetryOnNoSuchUpload(t *testing.T) {
-	log.Init("debug", false)
+	log.Init("debug", false, false)
 
 	noSuchUploadError := awserr.New(s3.ErrCodeNoSuchUpload, "The specified upload does not exist. The upload ID may be invalid, or the upload may have been aborted or completed. status code: 404, request id: PJXXXXX, host id: HOSTIDXX", nil)
 	testcases := []struct {
@@ -936,7 +936,7 @@ func TestS3listObjectsV2(t *testing.T) {
 }
 
 func TestSessionCreateAndCachingWithDifferentBuckets(t *testing.T) {
-	log.Init("error", false)
+	log.Init("error", false, false)
 	testcases := []struct {
 		bucket         string
 		alreadyCreated bool // sessions should not be created again if they already have been created before
@@ -1077,7 +1077,7 @@ func TestSessionAutoRegionValidateCredentials(t *testing.T) {
 }
 
 func TestSessionAutoRegion(t *testing.T) {
-	log.Init("error", false)
+	log.Init("error", false, false)
 
 	unitSession := func() *session.Session {
 		return session.Must(session.NewSession(&aws.Config{