support alternative output streams

cmd/etcdmtr/main.go

@@ -92,10 +92,6 @@ func (c *Client) GetEtcdWithEndpoints(endpoints []string) (*clientv3.Client, err
 	})
 }
 
-func getLogger() *etcdmtr.ELog {
-	return etcdmtr.NewRootLogger()
-}
-
 func main() {
 	args := &EtcdMtrArgs{}
 	positionals, err := flags.Parse(args)
@@ -109,7 +105,7 @@ func main() {
 		os.Exit(2)
 	}
 
-	logger := getLogger()
+	logger := etcdmtr.NewRootLogger()
 
 	if args.Version {
 		logger.Fatalc(1, "0.0.1")

pkg/etcdmtr/log.go

@@ -2,6 +2,7 @@ package etcdmtr
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"strings"
 	"time"
@@ -24,13 +25,6 @@ func (l LogLevel) String() string {
 	return LOGLEVEL_NAMES[l]
 }
 
-type ELog struct {
-	parent *ELog
-	name   string
-	Level  LogLevel
-	Format *LogFormat
-}
-
 type LogFormatTokenValue int64
 
 const (
@@ -91,6 +85,14 @@ func (f *LogFormat) AppendTimestamp(format string) *LogFormat {
 	})
 }
 
+type ELog struct {
+	parent *ELog
+	name   string
+	Level  LogLevel
+	Format *LogFormat
+	Output io.Writer
+}
+
 func NewRootLogger() *ELog {
 	return NewRootLoggerWithFormat(
 		NewFormat().
@@ -111,6 +113,7 @@ func NewRootLoggerWithFormat(format *LogFormat) *ELog {
 	return &ELog{
 		name:   "main",
 		Format: format,
+		Output: os.Stderr,
 	}
 }
 
@@ -161,7 +164,7 @@ func (e *ELog) _DoLog(level LogLevel, path []string, message string, a ...any) (
 		}
 		tail = tail.Next
 	}
-	return fmt.Println(log.String())
+	return fmt.Fprintln(e.Output, log.String())
 }
 
 func (e *ELog) Debug(message string, a ...any) (n int, err error) {