diff --git a/pkg/remote/remote.go b/pkg/remote/remote.go
index faf0ab26..4adf9864 100644
--- a/pkg/remote/remote.go
+++ b/pkg/remote/remote.go
@@ -263,7 +263,7 @@ func RunCommand(ctx context.Context, cmdId string, remoteId string, remoteState
 		DonePk:      nil,
 		RunOut:      nil,
 	}
-	err = sstore.AppendToCmdPtyBlob(ctx, cmd.SessionId, cmd.CmdId, nil, sstore.PosAppend)
+	err = sstore.AppendToCmdPtyBlob(ctx, cmd.SessionId, cmd.CmdId, nil, 0)
 	if err != nil {
 		return nil, err
 	}
@@ -359,6 +359,7 @@ func (runner *MShellProc) ProcessPackets() {
 		}
 		runner.notifyHangups_nolock()
 	})
+	var dataPos int64
 	for pk := range runner.ServerProc.Output.MainCh {
 		if pk.GetType() == packet.DataPacketStr {
 			dataPk := pk.(*packet.DataPacketType)
@@ -370,12 +371,13 @@ func (runner *MShellProc) ProcessPackets() {
 			}
 			var ack *packet.DataAckPacketType
 			if len(realData) > 0 {
-				err = sstore.AppendToCmdPtyBlob(context.Background(), dataPk.CK.GetSessionId(), dataPk.CK.GetCmdId(), realData, sstore.PosAppend)
+				err = sstore.AppendToCmdPtyBlob(context.Background(), dataPk.CK.GetSessionId(), dataPk.CK.GetCmdId(), realData, dataPos)
 				if err != nil {
 					ack = makeDataAckPacket(dataPk.CK, dataPk.FdNum, 0, err)
 				} else {
 					ack = makeDataAckPacket(dataPk.CK, dataPk.FdNum, len(realData), nil)
 				}
+				dataPos += int64(len(realData))
 			}
 			if ack != nil {
 				runner.ServerProc.Input.SendPacket(ack)
diff --git a/pkg/sstore/fileops.go b/pkg/sstore/fileops.go
index e296129c..ad89f4de 100644
--- a/pkg/sstore/fileops.go
+++ b/pkg/sstore/fileops.go
@@ -9,41 +9,24 @@ import (
 	"github.com/scripthaus-dev/sh2-server/pkg/scbase"
 )
 
-const PosAppend = -1
-
-// when calling with PosAppend, this is not multithread safe (since file could be modified).
-// we need to know the real position of the write to send a proper pty update to the frontends
-// in practice this is fine since we only use PosAppend in non-detached mode where
-//   we are reading/writing a stream in order with a single goroutine
 func AppendToCmdPtyBlob(ctx context.Context, sessionId string, cmdId string, data []byte, pos int64) error {
+	if pos < 0 {
+		return fmt.Errorf("invalid seek pos '%d' in AppendToCmdPtyBlob", pos)
+	}
 	ptyOutFileName, err := scbase.PtyOutFile(sessionId, cmdId)
 	if err != nil {
 		return err
 	}
-	var fd *os.File
-	var realPos int64
-	if pos == PosAppend {
-		fd, err = os.OpenFile(ptyOutFileName, os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0600)
-		if err != nil {
-			return err
-		}
-		finfo, err := fd.Stat()
-		if err != nil {
-			return err
-		}
-		realPos = finfo.Size()
-	} else {
-		fd, err = os.OpenFile(ptyOutFileName, os.O_WRONLY|os.O_CREATE, 0600)
-		if err != nil {
-			return err
-		}
-		realPos, err = fd.Seek(pos, 0)
-		if err != nil {
-			return err
-		}
-		if realPos != pos {
-			return fmt.Errorf("could not seek to pos:%d (realpos=%d)", pos, realPos)
-		}
+	fd, err := os.OpenFile(ptyOutFileName, os.O_WRONLY|os.O_CREATE, 0600)
+	if err != nil {
+		return err
+	}
+	realPos, err := fd.Seek(pos, 0)
+	if err != nil {
+		return err
+	}
+	if realPos != pos {
+		return fmt.Errorf("could not seek to pos:%d (realpos=%d)", pos, realPos)
 	}
 	defer fd.Close()
 	if len(data) == 0 {