Fix #522: add -volume-adjust-pct CLI arg

backend/cmdlineoptions.go

@@ -3,12 +3,14 @@ package backend
 import (
 	"flag"
 	"strconv"
+	"strings"
 )
 
 var (
-	VolumeCLIArg int     = -1
-	SeekToCLIArg float64 = -1
-	SeekByCLIArg float64 = 0
+	VolumeCLIArg    int     = -1
+	SeekToCLIArg    float64 = -1
+	SeekByCLIArg    float64 = 0
+	VolumePctCLIArg float64 = 0
 
 	FlagPlay      = flag.Bool("play", false, "unpause or begin playback")
 	FlagPause     = flag.Bool("pause", false, "pause playback")
@@ -36,6 +38,14 @@ func init() {
 		SeekByCLIArg = v
 		return err
 	})
+	flag.Func("volume-adjust-pct", "adjusts volume up or down by the given percentage (positive or negative)", func(s string) error {
+		if strings.HasSuffix(s, "%") {
+			s = s[:len(s)-1]
+		}
+		v, err := strconv.ParseFloat(s, 64)
+		VolumePctCLIArg = v
+		return err
+	})
 }
 
 func HaveCommandLineOptions() bool {