diff --git a/lib/envflag/envflag.go b/lib/envflag/envflag.go
index 1430ca8b4..598cf4dc0 100644
--- a/lib/envflag/envflag.go
+++ b/lib/envflag/envflag.go
@@ -22,9 +22,46 @@ var (
 //
 // This function must be called instead of flag.Parse() before using any flags in the program.
 func Parse() {
-	// Substitute %{ENV_VAR} inside args with the corresponding environment variable values
-	args := os.Args[1:]
-	dstArgs := args[:0]
+	ParseFlagSet(flag.CommandLine, os.Args[1:])
+}
+
+// ParseFlagSet parses the given args into the given fs.
+func ParseFlagSet(fs *flag.FlagSet, args []string) {
+	args = expandArgs(args)
+	if err := fs.Parse(args); err != nil {
+		// Do not use lib/logger here, since it is uninitialized yet.
+		log.Fatalf("cannot parse flags %q: %s", args, err)
+	}
+	if !*enable {
+		return
+	}
+	// Remember explicitly set command-line flags.
+	flagsSet := make(map[string]bool)
+	fs.Visit(func(f *flag.Flag) {
+		flagsSet[f.Name] = true
+	})
+
+	// Obtain the remaining flag values from environment vars.
+	fs.VisitAll(func(f *flag.Flag) {
+		if flagsSet[f.Name] {
+			// The flag is explicitly set via command-line.
+			return
+		}
+		// Get flag value from environment var.
+		fname := getEnvFlagName(f.Name)
+		if v, ok := envtemplate.LookupEnv(fname); ok {
+			if err := fs.Set(f.Name, v); err != nil {
+				// Do not use lib/logger here, since it is uninitialized yet.
+				log.Fatalf("cannot set flag %s to %q, which is read from env var %q: %s", f.Name, v, fname, err)
+			}
+		}
+	})
+}
+
+// expandArgs substitutes %{ENV_VAR} placeholders inside args
+// with the corresponding environment variable values.
+func expandArgs(args []string) []string {
+	dstArgs := make([]string, 0, len(args))
 	for _, arg := range args {
 		s, err := envtemplate.ReplaceString(arg)
 		if err != nil {
@@ -35,35 +72,7 @@ func Parse() {
 			dstArgs = append(dstArgs, s)
 		}
 	}
-	os.Args = os.Args[:1+len(dstArgs)]
-
-	// Parse flags
-	flag.Parse()
-	if !*enable {
-		return
-	}
-
-	// Remember explicitly set command-line flags.
-	flagsSet := make(map[string]bool)
-	flag.Visit(func(f *flag.Flag) {
-		flagsSet[f.Name] = true
-	})
-
-	// Obtain the remaining flag values from environment vars.
-	flag.VisitAll(func(f *flag.Flag) {
-		if flagsSet[f.Name] {
-			// The flag is explicitly set via command-line.
-			return
-		}
-		// Get flag value from environment var.
-		fname := getEnvFlagName(f.Name)
-		if v, ok := envtemplate.LookupEnv(fname); ok {
-			if err := flag.Set(f.Name, v); err != nil {
-				// Do not use lib/logger here, since it is uninitialized yet.
-				log.Fatalf("cannot set flag %s to %q, which is read from environment variable %q: %s", f.Name, v, fname, err)
-			}
-		}
-	})
+	return dstArgs
 }
 
 func getEnvFlagName(s string) string {