support reading from stdin.

helpers.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"bytes"
+	"io"
+	"os"
+
+	"github.com/urfave/cli/v2"
+)
+
+func getStdin() string {
+	stat, _ := os.Stdin.Stat()
+	if (stat.Mode() & os.ModeCharDevice) == 0 {
+		read := bytes.NewBuffer(make([]byte, 0, 1000))
+		_, err := io.Copy(read, os.Stdin)
+		if err == nil {
+			return read.String()
+		}
+	}
+	return ""
+}
+
+func getStdinOrFirstArgument(c *cli.Context) string {
+	target := c.Args().First()
+	if target != "" {
+		return target
+	}
+	return getStdin()
+}