Output format is better to be grep like

cmd/csvlint/main.go

@@ -18,6 +18,7 @@ func printHelpAndExit(code int) {
 func main() {
 	delimiter := flag.String("delimiter", ",", "field delimiter in the file, for instance '\\t' or '|'")
 	lazyquotes := flag.Bool("lazyquotes", false, "try to parse improperly escaped quotes")
+	verbose := flag.Bool("verbose", false, "verbose output")
 	help := flag.Bool("help", false, "print help and exit")
 	flag.Parse()
 
@@ -31,38 +32,49 @@ func main() {
 
 	convertedDelimiter, err := strconv.Unquote(`'` + *delimiter + `'`)
 	if err != nil {
-		fmt.Printf("error unquoting delimiter '%s', note that only one-character delimiters are supported\n\n", *delimiter)
+		fmt.Fprintf(os.Stderr, "error unquoting delimiter '%s', note that only one-character delimiters are supported\n", *delimiter)
 		printHelpAndExit(1)
 	}
 	// don't need to check size since Unquote returns one-character string
 	comma, _ := utf8.DecodeRuneInString(convertedDelimiter)
 
-	if len(flag.Args()) != 1 {
-		fmt.Print("csvlint accepts a single filepath as an argument\n\n")
+	if flag.NArg() > 1 {
+		fmt.Fprint(os.Stderr, "csvlint accepts stdin or a single filepath as an argument\n")
 		printHelpAndExit(1)
 	}
 
-	f, err := os.Open(flag.Args()[0])
-	if err != nil {
-		if os.IsNotExist(err) {
-			fmt.Printf("file '%s' does not exist\n", flag.Args()[0])
-			os.Exit(1)
-		} else {
-			panic(err)
+	var f *os.File
+	var fname string
+
+	if flag.NArg() == 0 {
+		fname = "stdin"
+		f = os.Stdin
+	} else {
+		fname = flag.Arg(0)
+		f, err = os.Open(fname)
+		if err != nil {
+			if os.IsNotExist(err) {
+				fmt.Fprintf(os.Stderr, "file '%s' does not exist\n", fname)
+				os.Exit(1)
+			} else {
+				panic(err)
+			}
 		}
+		defer f.Close()
 	}
-	defer f.Close()
 
 	invalids, halted, err := csvlint.Validate(f, comma, *lazyquotes)
 	if err != nil {
 		panic(err)
 	}
 	if len(invalids) == 0 {
-		fmt.Println("file is valid")
+		if *verbose {
+			fmt.Println("file is valid")
+		}
 		os.Exit(0)
 	}
 	for _, invalid := range invalids {
-		fmt.Println(invalid.Error())
+		fmt.Fprintf(os.Stderr, "%s:%d:%s\n", fname, invalid.Num, invalid.Error())
 	}
 	if halted {
 		fmt.Println("\nunable to parse any further")