feat: accept - or /dev/stdin instead of -in (#28)

Most command line utilities handle stdin with the `-` or `/dev/stdin`
argument. This PR changes the stdin operation detection from `-in` to
`-` or `/dev/stdin`. This detection is done manually to remain working
on all platforms.

README.md

@@ -65,7 +65,7 @@ The CLI supports 3 operation modes:
     - Format the matched files and output the diff to `stdout`
 * Lint (`-lint` flag)
     - Format the matched files and output the diff to `stdout`, exits with status 1 if there are any differences
-* Stdin (`-in` flag)
+* Stdin (just `-` or `/dev/stdin` argument)
     - Format the yaml data from `stdin` and output the result to `stdout`
 
 (NOTE: If providing paths as command line arguments, the flags must be specified before any paths)

cmd/yamlfmt/main.go

@@ -32,7 +32,6 @@ var (
 source yaml and formatted yaml.`)
 	dry *bool = flag.Bool("dry", false, `Perform a dry run; show the output of a formatting
 operation without performing it.`)
-	in *bool = flag.Bool("in", false, "Format yaml read from stdin and output to stdout")
 )
 
 const defaultConfigName = ".yamlfmt"
@@ -51,12 +50,14 @@ func run() error {
 		op = command.OperationLint
 	} else if *dry {
 		op = command.OperationDry
-	} else if *in {
-		op = command.OperationStdin
 	} else {
 		op = command.OperationFormat
 	}
 
+	if len(flag.Args()) == 1 && isStdin(flag.Args()[0]) {
+		op = command.OperationStdin
+	}
+
 	configData, err := readDefaultConfigFile()
 	if err != nil {
 		if errors.Is(err, os.ErrNotExist) {
@@ -101,3 +102,7 @@ func readConfig(path string) (map[string]interface{}, error) {
 func getFullRegistry() *yamlfmt.Registry {
 	return yamlfmt.NewFormatterRegistry(&basic.BasicFormatterFactory{})
 }
+
+func isStdin(arg string) bool {
+	return arg == "-" || arg == "/dev/stdin"
+}