cmd: Allow caddy fmt to read from stdin (#3680)

* Allow 'caddy fmt' to read from stdin

* fmt: use '-' as the file name for reading from stdin

* Minor adjustments

Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>

cmd/commandfuncs.go

@@ -533,7 +533,17 @@ func cmdFmt(fl Flags) (int, error) {
 	if formatCmdConfigFile == "" {
 		formatCmdConfigFile = "Caddyfile"
 	}
-	overwrite := fl.Bool("overwrite")
+
+	// as a special case, read from stdin if the file name is "-"
+	if formatCmdConfigFile == "-" {
+		input, err := ioutil.ReadAll(os.Stdin)
+		if err != nil {
+			return caddy.ExitCodeFailedStartup,
+				fmt.Errorf("reading stdin: %v", err)
+		}
+		fmt.Print(string(caddyfile.Format(input)))
+		return caddy.ExitCodeSuccess, nil
+	}
 
 	input, err := ioutil.ReadFile(formatCmdConfigFile)
 	if err != nil {
@@ -543,9 +553,8 @@ func cmdFmt(fl Flags) (int, error) {
 
 	output := caddyfile.Format(input)
 
-	if overwrite {
+	if fl.Bool("overwrite") {
-		err = ioutil.WriteFile(formatCmdConfigFile, output, 0644)
+		if err := ioutil.WriteFile(formatCmdConfigFile, output, 0600); err != nil {
-		if err != nil {
 			return caddy.ExitCodeFailedStartup, nil
 		}
 	} else {

cmd/commands.go

@@ -263,8 +263,12 @@ provisioning stages.`,
 Formats the Caddyfile by adding proper indentation and spaces to improve
 human readability. It prints the result to stdout.
 
-If --write is specified, the output will be written to the config file
+If --overwrite is specified, the output will be written to the config file
-directly instead of printing it.`,
+directly instead of printing it.
+
+If you wish you use stdin instead of a regular file, use - as the path.
+When reading from stdin, the --overwrite flag has no effect: the result
+is always printed to stdout.`,
 		Flags: func() *flag.FlagSet {
 			fs := flag.NewFlagSet("format", flag.ExitOnError)
 			fs.Bool("overwrite", false, "Overwrite the input file with the results")