feat: improve help message (#32)

* feat: improve help message

This PR will improve the help message for the command. It also moves the
flag definitions to a separate file, since the flag functionality is
likely to increase in complexity.

* fix slight formatting issue in help

cmd/yamlfmt/flags.go

@@ -0,0 +1,86 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"path"
+
+	"github.com/google/yamlfmt/command"
+)
+
+var (
+	flagLint *bool = flag.Bool("lint", false, `Check if there are any differences between
+source yaml and formatted yaml.`)
+	flagDry *bool = flag.Bool("dry", false, `Perform a dry run; show the output of a formatting
+operation without performing it.`)
+	flagConf *string = flag.String("conf", "", "Read yamlfmt config from this path")
+)
+
+func getOperation() command.Operation {
+	if len(flag.Args()) == 1 && isStdin(flag.Args()[0]) {
+		return command.OperationStdin
+	}
+	if *flagLint {
+		return command.OperationLint
+	}
+	if *flagDry {
+		return command.OperationDry
+	}
+	return command.OperationFormat
+}
+
+func isStdin(arg string) bool {
+	return arg == "-" || arg == "/dev/stdin"
+}
+
+func getConfigPath() (string, error) {
+	configPath := *flagConf
+	if configPath == "" {
+		configPath = defaultConfigName
+	}
+
+	if path.IsAbs(configPath) {
+		return configPath, nil
+	}
+
+	wd, err := os.Getwd()
+	if err != nil {
+		return "", err
+	}
+	return path.Join(wd, configPath), nil
+}
+
+func configureHelp() {
+	flag.Usage = printHelpMessage
+}
+
+func printHelpMessage() {
+	fmt.Println(`yamlfmt is a simple command line tool for formatting yaml files.
+
+Arguments:
+
+  Glob paths to yaml files
+        Send any number of paths to yaml files specified in doublestar glob format (see: https://github.com/bmatcuk/doublestar). 
+        Any flags must be specified before the paths.
+
+  - or /dev/stdin
+        Passing in a single - or /dev/stdin will read the yaml from stdin and output the formatted result to stdout
+	
+Flags:`)
+	flag.PrintDefaults()
+}

cmd/yamlfmt/main.go

@@ -19,22 +19,13 @@ import (
 	"flag"
 	"log"
 	"os"
-	"path"
 
 	"github.com/google/yamlfmt"
 	"github.com/google/yamlfmt/command"
 	"github.com/google/yamlfmt/formatters/basic"
 	"gopkg.in/yaml.v3"
 )
 
-var (
-	flagLint *bool = flag.Bool("lint", false, `Check if there are any differences between
-source yaml and formatted yaml.`)
-	flagDry *bool = flag.Bool("dry", false, `Perform a dry run; show the output of a formatting
-operation without performing it.`)
-	flagConf *string = flag.String("conf", "", "Read yamlfmt config from this path")
-)
-
 const defaultConfigName = ".yamlfmt"
 
 func main() {
@@ -44,6 +35,7 @@ func main() {
 }
 
 func run() error {
+	configureHelp()
 	flag.Parse()
 
 	operation := getOperation()
@@ -69,23 +61,6 @@ func run() error {
 	return command.RunCommand(operation, getFullRegistry(), configData)
 }
 
-func getConfigPath() (string, error) {
-	configPath := *flagConf
-	if configPath == "" {
-		configPath = defaultConfigName
-	}
-
-	if path.IsAbs(configPath) {
-		return configPath, nil
-	}
-
-	wd, err := os.Getwd()
-	if err != nil {
-		return "", err
-	}
-	return path.Join(wd, configPath), nil
-}
-
 func readConfig(path string) (map[string]interface{}, error) {
 	if _, err := os.Stat(path); err != nil {
 		return nil, err
@@ -102,23 +77,6 @@ func readConfig(path string) (map[string]interface{}, error) {
 	return configData, nil
 }
 
-func getOperation() command.Operation {
-	if len(flag.Args()) == 1 && isStdin(flag.Args()[0]) {
-		return command.OperationStdin
-	}
-	if *flagLint {
-		return command.OperationLint
-	}
-	if *flagDry {
-		return command.OperationDry
-	}
-	return command.OperationFormat
-}
-
-func isStdin(arg string) bool {
-	return arg == "-" || arg == "/dev/stdin"
-}
-
 func getFullRegistry() *yamlfmt.Registry {
 	return yamlfmt.NewFormatterRegistry(&basic.BasicFormatterFactory{})
 }