Feature: Add support for multiple env (#41)

Author: dearchap

README.md

@@ -52,7 +52,7 @@ And you can use your favorite flag or cli library!
  - [x] Required
  - [ ] Placeholders (by `name`)
  - [x] Deprecated and hidden options
- - [ ] Multiple ENV names
+ - [x] Multiple ENV names
  - [x] Interface for user types.
  - [x] [Validation](https://godoc.org/github.com/urfave/sflags/validator/govalidator#New) (using [govalidator](https://github.com/asaskevich/govalidator) package)
  - [x] Anonymous nested structure support (anonymous structures flatten by default)

flag.go

@@ -5,7 +5,7 @@ package sflags
 type Flag struct {
 	Name       string // name as it appears on command line
 	Short      string // optional short name
-	EnvName    string
+	EnvNames   []string
 	Usage      string // help message
 	Value      Value  // value as set
 	DefValue   string // default value (as text); for usage message

gen/gcli/gcli.go

@@ -16,7 +16,7 @@ func GenerateTo(src []*sflags.Flag, dst *[]cli.Flag) {
 		}
 		*dst = append(*dst, &cli.GenericFlag{
 			Name:     name,
-			EnvVars:  []string{srcFlag.EnvName},
+			EnvVars:  srcFlag.EnvNames,
 			Aliases:  aliases,
 			Hidden:   srcFlag.Hidden,
 			Usage:    srcFlag.Usage,

gen/gcli/gcliv3.go

@@ -41,7 +41,7 @@ func GenerateToV3(src []*sflags.Flag, dst *[]cli.Flag) {
 		}
 		*dst = append(*dst, &cli.GenericFlag{
 			Name:    name,
-			Sources: cli.EnvVars(srcFlag.EnvName),
+			Sources: cli.EnvVars(srcFlag.EnvNames...),
 			Aliases: aliases,
 			Hidden:  srcFlag.Hidden,
 			Usage:   srcFlag.Usage,

gen/gkingpin/gkingpin.go

@@ -17,8 +17,8 @@ func GenerateTo(src []*sflags.Flag, dst flagger) {
 	for _, srcFlag := range src {
 		flag := dst.Flag(srcFlag.Name, srcFlag.Usage)
 		flag.SetValue(srcFlag.Value)
-		if srcFlag.EnvName != "" {
-			flag.Envar(srcFlag.EnvName)
+		if len(srcFlag.EnvNames) > 0 && srcFlag.EnvNames[0] != "" {
+			flag.Envar(srcFlag.EnvNames[0])
 		}
 		if srcFlag.Hidden {
 			flag.Hidden()

parser.go

@@ -123,37 +123,47 @@ func parseFlagTag(field reflect.StructField, opt opts) *Flag {
 	return &flag
 }
 
-func parseEnv(flagName string, field reflect.StructField, opt opts) string {
-	ignoreEnvPrefix := false
-	envVar := flagToEnv(flagName, opt.flagDivider, opt.envDivider)
+func parseEnv(flagName string, field reflect.StructField, opt opts) []string {
+	var envVars []string
+	flagEnvVar := flagToEnv(flagName, opt.flagDivider, opt.envDivider)
 	if envTags := strings.Split(field.Tag.Get(defaultEnvTag), ","); len(envTags) > 0 {
 		switch envName := envTags[0]; envName {
 		case "-":
-			// if tag is `env:"-"` then won't fill flag from environment
-			envVar = ""
+			return envVars
 		case "":
 			// if tag is `env:""` then env var will be taken from flag name
+			envVars = append(envVars, opt.envPrefix+flagEnvVar)
 		default:
 			// if tag is `env:"NAME"` then env var is envPrefix_flagPrefix_NAME
 			// if tag is `env:"~NAME"` then env var is NAME
-			if strings.HasPrefix(envName, "~") {
-				envVar = envName[1:]
-				ignoreEnvPrefix = true
-			} else {
-				envVar = envName
-				if opt.prefix != "" {
-					envVar = flagToEnv(
-						opt.prefix,
-						opt.flagDivider,
-						opt.envDivider) + envVar
+			for _, envName := range envTags {
+				ignoreEnvPrefix := false
+				var envVar string
+				if strings.HasPrefix(envName, "~") {
+					envVar = envName[1:]
+					ignoreEnvPrefix = true
+				} else {
+					envVar = envName
+					if opt.prefix != "" {
+						envVar = flagToEnv(
+							opt.prefix,
+							opt.flagDivider,
+							opt.envDivider) + envVar
+					}
+				}
+				if envVar != "" {
+					if !ignoreEnvPrefix {
+						envVars = append(envVars, opt.envPrefix+envVar)
+					} else {
+						envVars = append(envVars, envVar)
+					}
 				}
 			}
 		}
+	} else if flagEnvVar != "" {
+		envVars = append(envVars, opt.envPrefix+flagEnvVar)
 	}
-	if envVar != "" && opt.envPrefix != "" && !ignoreEnvPrefix {
-		envVar = opt.envPrefix + envVar
-	}
-	return envVar
+	return envVars
 }
 
 // ParseStruct parses structure and returns list of flags based on this structure.
@@ -247,7 +257,7 @@ fields:
 			continue fields
 		}
 
-		flag.EnvName = parseEnv(flag.Name, field, opt)
+		flag.EnvNames = parseEnv(flag.Name, field, opt)
 		flag.Usage = field.Tag.Get(opt.descTag)
 		prefix := flag.Name + opt.flagDivider
 		if field.Anonymous && opt.flatten {