golang: how to support cli params, env vars and config file with viper but no cobra?

I am trying to write an app which only has one command, hence, I was thinking I might skip Cobra.

The app should be able to support all types of config:

• command line params
• env vars
• config file

I am using viper but I can't get it to read my cli params.

  v := viper.New()
  viper.SetConfigName("config") 
  viper.AddConfigPath(".")      
  v.SetDefault(pathKey, defaultPath)

  fs := pflag.NewFlagSet("app", pflag.ExitOnError)
  fs.String(pathKey, defaultPath, "Default path")
  fs.StringSlice(wordsKey, []string{""}, "Words")
  fs.String(loglevelKey, "info", "Log level")

  if err := v.BindPFlags(fs); err != nil {
    fmt.Println(err)
    os.Exit(1)
  }
  if err := fs.Parse(os.Args[1:]); err != nil {
    fmt.Println(err)
    os.Exit(1)
  }

  v.AutomaticEnv()

  if err := v.ReadInConfig(); err != nil {
    fmt.Println("no conf file") //ignore, it can be either cli params, or conf file
  }

  var c conf
  if err := v.Unmarshal(&c); err != nil {
    fmt.Println(err)
    os.Exit(1)
  }

But I never get the cli params into the conf struct. Printing v before the Unmarshal doesn't show any of the cli params I provide.

What am I missing? Do I need to use cobra for this?
Or do I have to assign each flagset flag, e.g. fs.String(pathKey, defaultPath, "Default path"), manually to the config struct?

For posterity, I think I found the issue:

My conf struct didn't have the correspondent key names as the flags do. Setting the json:"logLevel" for example while the field is called DisplayLogLevel is not enough, it must be:

const (
  pathKey = "path"
  wordsKey = "words"
  logLevelKey = "logLevel"
)

type conf struct {
   Path string `json:"path"`
   Words []string `json:"words"`
   LogLevel string `json:"logLevel"`
}

maybe you have to set config type. From https://github.com/spf13/viper:

viper.SetConfigType("yaml") // REQUIRED if the config file does not have the extension in the name