code: func setFlagFromEnv(name, envar string, fs *pflag.FlagSet) {
  	if fs.Changed(name) {
  		return
  	}
  	if v, ok := os.LookupEnv(envar); ok {
  		fs.Set(name, v)
  	}
  }
docs: // setFlagFromEnv looks up and sets a flag if the corresponding environment variable changed.
  // if the flag with the corresponding name was set during fs.Parse(), then the environment
  // variable is ignored.