kubernetes/vendor/go.etcd.io/etcd/pkg/v3/flags/flag.go

// SetFlagsFromEnv parses all registered flags in the given flagset,
// and if they are not already set it attempts to set their values from
// environment variables. Environment variables take the name of the flag but
// are UPPERCASE, have the given prefix  and any dashes are replaced by
// underscores - for example: some-flag => ETCD_SOME_FLAG
func SetFlagsFromEnv(lg *zap.Logger, prefix string, fs *flag.FlagSet) error { … }

// SetPflagsFromEnv is similar to SetFlagsFromEnv. However, the accepted flagset type is pflag.FlagSet
// and it does not do any logging.
func SetPflagsFromEnv(lg *zap.Logger, prefix string, fs *pflag.FlagSet) error { … }

// FlagToEnv converts flag string to upper-case environment variable key string.
func FlagToEnv(prefix, name string) string { … }

func verifyEnv(lg *zap.Logger, prefix string, usedEnvKey, alreadySet map[string]bool) { … }

type flagSetter …

func setFlagFromEnv(lg *zap.Logger, fs flagSetter, prefix, fname string, usedEnvKey, alreadySet map[string]bool, log bool) error { … }

func IsSet(fs *flag.FlagSet, name string) bool { … }