func (*User) lookUp(line string, f field, value interface{}) interface{} {
	_field := f.(userField)
	allField := strings.Split(line, ":")
	intField := make(map[int]int)

	// Check integers
	var err error
	if intField[2], err = strconv.Atoi(allField[2]); err != nil {
		panic(atoiError{_USER_FILE, line, "UID"})
	}
	if intField[3], err = strconv.Atoi(allField[3]); err != nil {
		panic(atoiError{_USER_FILE, line, "GID"})
	}

	// Check fields
	var isField bool
	if U_NAME&_field != 0 && allField[0] == value.(string) {
		isField = true
	} else if U_PASSWD&_field != 0 && allField[1] == value.(string) {
		isField = true
	} else if U_UID&_field != 0 && intField[2] == value.(int) {
		isField = true
	} else if U_GID&_field != 0 && intField[3] == value.(int) {
		isField = true
	} else if U_GECOS&_field != 0 && allField[4] == value.(string) {
		isField = true
	} else if U_DIR&_field != 0 && allField[5] == value.(string) {
		isField = true
	} else if U_SHELL&_field != 0 && allField[6] == value.(string) {
		isField = true
	} else if U_ALL&_field != 0 {
		isField = true
	}

	if isField {
		return &User{
			Name:     allField[0],
			password: allField[1],
			UID:      intField[2],
			GID:      intField[3],
			Gecos:    allField[4],
			Dir:      allField[5],
			Shell:    allField[6],
		}
	}
	return nil
}
func (*User) lookUp(line string, f field, value interface{}) interface{} {
	_field := f.(userField)
	allField := strings.Split(line, ":")
	intField := make(map[int]int)

	// Check integers
	var err error
	if intField[2], err = strconv.Atoi(allField[2]); err != nil {
		panic(atoiError{_USER_FILE, line, "UID"})
	}
	if intField[3], err = strconv.Atoi(allField[3]); err != nil {
		panic(atoiError{_USER_FILE, line, "GID"})
	}

	// Check fields
	var isField bool
	if U_NAME&_field != 0 && allField[0] == value.(string) {
		isField = true
	} else if U_PASSWD&_field != 0 && allField[1] == value.(string) {
		isField = true
	} else if U_UID&_field != 0 && intField[2] == value.(int) {
		isField = true
	} else if U_GID&_field != 0 && intField[3] == value.(int) {
		isField = true
	} else if U_GECOS&_field != 0 && allField[4] == value.(string) {
		isField = true
	} else if U_DIR&_field != 0 && allField[5] == value.(string) {
		isField = true
	} else if U_SHELL&_field != 0 && allField[6] == value.(string) {
		isField = true
	} else if U_ALL&_field != 0 {
		isField = true
	}

	if isField {
		return &User{
			Name:     allField[0],
			password: allField[1],
			UID:      intField[2],
			GID:      intField[3],
			Gecos:    allField[4],
			Dir:      allField[5],
			Shell:    allField[6],
		}
	}
	return nil
}
// == Lookup
//
// lookUp parses the user line searching a value into the field.
// Returns nil if is not found.