muesli/duf · error

error formatting the json output: %s

Error message

error formatting the json output: %s

What it means

renderJSON in main.go marshals the collected []Mount slice with json.MarshalIndent before printing. If marshaling fails (the value cannot be represented as JSON), it wraps the error as "error formatting the json output: %s". In this program Mount contains only basic types, so this error is extremely rare and usually indicates an internal invariant violation.

Source

Thrown at main.go:62

	width    = flag.Uint("width", 0, "max output width")
	themeOpt = flag.String("theme", defaultThemeName(), "color themes: dark, light, ansi")
	styleOpt = flag.String("style", defaultStyleName(), "style: unicode, ascii")

	availThreshold = flag.String("avail-threshold", "10G,1G", "specifies the coloring threshold (yellow, red) of the avail column, must be integer with optional SI prefixes")
	usageThreshold = flag.String("usage-threshold", "0.5,0.9", "specifies the coloring threshold (yellow, red) of the usage bars as a floating point number from 0 to 1")

	_          = flag.BoolP("human-readable", "h", false, "ignored, just for df compatibility")
	inodes     = flag.Bool("inodes", false, "list inode information instead of block usage")
	jsonOutput = flag.Bool("json", false, "output all devices in JSON format")
	warns      = flag.Bool("warnings", false, "output all warnings to STDERR")
	version    = flag.Bool("version", false, "display version")
)

// renderJSON encodes the JSON output and prints it.
func renderJSON(m []Mount) error {
	output, err := json.MarshalIndent(m, "", " ")
	if err != nil {
		return fmt.Errorf("error formatting the json output: %s", err)
	}

	fmt.Println(string(output))
	return nil
}

// parseColumns parses the supplied output flag into a slice of column indices.
func parseColumns(cols string) ([]int, error) {
	var i []int

	s := strings.Split(cols, ",")
	for _, v := range s {
		v = strings.TrimSpace(v)
		if len(v) == 0 {
			continue
		}

		col, err := stringToColumn(v)

View on GitHub (pinned to 4636deb4a7)

Solutions

  1. Check recently added fields on the Mount struct for types not supported by encoding/json and remove or convert them.
  2. Inspect the wrapped %s message to identify which value failed to marshal.
  3. Add a custom MarshalJSON implementation for any field with special encoding needs, ensuring it never returns an error for valid states.

Example fix

// before
type Mount struct {
	Device chan string // unsupported
}
// after
type Mount struct {
	Device string
}
Defensive patterns

Strategy: try-catch

Validate before calling

// ensure Mount fields are JSON-encodable before calling
func validateMounts(m []Mount) error {
	_, err := json.Marshal(m)
	return err
}

Try / catch

if err := renderJSON(mounts); err != nil {
	var encErr *json.UnsupportedTypeError
	if errors.As(err, &encErr) { /* fix struct field */ }
	log.Fatal(err)
}

Prevention

When it happens

Trigger: json.MarshalIndent(m, "", " ") returns an error when marshaling the []Mount slice fails, e.g. if a Mount field ever holds a channel, func, or cyclic data structure unsupported by encoding/json.

Common situations: A developer adds a field of an unencodable type (chan, func, sync.Mutex) to the Mount struct, or a field with a malformed MarshalJSON method that returns an error.

Understand the failure class

Background: json.Marshal / "failed to marshal" errors in Go: why "unsupported type" happens and how to fix it — this error's family across 22 libraries.

Related errors

AI-assisted analysis of muesli/duf@4636deb4a7 (2026-09-06). Data as JSON: /api/errors/906910da3309ea0d. Report an issue: GitHub.