muesli/duf · error

error formatting the json output

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.

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
}

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

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

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

• error parsing avail-threshold: invalid option
• error parsing usage-threshold: invalid option
• prefix ' ' not allowed, valid prefixes are K, M, G, T, P, E
• reading mountinfo
• ' ' is not valid, must have integer with optional SI prefix

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

Appendix: 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)

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