docker/cli · error

bad format

Error message

bad format: %s

What it means

Returned by ValidateWeightDevice (the validator behind --blkio-weight-device) when the input has no ':' separator or the device-path part is empty. Expected format: '<device-path>:<weight>'.

Solutions

1. Provide the full '<device-path>:<weight>' pair, e.g. '--blkio-weight-device=/dev/sda:200'.
2. Keep the device on the left, weight on the right.
3. Use multiple flags for multiple devices rather than comma-separating in one value.

Example fix

# before
docker run --blkio-weight-device=200 ubuntu
# after
docker run --blkio-weight-device=/dev/sda:200 ubuntu

// Require the '<device>:<weight>' shape before ValidateWeightDevice.
import "strings"

func hasWeightShape(s string) bool {
    k, v, ok := strings.Cut(s, ":")
    return ok && k != "" && v != ""
}

// if !hasWeightShape(val) { return fmt.Errorf("expected <device-path>:<weight>, got %q", val) }

if _, err := opts.ValidateWeightDevice(val); err != nil {
    return err // bad format: <val>
}

When it happens

Trigger: 'docker run --blkio-weight-device=200' (no device), '...=:200' (empty device key).

Common situations: User passes only a weight assuming a global default, or forgets the colon/device pair.

Related errors

• bad format for device path
• invalid weight for device
• bad format
• bad format for device path
• invalid rate for device

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/1518ccdf87dbdef0. Report an issue: GitHub.

Appendix: source

Thrown at opts/weightdevice.go:18

package opts

import (
	"fmt"
	"strconv"
	"strings"

	"github.com/moby/moby/api/types/blkiodev"
)

// ValidatorWeightFctType defines a validator function that returns a validated struct and/or an error.
type ValidatorWeightFctType func(val string) (*blkiodev.WeightDevice, error)

// ValidateWeightDevice validates that the specified string has a valid device-weight format.
func ValidateWeightDevice(val string) (*blkiodev.WeightDevice, error) {
	k, v, ok := strings.Cut(val, ":")
	if !ok || k == "" {
		return nil, fmt.Errorf("bad format: %s", val)
	}
	// TODO(thaJeztah): should we really validate this on the client?
	if !strings.HasPrefix(k, "/dev/") {
		return nil, fmt.Errorf("bad format for device path: %s", val)
	}
	weight, err := strconv.ParseUint(v, 10, 16)
	if err != nil {
		return nil, fmt.Errorf("invalid weight for device: %s", val)
	}
	if weight > 0 && (weight < 10 || weight > 1000) {
		return nil, fmt.Errorf("invalid weight for device: %s", val)
	}

	return &blkiodev.WeightDevice{
		Path:   k,
		Weight: uint16(weight),
	}, nil
}

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