docker/cli · error

error reading from STDIN: data is empty

Error message

error reading from STDIN: data is empty

What it means

Thrown by readSecretData when the data source is stdin ('-') and the read returns zero bytes. An empty secret is not permitted, so the create is refused (secret/create.go lines 133-135).

Solutions

1. Ensure the piped stdin contains non-empty secret data.
2. Use a file with actual content: 'docker secret create mysec ./secret.txt'.
3. Verify the upstream producer of the data emits content before piping.

Example fix

# before
printf '' | docker secret create mysec -
# after
printf '%s' "$SECRET_VALUE" | docker secret create mysec -

// Validate non-empty stdin before creating the secret
data, err := io.ReadAll(io.LimitReader(in, 2*maxSecretSize))
if err != nil { return err }
if len(data) == 0 {
    return fmt.Errorf("stdin secret data is empty")
}

When it happens

Trigger: Running 'echo -n "" | docker secret create mysec -' or piping an empty stream/file as the secret content via '-'.

Common situations: Empty env var piped in; upstream command produced no output; redirecting from /dev/null.

Related errors

• when using secret driver secret data must be empty
• secret file is required
• node ID not found in /info
• invalid field ' ' must be a key=value pair
• every ip-range or gateway must have a corresponding subnet

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/ab5e75635cc2035f. Report an issue: GitHub.

Appendix: source

Thrown at cli/command/secret/create.go:134

// as defined by [MaxSecretSize] in SwarmKit.
//
// [MaxSecretSize]: https://pkg.go.dev/github.com/moby/swarmkit/v2@v2.0.0-20250103191802-8c1959736554/api/validation#MaxSecretSize
const maxSecretSize = 500 * 1024 // 500KB

// readSecretData reads the secret from either stdin or the given fileName.
//
// It reads up to twice the maximum size of the secret ([maxSecretSize]),
// just in case swarm's limit changes; this is only a safeguard to prevent
// reading arbitrary files into memory.
func readSecretData(in io.Reader, fileName string) ([]byte, error) {
	switch fileName {
	case "-":
		data, err := io.ReadAll(io.LimitReader(in, 2*maxSecretSize))
		if err != nil {
			return nil, fmt.Errorf("error reading from STDIN: %w", err)
		}
		if len(data) == 0 {
			return nil, errors.New("error reading from STDIN: data is empty")
		}
		return data, nil
	case "":
		return nil, errors.New("secret file is required")
	default:
		// Open file with [FILE_FLAG_SEQUENTIAL_SCAN] on Windows, which
		// prevents Windows from aggressively caching it. We expect this
		// file to be only read once. Given that this is expected to be
		// a small file, this may not be a significant optimization, so
		// we could choose to omit this, and use a regular [os.Open].
		//
		// [FILE_FLAG_SEQUENTIAL_SCAN]: https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#FILE_FLAG_SEQUENTIAL_SCAN
		f, err := sequential.Open(fileName)
		if err != nil {
			return nil, fmt.Errorf("error reading from %s: %w", fileName, err)
		}
		defer f.Close()
		data, err := io.ReadAll(io.LimitReader(f, 2*maxSecretSize))

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