docker/cli · info

content size not available for stdin

Error message

content size not available for stdin

What it means

In copyToContainer, when the source is `-` the CLI reads a tar archive from stdin and sets sizeErr to this error instead of a byte count, because a stream's total size cannot be known up front. It is a sentinel used to disable the copy progress display, not a fatal failure of the copy itself — though it surfaces where code paths need the content size.

Source

Thrown at cli/command/container/cp.go:412

		// type of conflict (e.g., non-directory overwriting an existing directory
		// or vice versa) the extraction will fail. If the destination simply did
		// not exist, but the parent directory does, the extraction will still
		// succeed.
		_ = err // Intentionally ignore stat errors (see above)
	}

	var (
		content         io.ReadCloser
		resolvedDstPath string
		copiedSize      int64
		contentSize     int64
		sizeErr         error
	)

	if srcPath == "-" {
		content = os.Stdin
		resolvedDstPath = dstInfo.Path
		sizeErr = errors.New("content size not available for stdin")
		if !dstInfo.IsDir {
			return fmt.Errorf(`destination "%s:%s" must be a directory`, copyConfig.container, dstPath)
		}
	} else {
		// Prepare source copy info.
		srcInfo, err := archive.CopyInfoSourcePath(srcPath, copyConfig.followLink)
		if err != nil {
			return err
		}

		contentSize, sizeErr = localContentSize(srcInfo.Path)

		srcArchive, err := archive.TarResource(srcInfo)
		if err != nil {
			return err
		}
		defer srcArchive.Close()

View on GitHub (pinned to e9452d6e78)

Solutions

1. No fix needed for the copy itself — it proceeds; the size is simply unavailable for stdin, so progress reporting is skipped.
2. If you need size reporting, write the tar to a file first and copy the file/directory by path instead of streaming via `-`.
3. Ensure the stdin source is a valid tar archive and the destination is an existing directory (a non-directory destination fails separately).

When it happens

Trigger: `docker cp - mycontainer:/dest/dir` (tar archive piped on stdin). localContentSize is never called; sizeErr is pre-set at cp.go:412 since os.Stdin has no determinable length.

Common situations: Piping tar streams into containers (`tar -cf - . | docker cp - ctr:/app`) and wondering why no progress/size is shown; tooling that parses cp output expecting a size.

Related errors

• error reading from STDIN: data is empty
• destination can not be empty
• copying between containers is not supported
• must specify at least one container source
• invalid argument: can't use stdin for both build context and

AI-assisted analysis of docker/cli@e9452d6e78 (2026-08-01). Data as JSON: /data/errors/08bb95da4b54036a.json. Report an issue: GitHub.