docker/cli · error

must specify at least one container source

Error message

must specify at least one container source

What it means

The default branch of the direction switch in runCopy: neither the source nor the destination argument was recognized as a CONTAINER:PATH reference, so there is no container endpoint at all. docker cp only copies between a container and the local filesystem, so at least one side must name a container.

Source

Thrown at cli/command/container/cp.go:245

	var direction copyDirection
	if srcContainer != "" {
		direction |= fromContainer
		copyConfig.container = srcContainer
	}
	if destContainer != "" {
		direction |= toContainer
		copyConfig.container = destContainer
	}

	switch direction {
	case fromContainer:
		return copyFromContainer(ctx, dockerCli, copyConfig)
	case toContainer:
		return copyToContainer(ctx, dockerCli, copyConfig)
	case acrossContainers:
		return errors.New("copying between containers is not supported")
	default:
		return errors.New("must specify at least one container source")
	}
}

func resolveLocalPath(localPath string) (absPath string, _ error) {
	absPath, err := filepath.Abs(localPath)
	if err != nil {
		return "", err
	}
	return archive.PreserveTrailingDotOrSeparator(absPath, localPath), nil
}

func copyFromContainer(ctx context.Context, dockerCLI command.Cli, copyConfig cpConfig) (err error) {
	dstPath := copyConfig.destPath
	srcPath := copyConfig.sourcePath

	if dstPath != "-" {
		// Get an absolute destination path.
		dstPath, err = resolveLocalPath(dstPath)

View on GitHub (pinned to e9452d6e78)

Solutions

1. Prefix the container-side argument with the container name or ID, e.g. `docker cp mycontainer:/var/log/app.log ./`.
2. Verify shell variables holding the container name are non-empty before invoking the command.
3. Use plain `cp` if both paths are genuinely on the host.

Example fix

# before
docker cp /var/log/app.log ./app.log
# after
docker cp mycontainer:/var/log/app.log ./app.log

When it happens

Trigger: `docker cp ./a ./b` where both arguments are plain local paths (no `container:` prefix on either side), leaving both fromContainer and toContainer direction bits unset.

Common situations: Forgetting the `container:` prefix on the container-side path; scripts where the container name variable is empty so the argument degrades to a bare path (e.g. `docker cp "$CTR:/logs" .` with $CTR unset yields `:/logs` edge cases); confusing docker cp with plain cp.

Related errors

• destination can not be empty
• copying between containers is not supported
• content size not available for stdin
• you cannot start and attach multiple containers at once
• got a device

AI-assisted analysis of docker/cli@e9452d6e78 (2026-08-01). Data as JSON: /data/errors/1d70827a5ac5492b.json. Report an issue: GitHub.