docker/compose · error

output folder %s already exists

Error message

output folder %s already exists

What it means

bridge.CreateTransformer scaffolds a new transformer by materializing a folder from a container image (default DefaultTransformerImage). As a safety guard it refuses to run if the destination path already exists (os.Stat succeeds), so it never overwrites or merges into an existing folder. The message prints the absolute destination path.

Source

Thrown at pkg/bridge/transformers.go:53

	templatesPath = "/templates"
)

type CreateTransformerOptions struct {
	Dest string
	From string
}

func CreateTransformer(ctx context.Context, dockerCli command.Cli, options CreateTransformerOptions) error {
	if options.From == "" {
		options.From = DefaultTransformerImage
	}
	out, err := filepath.Abs(options.Dest)
	if err != nil {
		return err
	}

	if _, err := os.Stat(out); err == nil {
		return fmt.Errorf("output folder %s already exists", out)
	}

	tmpl := filepath.Join(out, "templates")
	err = os.MkdirAll(tmpl, 0o744)
	if err != nil && !os.IsExist(err) {
		return fmt.Errorf("cannot create output folder: %w", err)
	}

	if err := command.ValidateOutputPath(out); err != nil {
		return err
	}

	created, err := dockerCli.Client().ContainerCreate(ctx, client.ContainerCreateOptions{
		Image: options.From,
	})

	defer func() {
		_, _ = dockerCli.Client().ContainerRemove(context.Background(), created.ID, client.ContainerRemoveOptions{

View on GitHub (pinned to ddc4b044b6)

Solutions

1. Choose a fresh destination path that does not exist yet
2. Remove or rename the existing folder if its contents are disposable: rm -rf <dest>
3. Move the existing folder aside (mv dest dest.bak) if you need to compare or preserve it

Example fix

# before
$ create-transformer --dest ./my-transformer
output folder /path/to/my-transformer already exists
# after
$ mv ./my-transformer ./my-transformer.bak
$ create-transformer --dest ./my-transformer

// Before CreateTransformer:
if _, err := os.Stat(options.Dest); err == nil {
	return fmt.Errorf("destination %s already exists, pick a new path or remove it", options.Dest)
}

When it happens

Trigger: Calling CreateTransformer with options.Dest set to an existing directory or file; running the scaffolding command twice with the same destination; a previous partial run that left the folder behind.

Common situations: Re-running a transformer-creation command after an interrupted first attempt; pointing Dest at a checked-in project directory that already exists; CI caching that preserves the destination between runs.

Related errors

• cannot create output folder: %w
• cannot create output folder: %w
• failed to export container: %w
• source can not be empty
• destination can not be empty

AI-assisted analysis of docker/compose@ddc4b044b6 (2026-08-15). Data as JSON: /api/errors/db8882aeba9b8162. Report an issue: GitHub.