# gh-stack config.Config Struct and Test Overrides: Complete Configuration Guide > Explore gh-stack config.Config struct and test overrides to centralize runtime settings including I/O streams and service dependencies for deterministic testing. Learn configuration options now. - Repository: [GitHub/gh-stack](https://github.com/github/gh-stack) - Tags: api-reference - Published: 2026-08-03 --- **The `config.Config` struct centralizes all runtime settings for the gh-stack extension, bundling I/O streams, color formatting helpers, interactive prompt callbacks, and service overrides that enable deterministic testing through dependency injection.** The `github/gh-stack` repository implements a centralized configuration pattern to manage CLI behavior and facilitate unit testing. Understanding the **gh-stack config.Config struct and test overrides** is essential for extending functionality or debugging interaction flows. This configuration object aggregates everything from stdout/stderr writers to mockable GitHub client interfaces into a single injectable dependency consumed by all subcommands. ## Config Struct Fields Overview The `Config` struct defined in [`internal/config/config.go`](https://github.com/github/gh-stack/blob/main/internal/config/config.go) exposes several field categories that control both runtime behavior and testability. ### I/O Streams and Color Formatting The struct captures output destinations and text styling functions to ensure consistent terminal rendering across platforms: - **Stdout** and **Stderr** (`io.Writer`): Destination writers for normal and error output. In production, these map to `os.Stdout` and `os.Stderr`, while tests replace them with `bytes.Buffer` instances via `config.NewTestConfig`. - **ColorFn**, **BoldFn**, **ErrorFn**, **InfoFn**, **SuccessFn**, **WarningFn** (`func(string) string`): Helper functions that apply ANSI escape codes for terminal styling. Production implementations wrap text in color codes, whereas tests substitute identity functions (`func(s string) string { return s }`) to produce deterministic output. ### Interactive Prompt Hooks To enable non-interactive testing, the struct includes function fields that replace user input mechanisms: - **Interactive** (`bool`): Global flag indicating whether the environment supports user interaction. Set to `false` in CI environments and most test scenarios. - **SelectFn** (`func(prompt, defaultValue string, options []string) (int, error)`): Replaces the "select one of N options" UI. Tests stub this to return predetermined indices. - **ConfirmFn** (`func(prompt string, defaultValue bool) (bool, error)`): Hook for yes/no prompts. Test implementations return fixed boolean values. - **InputFn** (`func(prompt, defaultValue string) (string, error)`): Handles free-form text input. Tests provide deterministic string responses. ### Service and Repository Overrides The struct supports dependency injection for external services: - **RepoOverride** (`string`): Forces commands to target a specific repository instead of detecting the current working directory's remote. Used extensively in tests with temporary fixture repositories. - **GitHubClientOverride** (`github.ClientOps`): Swaps the live GitHub API client with a mock implementation (`github.MockClient`). The `config.NewTestConfig` helper automatically initializes this field for test configurations. - **GitOverride** (`git.Ops`): Though the field exists in `Config` primarily for completeness and is not always stored directly, it allows replacement of git operations with `git.MockOps` via the global `git.SetOps()` function. ### Internal Testing Flags - **TestMode** (`bool`): Signals execution under the test harness, enabling additional sanity checks such as strict mock restoration verification. ## Test Configuration Patterns The [`internal/config/testing.go`](https://github.com/github/gh-stack/blob/main/internal/config/testing.go) file provides the `NewTestConfig` constructor that standardizes test setup across the suite. ### Creating Test Configurations The helper function returns a fully initialized config along with capture buffers: ```go func NewTestConfig() (*Config, *bytes.Buffer, *bytes.Buffer) { stdout := new(bytes.Buffer) stderr := new(bytes.Buffer) cfg := &Config{ Stdout: stdout, Stderr: stderr, ColorFn: func(s string) string { return s }, BoldFn: func(s string) string { return s }, Interactive: false, } return cfg, stdout, stderr } ``` This implementation disables color formatting and interactivity by default, ensuring tests produce stable, parseable output regardless of terminal capabilities. ### Customizing Test Behavior Individual tests override specific hooks to simulate user interactions: ```go cfg, stdout, stderr := config.NewTestConfig() // Force selection of the second option (index 1) cfg.SelectFn = func(_, _ string, _ []string) (int, error) { return 1, nil } // Always confirm prompts cfg.ConfirmFn = func(_ string, _ bool) (bool, error) { return true, nil } // Provide deterministic text input cfg.InputFn = func(_, _ string) (string, error) { return "feature-branch", nil } // Inject mock GitHub client cfg.GitHubClientOverride = &github.MockClient{ ListStacksFn: func(_ context.Context, _, _ string) ([]github.Stack, error) { return []github.Stack{{ID: "S1", Number: 42}}, nil }, } ``` ## Implementation in Commands Command implementations in the `cmd/` directory receive `*config.Config` as their primary dependency. According to [`cmd/utils.go`](https://github.com/github/gh-stack/blob/main/cmd/utils.go) (lines 1420-1430 and 1760-1770), commands check whether prompt hooks are `nil` before invoking them, falling back to real interactive implementations when running in production. For example, a command consuming the configuration: ```go func runAdd(cfg *config.Config, args []string) error { // Use configured stdout fmt.Fprintln(cfg.Stdout, "Processing...") // Check if running in interactive mode if cfg.Interactive && cfg.SelectFn != nil { idx, err := cfg.SelectFn("Choose stack", "", []string{"A", "B"}) if err != nil { return err } // process selection... } // Use potentially mocked GitHub client client := cfg.GitHubClientOverride if client == nil { // initialize real client } } ``` ## Key Source File Locations Understanding the complete configuration system requires examining these specific files in the `github/gh-stack` repository: - **[`internal/config/config.go`](https://github.com/github/gh-stack/blob/main/internal/config/config.go)**: Defines the `Config` struct and its field types. - **[`internal/config/testing.go`](https://github.com/github/gh-stack/blob/main/internal/config/testing.go)**: Implements `NewTestConfig` and test-specific initialization logic. - **[`cmd/utils.go`](https://github.com/github/gh-stack/blob/main/cmd/utils.go)**: Contains UI helper functions that respect `SelectFn`, `ConfirmFn`, and `InputFn` overrides (see lines 1420-1430 and 1760-1770). - **[`internal/github/mock_client.go`](https://github.com/github/gh-stack/blob/main/internal/github/mock_client.go)**: Provides the `MockClient` type used with `GitHubClientOverride`. - **[`internal/git/mock_ops.go`](https://github.com/github/gh-stack/blob/main/internal/git/mock_ops.go)**: Defines `MockOps` for git operation substitution. ## Summary - **`config.Config`** aggregates I/O writers, color helpers, prompt callbacks, and service overrides into a single injectable configuration object defined in [`internal/config/config.go`](https://github.com/github/gh-stack/blob/main/internal/config/config.go). - **Test overrides** include `NewTestConfig` for standardized setup, buffer-based stdout/stderr capture, identity color functions, and mockable client interfaces. - **Interactive hooks** (`SelectFn`, `ConfirmFn`, `InputFn`) allow deterministic simulation of user input without actual terminal interaction. - **Service overrides** (`GitHubClientOverride`, `RepoOverride`) enable testing against mock GitHub API responses and temporary fixture repositories. - **Source locations** span [`internal/config/config.go`](https://github.com/github/gh-stack/blob/main/internal/config/config.go), [`internal/config/testing.go`](https://github.com/github/gh-stack/blob/main/internal/config/testing.go), and [`cmd/utils.go`](https://github.com/github/gh-stack/blob/main/cmd/utils.go) for the core configuration logic. ## Frequently Asked Questions ### How do I disable color output when testing gh-stack commands? Set the color helper fields to identity functions that return strings unchanged. The `config.NewTestConfig` helper does this automatically by assigning `func(s string) string { return s }` to `ColorFn`, `BoldFn`, and other styling fields in [`internal/config/testing.go`](https://github.com/github/gh-stack/blob/main/internal/config/testing.go), producing plain text suitable for assertions. ### What is the purpose of GitHubClientOverride in the Config struct? `GitHubClientOverride` allows tests to substitute the live GitHub API client with a `github.MockClient` implementation. This field accepts any type satisfying the `github.ClientOps` interface, enabling deterministic testing of stack operations without network calls or authentication requirements. ### How does the Interactive field affect command execution? When `Interactive` is `false`, commands skip user prompts entirely or use default values, making the tool suitable for CI pipelines. In test environments, `NewTestConfig` sets this to `false` by default, while individual tests can combine it with stubbed `SelectFn` or `ConfirmFn` functions to simulate specific user choices programmatically. ### Where are the mock implementations for git operations defined? The [`internal/git/mock_ops.go`](https://github.com/github/gh-stack/blob/main/internal/git/mock_ops.go) file defines `MockOps`, which implements the git operation interface used throughout gh-stack. While `Config` contains fields for service overrides, git operations are typically replaced using `git.SetOps(&git.MockOps{...})` in test setup, allowing commands to execute against synthetic repository states without modifying the filesystem.