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GitHub CLI project layout

At a high level, these areas make up the github.com/cli/cli project:

  • cmd/ - main packages for building binaries such as the gh executable
  • pkg/ - most other packages, including the implementation for individual gh commands
  • docs/ - documentation for maintainers and contributors
  • script/ - build and release scripts
  • internal/ - Go packages highly specific to our needs and thus internal
  • go.mod - external Go dependencies for this project, automatically fetched by Go at build time

Some auxiliary Go packages are at the top level of the project for historical reasons:

  • api/ - main utilities for making requests to the GitHub API
  • context/ - DEPRECATED: use only for referencing git remotes
  • git/ - utilities to gather information from a local git repository
  • test/ - DEPRECATED: do not use
  • utils/ - DEPRECATED: use only for printing table output

Command-line help text

Running gh help issue list displays help text for a topic. In this case, the topic is a specific command, and help text for every command is embedded in that command's source code. The naming convention for gh commands is:

pkg/cmd/<command>/<subcommand>/<subcommand>.go

Following the above example, the main implementation for the gh issue list command, including its help text, is in pkg/cmd/issue/list/list.go

Other help topics not specific to any command, for example gh help environment, are found in pkg/cmd/root/help_topic.go.

During our release process, these help topics are automatically converted to manual pages and published under https://cli.github.com/manual/.

How GitHub CLI works

To illustrate how GitHub CLI works in its typical mode of operation, let's build the project, run a command, and talk through which code gets run in order.

  1. go run script/build.go - Makes sure all external Go dependencies are fetched, then compiles the cmd/gh/main.go file into a bin/gh binary.
  2. bin/gh issue list --limit 5 - Runs the newly built bin/gh binary (note: on Windows you must use backslashes like bin\gh) and passes the following arguments to the process: ["issue", "list", "--limit", "5"].
  3. func main() inside cmd/gh/main.go is the first Go function that runs. The arguments passed to the process are available through os.Args.
  4. The main package initializes the "root" command with root.NewCmdRoot() and dispatches execution to it with rootCmd.ExecuteC().
  5. The root command represents the top-level gh command and knows how to dispatch execution to any other gh command nested under it.
  6. Based on ["issue", "list"] arguments, the execution reaches the RunE block of the cobra.Command within pkg/cmd/issue/list/list.go.
  7. The --limit 5 flag originally passed as arguments be automatically parsed and its value stored as opts.LimitResults.
  8. func listRun() is called, which is responsible for implementing the logic of the gh issue list command.
  9. The command collects information from sources like the GitHub API then writes the final output to standard output and standard error streams available at opts.IO.
  10. The program execution is now back at func main() of cmd/gh/main.go. If there were any Go errors as a result of processing the command, the function will abort the process with a non-zero exit status. Otherwise, the process ends with status 0 indicating success.

How to add a new command

  1. First, check on our issue tracker to verify that our team had approved the plans for a new command.
  2. Create a package for the new command, e.g. for a new command gh boom create the following directory structure: pkg/cmd/boom/
  3. The new package should expose a method, e.g. NewCmdBoom(), that accepts a *cmdutil.Factory type and returns a *cobra.Command.
    • Any logic specific to this command should be kept within the command's package and not added to any "global" packages like api or utils.
  4. Use the method from the previous step to generate the command and add it to the command tree, typically somewhere in the NewCmdRoot() method.

How to write tests

This task might be tricky. Typically, gh commands do things like look up information from the git repository in the current directory, query the GitHub API, scan the user's ~/.ssh/config file, clone or fetch git repositories, etc. Naturally, none of these things should ever happen for real when running tests, unless you are sure that any filesystem operations are strictly scoped to a location made for and maintained by the test itself. To avoid actually running things like making real API requests or shelling out to git commands, we stub them. You should look at how that's done within some existing tests.

To make your code testable, write small, isolated pieces of functionality that are designed to be composed together. Prefer table-driven tests for maintaining variations of different test inputs and expectations when exercising a single piece of functionality.