subcommand

Subcommand is a convention in command-line interfaces where an operation is invoked as a sub-level of a larger program, enabling a hierarchical namespace for commands. A subcommand represents a specific action within the context of the main program. For example, the Git command line exposes subcommands such as commit, push, and branch, while kubectl offers subcommands like get, describe, and apply. The general syntax is: program subcommand [options] [arguments].

Subcommands typically have their own options and argument semantics, in addition to any global options that may apply to the program as a whole. This separation helps organize functionality and keeps the top-level interface compact. Users first select a subcommand, then pass flags and parameters relevant to that subcommand.

Design considerations include naming consistency, discoverability, and helpful help messages. Subcommands are often implemented with a command-line parsing framework that supports nesting, such as subparsers in Python's argparse, Cobra in Go, Clap in Rust, or similar libraries in other ecosystems. Good tools provide concise help for each subcommand, alias support, and useful error messages when a subcommand is missing or unknown.

Pros include modularity, easier maintenance, and clearer documentation. Potential drawbacks are learning curve for new users and the risk of over-nesting, which can hamper usability and scriptability. Subcommands are a foundational pattern in modern CLIs, enabling large tools to expose a coherent, extensible set of actions with focused options and documentation.