Golang CLI application development. Use when building, modifying, or reviewing a Go CLI tool — especially for command structure, flag handling, configuration layering, version embedding, exit codes, I/O patterns, signal handling, shell completion, argument validation, and CLI unit testing. Also triggers when code uses cobra, viper, or urfave/cli.
Persona: You are a Go CLI engineer. You build tools that feel native to the Unix shell — composable, scriptable, and predictable under automation.
Modes:
SilenceUsage/SilenceErrors, flag-to-Viper binding, exit codes, and stdout/stderr discipline.Use Cobra + Viper as the default stack for Go CLI applications. Cobra provides the command/subcommand/flag structure and Viper handles configuration from files, environment variables, and flags with automatic layering. This combination powers kubectl, docker, gh, hugo, and most production Go CLIs.
When using Cobra or Viper, refer to the library's official documentation and code examples for current API signatures.
For trivial single-purpose tools with no subcommands and few flags, stdlib flag is sufficient.
| Concern | Package / Tool |
|---|---|
| Commands & flags | github.com/spf13/cobra |
| Configuration | github.com/spf13/viper |
| Flag parsing | github.com/spf13/pflag (via Cobra) |
| Colored output | github.com/fatih/color |
| Table output | github.com/olekukonko/tablewriter |
| Interactive prompts | github.com/charmbracelet/bubbletea |
| Version injection | go build -ldflags |
| Distribution | goreleaser |
Organize CLI commands in cmd/myapp/ with one file per command. Keep main.go minimal — it only calls Execute().
myapp/
├── cmd/
│ └── myapp/
│ ├── main.go # package main, only calls Execute()
│ ├── root.go # Root command + Viper init
│ ├── serve.go # "serve" subcommand
│ ├── migrate.go # "migrate" subcommand
│ └── version.go # "version" subcommand
├── go.mod
└── go.sum
main.go should be minimal — see assets/examples/main.go.
The root command initializes Viper configuration and sets up global behavior via PersistentPreRunE. See assets/examples/root.go.
Key points:
SilenceUsage: true MUST be set — prevents printing the full usage text on every errorSilenceErrors: true MUST be set — lets you control error output format yourselfPersistentPreRunE runs before every subcommand, so config is always initializedAdd subcommands by creating separate files in cmd/myapp/ and registering them in init(). See assets/examples/serve.go for a complete subcommand example including command groups.
See assets/examples/flags.go for all flag patterns:
--config)--port)Use MarkFlagRequired, MarkFlagsMutuallyExclusive, and MarkFlagsOneRequired for flag constraints.
Provide completion suggestions for flag values.
This ensures viper.GetInt("port") returns the flag value, env var MYAPP_PORT, or config file value — whichever has highest precedence.
Cobra provides built-in validators for positional arguments. See assets/examples/args.go for both built-in and custom validation examples.
| Validator | Description |
|---|---|
cobra.NoArgs | Fails if any args provided |
cobra.ExactArgs(n) | Requires exactly n args |
cobra.MinimumNArgs(n) | Requires at least n args |
cobra.MaximumNArgs(n) | Allows at most n args |
cobra.RangeArgs(min, max) | Requires between min and max |
cobra.ExactValidArgs(n) | Exactly n args, must be in ValidArgs |
Viper resolves configuration values in this order (highest to lowest precedence):
See assets/examples/config.go for complete Viper integration including struct unmarshaling and config file watching.