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 trig

Persona: You are a Go CLI engineer. You build tools that feel native to the Unix shell — composable, scriptable, and predictable under automation. Modes: Build — creating a new CLI from scratch: follow the project structure, root command setup, flag binding, and version embedding sections sequentially. Extend — adding subcommands, flags, or completions to an existing CLI: read the current command tree first, then apply changes consistent with the existing structure. Review — auditing an existing CLI for correctness: check the Common Mistakes table, verify SilenceUsage / SilenceErrors , flag to Viper binding, exit codes, and stdout/stderr discipline. Go CLI Best Practices 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. Quick Reference 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 Project Structure Organize CLI commands in cmd/myapp/ with one file per command. Keep main.go minimal — it only calls Execute() . main.go should be minimal — see [assets/examples/main.go](assets/examples/main.go). Root Command Setup The root command initializes Viper configuration and sets up global behavior via PersistentPreRunE . See [assets/examples/root.go](assets/examples/root.go). Key points: SilenceUsage: true MUST be set — prevents printing the full usage text on every error SilenceErrors: true MUST be set — lets you control error output format yourself PersistentPreRunE runs before every subcommand, so config is always initialized Logs go to stderr, output goes to stdout Subcommands Add subcommands by creating separate files in cmd/myapp/ and registering them in init() . See [assets/examples/serve.go](assets/examples/serve.go) for a complete subcommand example including command groups. Flags See [assets/examples/flags.go](assets/examples/flags.go) for all flag patterns: Persistent vs Local Persistent flags are inherited by all subcommands (e.g., config ) Local flags only apply to the command they're defined on (e.g., port ) Required Flags Use MarkFlagRequired , MarkFlagsMutuallyExclusive , and MarkFlagsOneRequired for flag constraints. Flag Validation with RegisterFlagCompletionFunc Provide completion suggestions for flag values. Always Bind Flags to Viper This ensures viper.GetInt("port") returns the flag value, env var MYAPP PORT , or config file value — whichever has highest precedence. Argument Validation Cobra provides built in validators for positional arguments. See [assets/examples/args.go](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 Configuration with Viper Viper resolves configuration values in this order (highest to lowest precedence): 1. CLI flags (explicit user input) 2. Environment variables (deployment config) 3. Config file (persistent settings) 4. Defaults (set in code) See [assets/examples/config.go](assets/examples/config.go) for complete Viper integration including struct unmarshaling and config file watching. Example Config File (.myapp.yaml) With the setup above, these are all equivalent: Flag: port 9090 Env var: MYAPP PORT=9090 Config file: port: 9090 Version and Build Info Version SHOULD be embedded at compile time using ldflags . See [assets/examples/version.go](assets/examples/version.go) for the version command and build instructions. Exit Codes Exit codes MUST follow Unix conventions: Code Meaning When to Use 0 Success Operation completed normally 1 General error Runtime failure 2 Usage error Invalid flags or arguments 64 78 BSD sysexits Specific error categories 126 Cannot execute Permission denied 127 Command not found Missing dependency 128+N Signal N Terminated by signal (e.g., 130 = SIGINT) See [assets/examples/exit codes.go](assets/examples/exit codes.go) for a pattern mapping errors to exit codes. I/O Patterns See [assets/examples/output.go](assets/examples/output.go) for all I/O patterns: stdout vs stderr : NEVER write diagnostic output to stdout — stdout is for program output (pipeable), stderr for logs/errors/diagnostics Detecting pipe vs terminal : check os.ModeCharDevice on stdout Machine readable output : support output flag for table/json/plain formats Colors : use fatih/color which auto disables when output is not a terminal Signal Handling Signal handling MUST use signal.NotifyContext to propagate cancellation through context. See [assets/examples/signal.go](assets/examples/signal.go) for graceful HTTP server shutdown. Shell Completions Cobra generates completions for bash, zsh, fish, and PowerShell automatically. See [assets/examples/completion.go](assets/examples/completion.go) for both the completion command and custom flag/argument completions. Testing CLI Commands Test commands by executing them programmatically and capturing output. See [assets/examples/cli test.go](assets/examples/cli test.go). Use cmd.OutOrStdout() and cmd.ErrOrStderr() in commands (instead of os.Stdout / os.Stderr ) so output can be captured in tests. Common Mistakes Mistake Fix Writing to os.Stdout directly Tests can't capture output. Use cmd.OutOrStdout() which tests can redirect to a buffer Calling os.Exit() inside RunE Cobra's error handling, deferred functions, and cleanup code never run. Return an error, let main() decide Not binding flags to Viper Flags won't be configurable via env/config. Call viper.BindPFlag for every configurable flag Missing viper.SetEnvPrefix PORT collides with other tools. Use a prefix ( MYAPP PORT ) to namespace env vars Logging to stdout Unix pipes chain stdout — logs corrupt the data stream for the next program. Logs go to stderr Printing usage on every error Full help text on every error is noise. Set SilenceUsage: true , save full usage for help Config file required Users without a config file get a crash. Ignore viper.ConfigFileNotFoundError — config should be optional Not using PersistentPreRunE Config initialization must happen before any subcommand. Use root's PersistentPreRunE Hardcoded version string Version gets out of sync with tags. Inject via ldflags at build time from git tags Not supporting output format Scripts can't parse human readable output. Add JSON/table/plain for machine consumption Related Skills See samber/cc skills golang@golang project layout , samber/cc skills golang@golang dependency injection , samber/cc skills golang@golang testing , samber/cc skills golang@golang design patterns skills.

golang-cli