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#

ConcernPackage / ToolCommands & flagsgithub.com/spf13/cobraConfigurationgithub.com/spf13/viperFlag parsinggithub.com/spf13/pflag (via Cobra)Colored outputgithub.com/fatih/colorTable outputgithub.com/olekukonko/tablewriterInteractive promptsgithub.com/charmbracelet/bubbleteaVersion injectiongo build -ldflagsDistributiongoreleaser

Project Structure#

Organize CLI commands in cmd/myapp/ with one file per command. Keep main.go minimal — it only calls Execute().

text

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.

Root Command Setup#

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 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 for a complete subcommand example including command groups.

Flags#

See 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 for both built-in and custom validation examples.

ValidatorDescriptioncobra.NoArgsFails if any args providedcobra.ExactArgs(n)Requires exactly n argscobra.MinimumNArgs(n)Requires at least n argscobra.MaximumNArgs(n)Allows at most n argscobra.RangeArgs(min, max)Requires between min and maxcobra.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 for complete Viper integration including struct unmarshaling and config file watching.

Example Config File (.myapp.yaml)#

yaml

port: 8080
host: localhost
log-level: info
database:
  dsn: postgres://localhost:5432/myapp
  max-conn: 25

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 for the version command and build instructions.

Exit Codes#

Exit codes MUST follow Unix conventions:

CodeMeaningWhen to Use0SuccessOperation completed normally1General errorRuntime failure2Usage errorInvalid flags or arguments64-78BSD sysexitsSpecific error categories126Cannot executePermission denied127Command not foundMissing dependency128+NSignal NTerminated by signal (e.g., 130 = SIGINT)

See assets/examples/exit_codes.go for a pattern mapping errors to exit codes.

I/O Patterns#

See 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 for graceful HTTP server shutdown.

Shell Completions#

Cobra generates completions for bash, zsh, fish, and PowerShell automatically. See 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.

Use cmd.OutOrStdout() and cmd.ErrOrStderr() in commands (instead of os.Stdout / os.Stderr) so output can be captured in tests.

Common Mistakes#

MistakeFixWriting to os.Stdout directlyTests can't capture output. Use cmd.OutOrStdout() which tests can redirect to a bufferCalling os.Exit() inside RunECobra's error handling, deferred functions, and cleanup code never run. Return an error, let main() decideNot binding flags to ViperFlags won't be configurable via env/config. Call viper.BindPFlag for every configurable flagMissing viper.SetEnvPrefix``PORT collides with other tools. Use a prefix ( MYAPP_PORT) to namespace env varsLogging to stdoutUnix pipes chain stdout — logs corrupt the data stream for the next program. Logs go to stderrPrinting usage on every errorFull help text on every error is noise. Set SilenceUsage: true, save full usage for --helpConfig file requiredUsers without a config file get a crash. Ignore viper.ConfigFileNotFoundError — config should be optionalNot using PersistentPreRunEConfig initialization must happen before any subcommand. Use root's PersistentPreRunEHardcoded version stringVersion gets out of sync with tags. Inject via ldflags at build time from git tagsNot supporting --output formatScripts 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.