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

Quick Reference

github.com/spf13/cobra

github.com/spf13/viper

github.com/spf13/pflag

github.com/fatih/color

github.com/olekukonko/tablewriter

github.com/charmbracelet/bubbletea

go build -ldflags

goreleaser

Project Structure

Organize CLI commands in

cmd/myapp/

main.go

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

Root Command Setup

The root command initializes Viper configuration and sets up global behavior via

PersistentPreRunE

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/

init()

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

MarkFlagsOneRequired

Flag Validation with RegisterFlagCompletionFunc

Provide completion suggestions for flag values.

Always Bind Flags to Viper

This ensures

viper.GetInt("port")

MYAPP_PORT

Argument Validation

Cobra provides built-in validators for positional arguments. See assets/examples/args.go for both built-in and custom validation examples.

cobra.NoArgs

cobra.ExactArgs(n)

cobra.MinimumNArgs(n)

cobra.MaximumNArgs(n)

cobra.RangeArgs(min, max)

cobra.ExactValidArgs(n)

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)

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

Exit Codes

Exit codes MUST follow Unix conventions:

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

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()

cmd.ErrOrStderr()

os.Stdout

os.Stderr

Common Mistakes

os.Stdout

cmd.OutOrStdout()

os.Exit()

RunE

main()

viper.BindPFlag

viper.SetEnvPrefix

PORT

MYAPP_PORT

SilenceUsage: true

--help

viper.ConfigFileNotFoundError

PersistentPreRunE

PersistentPreRunE

ldflags

--output

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