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/olekukonez/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()

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.

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):

CLI flags (explicit user input)
Environment variables (deployment config)
Config file (persistent settings)
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:

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 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

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

NPX Install

Tags

SKILL.md Content