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()

.

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)

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