golang-cli

$27

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

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.