Go CLI Structure

Go CLI Structure

How a Cobra command tree is put together: the entry point, the root, the commands hanging off it, and where a command's work goes.

CLI Only projects and Web Only projects use different roots, and a CLI + Web hybrid uses the CLI Only root plus a serve command.

Both set Use, Short, Version from the AppVersion ldflag, and CompletionOptions.HiddenDefaultCmd: true.

Entry Point

package main

import "github.com/[GITHUB_USER]/REPO_NAME/cmd"

func main() {
    cmd.Execute()
}

main.go holds nothing else, so no logic ends up in the one function a test cannot call.

Root Command, CLI Only

package cmd

import (
    "os"
    "time"

    "github.com/rs/zerolog"
    "github.com/rs/zerolog/log"
    "github.com/spf13/cobra"
    "github.com/[GITHUB_USER]/REPO_NAME/utils"

    featureCmd "github.com/[GITHUB_USER]/REPO_NAME/cmd/feature-cmd"
)

var AppVersion = "dev-build" // set at build time via ldflags

var debugFlag bool

var rootCmd = &cobra.Command{
    Use:               "appname",
    Short:             "Brief description of the application",
    Version:           AppVersion,
    CompletionOptions: cobra.CompletionOptions{HiddenDefaultCmd: true},
}

func Execute() {
    if err := rootCmd.Execute(); err != nil {
        os.Exit(1)
    }
}

func setupLogs() {
    zerolog.TimeFieldFormat = zerolog.TimeFormatUnix
    output := zerolog.ConsoleWriter{Out: os.Stdout, TimeFormat: time.DateTime, NoColor: !utils.StdoutIsTerminal}
    log.Logger = zerolog.New(output).With().Timestamp().Logger()
    zerolog.SetGlobalLevel(zerolog.InfoLevel)
    if debugFlag {
        zerolog.SetGlobalLevel(zerolog.DebugLevel)
        utils.GlobalDebugFlag = true
    }
}

func init() {
    rootCmd.SetHelpCommand(&cobra.Command{Hidden: true})

    rootCmd.PersistentFlags().BoolVar(&debugFlag, "debug", false, "Enable debug logging")

    cobra.OnInitialize(setupLogs)

    rootCmd.AddCommand(serveCmd)
    rootCmd.AddCommand(featureCmd.FeatureCmd)
}

setupLogs runs through cobra.OnInitialize rather than at package init, because the flag values it reads are not parsed until Cobra has matched the command.

Execute sets the exit code and prints nothing. Cobra has already written the error to stderr by the time it returns one (cobra@v1.10.2 command.go:1159-1161), so a wrapper that prints it again shows the user the same line twice.

The help and completion commands are hidden so appname --help lists only the commands the tool actually offers.

A root declaring cobra.Groups needs one line more. The hidden help command still counts toward AllChildCommandsHaveGroup (cobra@v1.10.2 command.go:1376-1383), and leaving it ungrouped renders an empty Additional Commands: header:

rootCmd.AddGroup(&cobra.Group{ID: "main", Title: "Main Commands:"})
rootCmd.SetHelpCommandGroupID("main")

An ungrouped root needs nothing extra, since the usage template reaches that header only when len .Groups is non-zero.

Root Command, Web Only

package cmd

import (
    "io"
    "os"
    "time"

    "github.com/charmbracelet/x/term"
    "github.com/rs/zerolog"
    "github.com/rs/zerolog/log"
    "github.com/spf13/cobra"
)

var AppVersion = "dev-build"

var debugFlag bool

var rootCmd = &cobra.Command{
    Use:               "appname",
    Short:             "Brief description of the application",
    Version:           AppVersion,
    CompletionOptions: cobra.CompletionOptions{HiddenDefaultCmd: true},
}

func Execute() {
    if err := rootCmd.Execute(); err != nil {
        os.Exit(1)
    }
}

func setupLogs() {
    zerolog.TimeFieldFormat = zerolog.TimeFormatUnix
    var out io.Writer = os.Stdout
    if term.IsTerminal(os.Stdout.Fd()) {
        out = zerolog.ConsoleWriter{Out: os.Stdout, TimeFormat: time.DateTime}
    }
    log.Logger = zerolog.New(out).With().Timestamp().Logger()
    zerolog.SetGlobalLevel(zerolog.InfoLevel)
    if debugFlag {
        zerolog.SetGlobalLevel(zerolog.DebugLevel)
    }
}

func init() {
    rootCmd.SetHelpCommand(&cobra.Command{Hidden: true})
    rootCmd.PersistentFlags().BoolVar(&debugFlag, "debug", false, "Enable debug logging")
    cobra.OnInitialize(setupLogs)
    rootCmd.AddCommand(serveCmd)
}

Same logger and same --debug as CLI Only, without the utils import. What a server has no use for is the printers, since nobody is watching a container's stdout for a styled checkmark.

Simple Command

A command without subcommands is defined directly in cmd/, with its own flag struct and init(). Grouping flags in a struct per command keeps two commands from colliding over a variable named output in the shared cmd package.

// cmd/serve.go
package cmd

import (
    "fmt"

    "github.com/spf13/cobra"
    "github.com/[GITHUB_USER]/REPO_NAME/internal/server"
    u "github.com/[GITHUB_USER]/REPO_NAME/utils"
)

var serveFlags struct {
    port int
    host string
}

var serveCmd = &cobra.Command{
    Use:   "serve",
    Short: "Start the web server",
    Run: func(cmd *cobra.Command, args []string) {
        srv := server.New(serveFlags.host, serveFlags.port)
        if err := srv.Setup(); err != nil {
            u.PrintFatal("Failed to set up server", err)
        }
        u.PrintInfo(fmt.Sprintf("Starting server on %s:%d", serveFlags.host, serveFlags.port))
        if err := srv.Run(); err != nil {
            u.PrintFatal("Server error", err)
        }
    },
}

func init() {
    serveCmd.Flags().IntVarP(&serveFlags.port, "port", "p", 8080, "Port to listen on")
    serveCmd.Flags().StringVarP(&serveFlags.host, "host", "H", "0.0.0.0", "Host to bind to")
}

The same command in a Web Only project swaps u.PrintFatal(msg, err) for log.Fatal().Err(err).Msg(msg) and u.PrintInfo for log.Info(), and drops the utils import.

Run Function Shape

A Run body validates flags, builds a config struct, calls into internal/, and reports the result. Keeping the work in internal/ leaves the command as the only layer that knows about flags and terminals, which is what lets the logic be tested without one.

Run: func(cmd *cobra.Command, args []string) {
    if flags.required == "" {
        u.PrintFatal("--required flag is required", nil)
    }

    cfg := internal.Config{Field1: flags.field1, Field2: flags.field2}

    result, err := internal.DoThing(cfg)
    if err != nil {
        u.PrintFatal("Failed to do thing", err)
    }

    u.PrintSuccess("Thing completed")
    u.PrintGeneric(result)
}

Run with u.PrintFatal is the default. RunE is for a command holding something a deferred function has to release, such as a lock file, a temporary directory, or a half-written output file, since PrintFatal calls os.Exit(1) and skips defers.

Everywhere else RunE costs two things. PrintFatal(msg, err) carries a human label and the wrapped error separately, which is what lets the normal tier show the label alone while --debug shows the chain, and a returned error collapses both into one string. Cobra then prints Error: <err> followed by the entire usage block (cobra@v1.10.2 command.go:1159-1167), burying the message under a wall of flags.

A tree that uses RunE anywhere sets both silences on the root and reports the error itself, which arrives back at PrintFatal:

func init() {
    rootCmd.SilenceErrors = true
    rootCmd.SilenceUsage = true
}

func Execute() {
    if err := rootCmd.Execute(); err != nil {
        u.PrintFatal("Command failed", err)
    }
}

Subcommand Package

A group of related subcommands gets a package under cmd/. The parent command is exported and has no Run, so invoking it bare prints help instead of doing something arbitrary; the children are unexported and each carry a Run.

// cmd/feature-cmd/feature.go
package featureCmd

import (
    "fmt"

    "github.com/spf13/cobra"
    "github.com/[GITHUB_USER]/REPO_NAME/internal/feature"
    u "github.com/[GITHUB_USER]/REPO_NAME/utils"
)

var createFlags struct {
    name   string
    config string
}

var FeatureCmd = &cobra.Command{
    Use:   "feature",
    Short: "Feature management commands",
}

var createCmd = &cobra.Command{
    Use:   "create",
    Short: "Create a new feature",
    Run: func(cmd *cobra.Command, args []string) {
        cfg := feature.CreateConfig{Name: createFlags.name, Config: createFlags.config}
        if err := feature.Create(cfg); err != nil {
            u.PrintFatal("Failed to create feature", err)
        }
        u.PrintSuccess(fmt.Sprintf("Created feature: %s", createFlags.name))
    },
}

func init() {
    FeatureCmd.AddCommand(createCmd)

    createCmd.Flags().StringVarP(&createFlags.name, "name", "n", "", "Feature name (required)")
    createCmd.MarkFlagRequired("name")
    createCmd.Flags().StringVarP(&createFlags.config, "config", "c", "", "Config file path")
}