GitHub - leodido/structcli: Eliminate Cobra boilerplate: build powerful, feature-rich CLIs declaratively from Go structs. (original) (raw)

CLI generation from Go structs

Transform your Go structs into fully-featured command-line interfaces with configuration files, environment variables, flags, validation, and beautiful help output.

Declare your options in a struct, and let structcli do the rest

You don't need much: just a few struct tags + structcli.

Stop writing boilerplate. Start building features.

⚡ Quick Start

package main

import ( "fmt" "log"

"github.com/leodido/structcli"
"github.com/spf13/cobra"
"go.uber.org/zap/zapcore"

)

type Options struct { LogLevel zapcore.Level Port int }

func (o *Options) Attach(c *cobra.Command) error { return structcli.Define(c, o) // This is it }

func main() { log.SetFlags(0) opts := &Options{} cli := &cobra.Command{Use: "myapp"}

// This single line creates all the options (flags, env vars, config keys)
if err := opts.Attach(cli); err != nil {
    log.Fatalln(err)
}

cli.PreRunE = func(c *cobra.Command, args []string) error {
    return structcli.Unmarshal(c, opts) // Populates struct from config keys, env variables, flags
}

cli.RunE = func(c *cobra.Command, args []string) error {
    fmt.Println(opts)

    return nil
}

if err := cli.Execute(); err != nil {
    log.Fatalln(err)
}

}

That's it!

❯ go run examples/minimal/main.go --help

Usage:

myapp [flags]

Flags:

--loglevel zapcore.Level {debug,info,warn,error,dpanic,panic,fatal} (default info)

--port int

Want automatic environment variables, aliases, shorthand, flag description in usage?

Just annotate your struct with structcli tags.

type Options struct { LogLevel zapcore.Level flag:"level" flagdescr:"Set logging level" flagenv:"true" Port int flagshort:"p" flagdescr:"Server port" flagenv:"true" default:"3000" }

Here it is.

❯ go run examples/simple/main.go -h

Usage:

myapp [flags]

Flags:

--level zapcore.Level Set logging level {debug,info,warn,error,dpanic,panic,fatal} (default info)

-p, --port int Server port (default 3000)

You got your environment variables.

❯ MYAPP_LOGLEVEL=debug go run examples/simple/main.go

&{debug 3000}

❯ MYAPP_LEVEL=warn go run examples/simple/main.go

&{warn 3000}

Flags override environment variables, of course.

❯ MYAPP_LOGLEVEL=error MYAPP_PORT=9000 go run examples/simple/main.go --level dpanic

&{dpanic 9000}

Built-in custom types like zapcore.LogLevel comes with automatic validation.

❯ MYAPP_LOGLEVEL=debug MYAPP_PORT=9000 go run examples/simple/main.go --level what

Error: invalid argument "what" for "--level" flag: must be 'debug', 'dpanic', 'error', 'fatal', 'info', 'panic', 'warn'

Usage:

myapp [flags]

Flags:

--level zapcore.Level Set logging level {debug,info,warn,error,dpanic,panic,fatal} (default info)

-p, --port int Server port (default 3000)

invalid argument "what" for "--level" flag: must be 'debug', 'dpanic', 'error', 'fatal', 'info', 'panic', 'warn'

exit status 1

Your CLI now supports:

• 📝 Command-line flags (--level info, -p 8080)
• 🌍 Environment variables (MYAPP_PORT=8080)
• 💦 Options precedence (flags > env vars > config file)
• ✅ Automatic validation and type conversion
• 📚 Beautiful help output with proper grouping

⬇️ Install

go get github.com/leodido/structcli

📦 Key Features

🧩 Declarative Flags Definition

Define flags once using Go struct tags.

No more boilerplate for Flags().StringVarP, Flags().IntVar, viper.BindPFlag, etc.

Yes, you can nest structs too.

type ServerOptions struct { // Basic flags Host string flag:"host" flagdescr:"Server host" default:"localhost" Port int flagshort:"p" flagdescr:"Server port" flagrequired:"true" flagenv:"true"

// Environment variable binding
APIKey string `flagenv:"true" flagdescr:"API authentication key"`

// Flag grouping for organized help
LogLevel zapcore.Level `flag:"log-level" flaggroup:"Logging" flagdescr:"Set log level"`
LogFile  string        `flag:"log-file" flaggroup:"Logging" flagdescr:"Log file path" flagenv:"true"`

// Nested structs for organization
Database DatabaseConfig `flaggroup:"Database"`

// Custom type
TargetEnv Environment `flagcustom:"true" flag:"target-env" flagdescr:"Set the target environment"`

}

type DatabaseConfig struct { URL string flag:"db-url" flagdescr:"Database connection URL" MaxConns int flagdescr:"Max database connections" default:"10" flagenv:"true" }

See full example for more details.

🛠️ Automatic Environment Variable Binding

Automatically generate environment variables binding them to configuration files (YAML, JSON, TOML, etc.) and flags.

From the previous options struct, you get the following env vars automatically:

• FULL_SRV_PORT
• FULL_SRV_APIKEY
• FULL_SRV_DATABASE_MAXCONNS
• FULL_SRV_LOGFILE, FULL_SRV_LOG_FILE

Every struct field with the flagenv:"true" tag gets an environment variable (two if the struct field also has the flag:"..." tag, see struct field LogFile).

The prefix of the environment variable name is the CLI name plus the command name to which those options are attached to.

⚙️ Configuration File Support

Easily set up configuration file discovery (flag, environment variable, and fallback paths) with a single line of code.

structcli.SetupConfig(rootCmd, config.Options{AppName: "full"})

The line above:

• creates --config global flag
• creates FULL_CONFIG env var
• sets /etc/full/, $HOME/.full/, $PWD/.full/ as fallback paths for config.yaml

Magic, isn't it?

What's left? Tell your CLI to load the configuration file (if any).

rootC.PersistentPreRunE = func(c *cobra.Command, args []string) error { _, configMessage, configErr := structcli.UseConfigSimple(c) if configErr != nil { return configErr } if configMessage != "" { c.Println(configMessage) }

return nil

}

📜 Configuration Is First-Class Citizen

Configuration can mirror your command hierarchy.

Settings can be global (at the top level) or specific to a command or subcommand. The most specific section always takes precedence.

Global settings apply to all commands unless overridden by a specific section.

dryrun matches the DryRun struct field name.

dryrun: true verbose: 1 # A default verbosity level for all commands.

Config for the srv command (full srv)

srv:

port matches the Port field name.

port: 8433

log-level matches the flag:"log-level" tag.

log-level: "warn"

logfile matches the LogFile field name.

logfile: /var/log/mysrv.log

Flattened keys can set options in nested structs.

db-url (from flag:"db-url" tag) maps to ServerOptions.Database.URL.

db-url: "postgres://user:pass@db/prod"

Config for the usr command group.

usr:

This nested section matches the usr add command (full usr add).

Its settings are ONLY applied to 'usr add'.

add: name: "Config User" email: "config.user@example.com" age: 42 # Command specific override dry: false

NOTE: Per the library's design, there is no other fallback other than from the top-level.

A command like 'usr delete' would ONLY use the global keys above (if those keys/flags are attached to it),

as an exact 'usr.delete' section is not defined.

This configuration system supports:

• Hierarchical Structure: Nest keys to match your command path (e.g., usr: { add: { ... } }).
• Strict Precedence: Only settings from the global scope and the exact command path section are merged. There is no automatic fallback to parent command sections.
• Flexible Keys: Use either the struct field name (lowercase(DryRun)) or the flag tag (flag:"log-level") as keys.
• Flattened Keys: Set options in nested structs easily using a single, flattened key (e.g., db-url).

✅ Built-in Validation & Transformation

Supports validation, transformation, and custom flag type definitions through simple interfaces.

Just make your struct implement ValidatableOptions and TransformableOptions interfaces.

type UserConfig struct { Email string flag:"email" flagdescr:"User email" validate:"email" Age int flag:"age" flagdescr:"User age" validate:"min=18,max=120" Name string flag:"name" flagdescr:"User name" mod:"trim,title" }

func (o *ServerOptions) Validate(ctx context.Context) []error { // Automatic validation }

func (o *ServerOptions) Transform(ctx context.Context) error { // Automatic transformation }

See a full working example here.

🚧 Automatic Debugging Support

Create a --debug-options flag (plus a FULL_DEBUG_OPTIONS env var) for troubleshooting config/env/flags resolution.

structcli.SetupDebug(rootCmd, debug.Options{})

❯ go run examples/full/main.go srv --debug-options --config examples/full/config.yaml -p 3333 #

Aliases:

map[string]string{"database.url":"db-url", "logfile":"log-file", "loglevel":"log-level", "targetenv":"target-env"}

Override:

map[string]interface {}{}

PFlags:

map[string]viper.FlagValue{"apikey":viper.pflagValue{flag:(*pflag.Flag)(0x14000109ea0)}, "database.maxconns":viper.pflagValue{flag:(*pflag.Flag)(0x140002181e0)}, "db-url":viper.pflagValue{flag:(*pflag.Flag)(0x14000218140)}, "host":viper.pflagValue{flag:(*pflag.Flag)(0x14000109d60)}, "log-file":viper.pflagValue{flag:(*pflag.Flag)(0x140002180a0)}, "log-level":viper.pflagValue{flag:(*pflag.Flag)(0x14000218000)}, "port":viper.pflagValue{flag:(*pflag.Flag)(0x14000109e00)}, "target-env":viper.pflagValue{flag:(*pflag.Flag)(0x14000218320)}}

Env:

map[string][]string{"apikey":[]string{"SRV_APIKEY"}, "database.maxconns":[]string{"SRV_DATABASE_MAXCONNS"}, "log-file":[]string{"SRV_LOGFILE", "SRV_LOG_FILE"}}

Key/Value Store:

map[string]interface {}{}

Config:

map[string]interface {}{"apikey":"secret-api-key", "database":map[string]interface {}{"maxconns":3}, "db-url":"postgres://user:pass@localhost/mydb", "host":"production-server", "log-file":"/var/log/mysrv.log", "log-level":"debug", "port":8443}

Defaults:

map[string]interface {}{"database":map[string]interface {}{"maxconns":"10"}, "host":"localhost"}

Values:

map[string]interface {}{"apikey":"secret-api-key", "database":map[string]interface {}{"maxconns":3, "url":"postgres://user:pass@localhost/mydb"}, "db-url":"postgres://user:pass@localhost/mydb", "host":"production-server", "log-file":"/var/log/mysrv.log", "log-level":"debug", "logfile":"/var/log/mysrv.log", "loglevel":"debug", "port":3333, "target-env":"dev", "targetenv":"dev"}

↪️ Sharing Options Between Commands

In complex CLIs, multiple commands often need access to the same global configuration and shared resources (like a logger or a database connection). structcli provides a powerful pattern using the ContextOptions interface to achieve this without resorting to global variables, by propagating a single "source of truth" through the command context.

The pattern allows you to:

• Populate a shared options struct once from flags, environment variables, or a config file.
• Initialize "computed state" (like a logger) based on those options.
• Share this single, fully-prepared "source of truth" with any subcommand that needs it.

🍩 In a Nutshell

Create a shared struct that implements the ContextOptions interface. This struct will hold both the configuration flags and the computed state (e.g., the logger).

// This struct holds our shared state. type CommonOptions struct { LogLevel zapcore.Level flag:"loglevel" flagdescr:"Logging level" default:"info" Logger *zap.Logger flagignore:"true" // This field is computed, not a flag. }

// The Context/FromContext methods enable the propagation pattern. func (o CommonOptions) Context(ctx context.Context) context.Context { / ... */ } func (o CommonOptions) FromContext(ctx context.Context) error { / ... */ }

// Initialize is a custom method to create the computed state. func (o CommonOptions) Initialize() error { / ... */ }

Initialize the state in the root command. Use a PersistentPreRunE hook on your root command to populate your struct and initialize any resources. Invoking structcli.Unmarshal will automatically inject the prepared object into the context for all subcommands to use.

rootC.PersistentPreRunE = func(c *cobra.Command, args []string) error { // Populate the master commonOpts from flags, env, and config file. if err := structcli.Unmarshal(c, commonOpts); err != nil { return err } // Use the populated values to initialize the computed state (the logger). if err := commonOpts.Initialize(); err != nil { return err }

return nil

}

Finally, retrieve the state in subcommands. In your subcommand's RunE, simply call .FromContext() to retrieve the shared, initialized object.

func(c *cobra.Command, args []string) error { // Create a receiver and retrieve the master state from the context. config := &CommonOptions{} if err := config.FromContext(c.Context()); err != nil { return err } config.Logger.Info("Executing subcommand...")

return nil

},

This pattern ensures that subcommands remain decoupled while having access to a consistent, centrally-managed state.

For a complete, runnable implementation of this pattern, see the loginsvc example located in the /examples/loginsvc directory.

🪃 Custom Type Handlers

Declare options (flags, env vars, config file keys) with custom types by implementing two methods on your options struct.

Just implement two methods on your options structs:

• Define<FieldName>: return a pflag.Value that knows how to handle your custom type, along with an enhanced description.
• Decode<FieldName>: decode the input into your custom type.

type Environment string

const ( EnvDevelopment Environment = "dev" EnvStaging Environment = "staging" EnvProduction Environment = "prod" )

type ServerOptions struct { ... // Custom type TargetEnv Environment flagcustom:"true" flag:"target-env" flagdescr:"Set the target environment" }

// DefineTargetEnv returns a pflag.Value for the custom Environment type. func (o *ServerOptions) DefineTargetEnv(name, short, descr string, structField reflect.StructField, fieldValue reflect.Value) (pflag.Value, string) { enhancedDesc := descr + " {dev,staging,prod}" fieldPtr := fieldValue.Addr().Interface().(*Environment) *fieldPtr = "dev" // Set default

return structclivalues.NewString((*string)(fieldPtr)), enhancedDesc

}

// DecodeTargetEnv converts the string input to the Environment type. func (o *ServerOptions) DecodeTargetEnv(input any) (any, error) { // ... (validation and conversion logic) return EnvDevelopment, nil }

// Attach handles flag definition and shell completion for our custom type. func (o *ServerOptions) Attach(c *cobra.Command) error { if err := structcli.Define(c, o); err != nil { return err }

// Register shell completion after the flag has been defined.
c.RegisterFlagCompletionFunc("target-env", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
    return []string{"dev", "staging", "prod"}, cobra.ShellCompDirectiveNoFileComp
})

return nil

}

In values we provide pflag.Value implementations for standard types.

See full example for more details.

🧱 Built-in Custom Types

All built-in types support:

• Command-line flags with validation and help text
• Environment variables with automatic binding
• Configuration files (YAML, JSON, TOML)
• Type validation with helpful error messages

🎨 Beautiful, Organized Help Output

Organize your --help output into logical groups for better readability.

❯ go run examples/full/main.go --help

A demonstration of the structcli library with beautiful CLI features

Usage:

full [flags]

full [command]

Available Commands:

completion Generate the autocompletion script for the specified shell

help Help about any command

srv Start the server

usr User management

Global Flags:

--config string config file (fallbacks to: {/etc/full,{executable_dir}/.full,$HOME/.full}/config.{yaml,json,toml})

--debug-options enable debug output for options

Utility Flags:

--dry-run

-v, --verbose count

❯ go run examples/full/main.go srv --help

Start the server with the specified configuration

Usage:

full srv [flags]

full srv [command]

Available Commands:

version Print version information

Flags:

--apikey string API authentication key

--host string Server host (default "localhost")

-p, --port int Server port

--target-env string Set the target environment {dev,staging,prod} (default "dev")

Database Flags:

--database.maxconns int Max database connections (default 10)

--db-url string Database connection URL

Logging Flags:

--log-file string Log file path

--log-level zapcore.Level Set log level {debug,info,warn,error,dpanic,panic,fatal} (default info)

Global Flags:

--config string config file (fallbacks to: {/etc/full,{executable_dir}/.full,$HOME/.full}/config.{yaml,json,toml})

--debug-options enable debug output for options

Use "full srv [command] --help" for more information about a command.

🏷️ Available Struct Tags

Use these tags in your struct fields to control the behavior:

📖 Documentation

For comprehensive documentation and advanced usage patterns, visit the documentation.

Or take a look at the examples.

🤝 Contributing

Contributions are welcome!

Please feel free to submit a Pull Request.