Flag-rs - A Cobra-inspired CLI Framework for Rust

Flag-rs is a command-line interface (CLI) framework for Rust inspired by Go's Cobra library. It provides dynamic command completion, self-registering commands, and a clean, modular architecture for building sophisticated CLI applications.

Key Features

• Zero Dependencies - Pure Rust implementation with no external crates
• Dynamic Runtime Completions - Generate completions based on runtime state (like kubectl)
• Self-Registering Commands - Commands can register themselves with parent commands
• Modular Architecture - Organize commands in separate files/modules
• Shell Completion Support - Generate completion scripts for bash, zsh, and fish
• Hierarchical Flag Inheritance - Global flags are available to all subcommands
• Idiomatic Error Handling - Uses standard Rust Result types
• Colored Output - Beautiful help messages with ANSI color support (respects NO_COLOR and terminal detection)

Why Flag-rs?

Flag-rs enables dynamic completions that can query APIs, databases, or any runtime state - just like kubectl does when completing pod names. I've struggled and failed for a long time at modifying the leading command line processing crate to work this way - hence this new crate.

Why Not Flag-rs?

The Flag-rs implementation may be naive - the leading crate, Clap, is very well regarded and meets the needs of a huge knowledgeable user base.

Installation

Add this to your Cargo.toml:

[dependencies]
flag-rs = "0.6"

Quick Start

use flag_rs::{Command, CommandBuilder, Context, Flag, FlagType, FlagValue, CompletionResult};

fn main() {
    let app = CommandBuilder::new("myapp")
        .short("A simple CLI application")
        .long("This is a longer description of my application")
        .flag(
            Flag::new("verbose")
                .short('v')
                .usage("Enable verbose output")
                .value_type(FlagType::Bool)
                .default(FlagValue::Bool(false))
        )
        .subcommand(build_serve_command())
        .build();

    let args: Vec<String> = std::env::args().skip(1).collect();
    if let Err(e) = app.execute(args) {
        eprintln!("Error: {}", e);
        std::process::exit(1);
    }
}

fn build_serve_command() -> Command {
    CommandBuilder::new("serve")
        .short("Start the server")
        .flag(
            Flag::new("port")
                .short('p')
                .usage("Port to listen on")
                .value_type(FlagType::Int)
                .default(FlagValue::Int(8080))
        )
        .run(|ctx| {
            let port = ctx.flag("port")
                .and_then(|s| s.parse::<i64>().ok())
                .unwrap_or(8080);

            println!("Starting server on port {}", port);
            Ok(())
        })
        .build()
}

Dynamic Completions

The killer feature of Flag-rs is dynamic completions. Unlike static completions, these run at completion time and can return different values based on current state:

CommandBuilder::new("get")
    .arg_completion(|ctx, prefix| {
        // This runs when the user presses TAB!
        let namespace = ctx.flag("namespace").unwrap_or("default");

        // In a real app, you'd query an API here
        let items = fetch_items_from_api(namespace);

        Ok(CompletionResult::new()
            .extend(items.into_iter()
                .filter(|item| item.starts_with(prefix))
                .collect::<Vec<_>>()))
    })
    .build()

Completions with Descriptions

Flag-rs supports rich completions with descriptions, similar to Cobra. When descriptions are provided, they are handled appropriately by each shell:

.flag_completion("environment", |_ctx, prefix| {
    Ok(CompletionResult::new()
        .add_with_description("dev", "Development environment - safe for testing")
        .add_with_description("staging", "Staging environment - production mirror")
        .add_with_description("prod", "Production environment - BE CAREFUL!"))
})

Shell-specific behavior:

• Bash: Shows only the completion values (descriptions not supported natively)
• Zsh: Shows descriptions using native format: value:description
• Fish: Shows descriptions using tab-separated format: value[TAB]description

For Zsh and Fish, descriptions appear alongside completions in the shell's native format, providing helpful context when selecting options.

Modular Command Structure

For larger applications, Flag supports a modular structure where commands register themselves:

// src/cmd/mod.rs
pub fn register_commands(root: &mut Command) {
    serve::register(root);
    deploy::register(root);
    status::register(root);
}

// src/cmd/serve.rs
pub fn register(parent: &mut Command) {
    let cmd = CommandBuilder::new("serve")
        .short("Start the server")
        .run(|ctx| {
            // Implementation
            Ok(())
        })
        .build();

    parent.add_command(cmd);
}

Shell Completions

Generate completion scripts for popular shells:

// Add a completion command to your CLI
CommandBuilder::new("completion")
    .short("Generate shell completion scripts")
    .run(|ctx| {
        let shell = ctx.args().first()
            .ok_or("Shell name required")?;

        let script = match shell.as_str() {
            "bash" => app.generate_completion(Shell::Bash),
            "zsh" => app.generate_completion(Shell::Zsh),
            "fish" => app.generate_completion(Shell::Fish),
            _ => return Err("Unsupported shell"),
        };

        println!("{}", script);
        Ok(())
    })
    .build()

Then users can enable completions:

# Bash
source <(myapp completion bash)

# Zsh
source <(myapp completion zsh)

# Fish
myapp completion fish | source

Flag Types

Flag-rs supports multiple value types:

• String - Text values
• Bool - Boolean flags
• Int - Integer values
• Float - Floating point values
• StringSlice - Multiple string values

Error Handling

Flag uses idiomatic Rust error handling with no forced dependencies:

pub enum Error {
    CommandNotFound(String),
    SubcommandRequired(String),
    FlagParsing(String),
    ArgumentParsing(String),
    ValidationError(String),
    Completion(String),
    Io(std::io::Error),
    Custom(Box<dyn std::error::Error + Send + Sync>),
}

Examples

See the examples/ directory for complete examples:

• kubectl.rs - A simple kubectl-like CLI demonstrating dynamic completions
• kubectl_modular/ - A modular kubectl implementation showing command self-registration

Design Philosophy

Flag is designed to be a foundational library with zero dependencies. This ensures:

• Fast compilation times
• No dependency conflicts
• Maximum flexibility for users
• Minimal binary size

Users can add their own dependencies for features like colored output, async runtime, or configuration file support without conflicts.

Development

Running Clippy Locally

Flag uses opinionated clippy linting to maintain code quality. Here are the most useful commands:

# Basic clippy check
cargo clippy

# Strict clippy with all lints (what CI uses)
cargo clippy -- \
  -D clippy::all \
  -D clippy::pedantic \
  -D clippy::nursery \
  -D clippy::cargo

# Fix clippy warnings automatically
cargo clippy --fix

# Check specific target
cargo clippy --tests     # Just tests
cargo clippy --examples  # Just examples
cargo clippy --all       # Everything

Continuous Integration

The project uses GitHub Actions for CI with:

• Rust stable toolchain
• Format checking (cargo fmt)
• Comprehensive clippy linting
• Test execution
• Documentation build verification

The clippy configuration in CI is very strict to catch potential issues early. See .github/workflows/ci.yml for the full configuration.

Publishing Releases

To publish a new release to crates.io:

1. Update the version in Cargo.toml
2. Commit and push the version change
3. Create a GitHub release with a tag like v0.6.0 (matching the Cargo.toml version)
4. GitHub Actions will automatically publish to crates.io

Setup Required:

• Add your crates.io API token as a GitHub secret named CRATES_IO_TOKEN
• Get your token from: https://crates.io/settings/tokens

License

MIT