esh - Embeddable Shell

Note: This is pre-release / alpha software, use at your own discretion. Notably, at the time of writing, the actual REPL mode is missing, as well as sufficient testing, panic-safety, and library documention.

A Rust library for building interactive, command-driven CLI applications. esh provides a shell framework that handles argument parsing, command dispatch, VFS integration, and structured logging -- so you can focus on defining commands rather than wiring up boilerplate.

The crate ships both a library (esh) and a reference binary (esh) that demonstrates how to assemble a working shell from the library's building blocks.

Library

The library is backend-agnostic. It has no dependency on any concrete VFS crate -- it defines a Vfs trait that consumers implement.

Modules:

• shell -- The core framework. ShellConfig is a builder that registers CLI arguments, subcommands, command handlers, and an optional VFS lookup. Calling .build() produces an Arc<dyn Shell> that can be .run()'d. Built-in commands include version, exit, and (when a VFS is configured) pwd.
• parse -- A POSIX-like shell parser. shell_parse_line() splits a string into words honoring single quotes, double quotes, backslash escapes, # comments, and line continuations. shell_parse_arg() processes escape sequences in a single argument. Supported escapes include \n, \t, \xHH (hex bytes), \u{H..H} (Unicode scalars), and \0ooo (octal).
• util -- Logging initialization via tracing + tracing-subscriber with ENV_FILTER support, the die! macro for fatal exits, and pluralize! for simple English pluralization.

Key traits:

• Shell -- Run a shell from env::args() or a supplied arg iterator.
• Vfs -- Backend-agnostic virtual filesystem interface (fn cwd(&self) -> &Path). Implement this for any FS backend you like.

Binary

The reference binary demonstrates how to wire up the library:

1. Defines a -p/--path CLI argument that validates and canonicalizes a directory path.
2. Implements Vfs for vfs-kit's DirFS via a thin DirFsVfs newtype adapter.
3. Registers a VFS lookup function that creates the DirFS from the parsed path.
4. Builds and runs the shell.

Usage

# Run a command against a directory
esh -p /some/directory pwd

# Print version
esh version

# Verbose logging (-v, -vv, -vvv for increasing detail)
esh -v -p . pwd

# Quiet mode (errors only)
esh -q -p . pwd

Shell Parser

The parser in parse.rs follows POSIX shell quoting conventions:

use esh::{shell_parse_line, shell_parse_arg};

let words = shell_parse_line(r#"echo "hello world" foo\ bar"#)?;
// => ["echo", "hello world", "foo bar"]

let arg = shell_parse_arg(r"\x48\x65\x6c\x6c\x6f")?;
// => "Hello"

Extending with Custom Commands

Use the ShellConfig builder to register your own arguments, subcommands, and handlers:

use std::sync::Arc;
use esh::{shell_config, Shell};

let cfg = shell_config!()
    .cli_args(Arc::new(MyArgs::augment_args))
    .cli_cmds(Arc::new(MyCommands::augment_subcommands))
    .cli_handler(Arc::new(my_handler));

let sh = cfg.build();
sh.run();

Plugging in a VFS Backend

The library's Vfs trait is intentionally minimal so you can bring any filesystem backend:

use std::path::Path;
use esh::Vfs;

struct MyVfs { /* ... */ }

impl Vfs for MyVfs {
    fn cwd(&self) -> &Path {
        // return current working directory within the virtual FS
        todo!()
    }
}

Register it via .vfs_lookup() on ShellConfig:

fn create_my_vfs(matches: &ArgMatches) -> Option<Box<dyn Vfs>> {
    let path = matches.get_one::<PathBuf>("my_path")?;
    Some(Box::new(MyVfs::new(path)))
}

let cfg = shell_config!()
    .vfs_lookup(Arc::new(create_my_vfs));

When a VFS is configured, the shell automatically enables the vfs-aware command, e.g. pwd.

Building

cargo build
cargo test
cargo run -- -p . version

Authors

This library is written by Michael Wildpaner.

Acknowledgments

This project is made possible thanks to the incredible work of the Rust community and the maintainers of the following crates:

• clap – For providing the gold standard in command-line argument parsing.
• tracing – For structured, scoped, and async-aware diagnostics.
• vfs-kit – For the elegant virtual file system abstractions so I don't have to.

I'm grateful to the authors and contributors of these libraries for their dedication to the Rust ecosystem.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE)
• MIT license (LICENSE-MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.