Rable

A complete GNU Bash 5.3-compatible parser, written in Rust.

Rable is a from-scratch reimplementation of Parable — the excellent Python-based bash parser by @ldayton. It produces identical S-expression output and provides a drop-in replacement Python API via PyO3.

Acknowledgments

This project would not exist without Parable.

Parable is a remarkable piece of work — a complete, well-tested bash parser that produces clean S-expression AST output validated against bash's own internal parser. Its comprehensive test suite (1,604 tests across 36 files) defines the gold standard for bash parsing correctness, and Rable's compatibility is measured entirely against it.

We are deeply grateful to @ldayton for:

• Building a high-quality, MIT-licensed bash parser that others can learn from and build upon
• Creating the bash-oracle approach that validates parser output against bash itself
• Maintaining the extensive .tests corpus that made Rable's development possible
• Designing the clean S-expression output format that Rable faithfully reproduces

Rable exists because Parable showed the way. Thank you.

Compatibility

Performance

Rable is approximately 9.5x faster than Parable across all test inputs:

Run just benchmark to reproduce these results on your machine.

Installation

As a Rust library

[dependencies]
rable = "0.1"

As a Python package

pip install rable

Or build from source:

just setup       # creates venv, builds bindings, installs Parable
just develop     # rebuild after code changes

Usage

Rust

use rable::parse;

fn main() {
    let nodes = parse("echo hello | grep h", false).unwrap();
    for node in &nodes {
        println!("{node}");
    }
    // Output: (pipe (command (word "echo") (word "hello")) (command (word "grep") (word "h")))
}

Python

from rable import parse, ParseError, MatchedPairError

# Parse bash source into AST nodes
nodes = parse('if [ -f file ]; then cat file; fi')
for node in nodes:
    print(node.to_sexp())
# Output: (if (command (word "[") (word "-f") (word "file") (word "]")) (command (word "cat") (word "file")))

# Errors are raised as exceptions
try:
    parse('if')
except ParseError as e:
    print(f"Syntax error: {e}")

# Enable extended glob patterns
nodes = parse('echo @(foo|bar)', extglob=True)

The Python API is a drop-in replacement for Parable:

# Before (Parable)
from parable import parse, ParseError, MatchedPairError

# After (Rable) — same API, ~10x faster
from rable import parse, ParseError, MatchedPairError

Development

Prerequisites

• Rust 1.93+ (see rust-toolchain.toml)
• Python 3.12+ (for Python bindings)
• just (task runner)

Quick start

just              # format, lint, test
just check        # same as above
just test-parable # run full Parable compatibility suite
just setup        # set up Python environment + benchmarks
just benchmark    # compare performance vs Parable

Available commands

Architecture

Rable is a hand-written recursive descent parser with a context-sensitive lexer:

Design principles

1. Compatibility is correctness — output matches Parable's S-expressions exactly
2. If it is not tested, it is not shipped — 1,604 integration tests + unit tests
3. Simplicity is king — solve problems with least complexity
4. Correctness over speed — match bash-oracle behavior, optimize later

Contributing

Contributions are welcome! Please ensure:

1. All tests pass: just check must succeed
2. Parable compatibility: just test-parable must show 1604/1604
3. Code quality: No clippy warnings (just clippy)
4. Formatting: Code is formatted (just fmt)

Adding new features

If you're adding support for new bash syntax:

1. Add test cases to the appropriate .tests file in tests/parable/
2. Implement the lexer/parser changes
3. Verify S-expression output matches what Parable would produce
4. Run just test-parable to confirm no regressions

Code limits

License

MIT License. See LICENSE for details.

Disclosure

Rable is a complete reimplementation of Parable in Rust. It was built by studying Parable's test suite and output format, not by translating Parable's Python source code. The test corpus (tests/parable/*.tests) originates from the Parable project and is used under its MIT license.