just-bash is a TypeScript-based simulated bash environment with an in-memory virtual filesystem, designed specifically for AI agents that need secure, sandboxed command-line access. The project, developed by Vercel Labs, provides a robust alternative to running actual bash shells in production AI applications.

Built with security as a primary concern, just-bash implements multiple protective layers including execution protection against infinite loops, network access controls through URL prefix allow-lists, and filesystem isolation. The environment supports optional network access via `curl` with secure-by-default filtering, while binaries and WASM are intentionally unsupported to prevent security risks. Developers can choose from four filesystem implementations: InMemoryFs for pure memory operations, OverlayFs for copy-on-write over real directories, ReadWriteFs for direct disk access, and MountableFs for combining multiple filesystems.

The tool offers extensive configuration options including custom commands via TypeScript, lazy file loading for expensive content, and compatibility with the Vercel Sandbox API. With 1.2k stars on GitHub and active development, just-bash provides AI developers with a production-ready solution for safely executing bash commands without the security risks of traditional shell environments.