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Overview

Capsule Bash is a command interpreter built for executing untrusted commands. It provides a bash-like interface to interact with the filesystem and run commands in a sandboxed environment.

  • Commands and sandboxes: Bash commands are reimplemented in TypeScript and run code inside isolated sandboxes. The sandbox layer is modular, so we can plug in any runtime that implements the interface. The default WasmRuntime uses Capsule to run commands inside WebAssembly sandboxes.

  • Instant feedback: Traditional bash treats silence as success. In agentic workflows for example, that forces a second call just to confirm the first one worked. Capsule Bash returns structured output for every command. Exit code, stdout, stderr, and a diff of filesystem changes.

  • Workspace isolation: Commands operates in a mounted workspace directory. The host filesystem is not accessible from inside the sandbox. You get full visibility into what is executed without exposing your system. The workspace is persistent and stays alive until you reset it manually.

Getting Started

TypeScript SDK

npm install @capsule-run/bash @capsule-run/bash-wasm

Run it:

import { Bash } from "@capsule-run/bash";
import { WasmRuntime } from "@capsule-run/bash-wasm";

const bash = new Bash({ runtime: new WasmRuntime() });

const result = await bash.run("mkdir src && touch src/index.ts");

console.log(result);

/**
Result {
  stdout: "Folder created ✔\nFile created ✔",
  stderr: "",
  diff: { created: ['src/index.ts'], modified: [], deleted: [] },
  duration: 10,
  exitCode: 0,
}
**/

Interactive shell

Clone the repository, then run from the project root:

pnpm -s bash-wasm-shell

Important

Python and pip are required to compile the Python sandbox. Both sandboxes (JS and Python) are needed to run the shell.

MCP server

{
  "mcpServers": {
    "capsule": {
      "command": "npx",
      "args": ["-y", "@capsule-run/bash-mcp"]
    }
  }
}

See the MCP Readme for configuration details.

Documentation

Bash Options

Parameter Description Type Default
runtime Runtime to use for the sandbox Runtime class None
customCommands Custom commands to add to the bash instance CustomCommand[] []
hostWorkspace Host workspace directory string ".capsule/session/workspace"
initialCwd Initial working directory string "/workspace"

Runtime

The runtime is the engine that runs the bash commands. WasmRuntime is available by default to run the commands in a WebAssembly sandbox.

import { Bash } from "@capsule-run/bash";
import { WasmRuntime } from "@capsule-run/bash-wasm";

const bash = new Bash({ runtime: new WasmRuntime() });

Custom Commands

import { Bash, createCommand } from "@capsule-run/bash";
import { WasmRuntime } from "@capsule-run/bash-wasm";

const firstCustomCommand = createCommand("hello", async (opts, state) => {
  return { stdout: "Hello", stderr: "", exitCode: 0 };
});

const bash = new Bash({
  runtime: new WasmRuntime(),
  customCommands: [
    firstCustomCommand,
  ],
});

Host Workspace

The host workspace is the directory on the host system that is mounted to the sandbox. It can be any folder in the project directory. By default, it is set to .capsule/session/workspace.

const bash = new Bash({ runtime: new WasmRuntime(), hostWorkspace: "customFolder" });

Initial Working Directory

The initial working directory is where bash commands are executed. By default it is set to /workspace. You can set it to any directory inside the sandbox filesystem.

const bash = new Bash({ runtime: new WasmRuntime(), initialCwd: "/" });

Run

Use the run method to execute a command in the sandbox.

const bash = new Bash({ runtime: new WasmRuntime() });

const result = await bash.run("mkdir src && touch src/index.ts");

console.log(result);
Response Format
{
  stdout: string,
  stderr: string,
  diff: { created: string[], modified: string[], deleted: string[] }, // Files and directories changes
  duration: number, // Duration in milliseconds
  exitCode: number
}

Preload

Use the preload method to warm up a sandbox before running commands. By default it preloads the JS sandbox.

const bash = new Bash({ runtime: new WasmRuntime() });

await bash.preload(); // js by default
await bash.preload("python"); // python

Reset

Use the reset method to clear the bash instance and restore the sandbox filesystem to its initial state.

const bash = new Bash({ runtime: new WasmRuntime() });

await bash.reset();

Limitations

  • WasmRuntime runs in Node.js only, browser environments are not supported with the existing runtime yet
  • Not all bash commands are implemented yet. See packages/bash/src/commands/ for the current list. Feel free to open an issue to request a new one.

Contributing

Contributions are welcome, whether it's documentation, new commands, or bug reports.

Adding or improving commands

Commands live in packages/bash/src/commands/. To contribute:

  1. Fork the repository
  2. Create a feature branch: git checkout -b feature/amazing-command
  3. Add or update your command in packages/bash/src/commands/
  4. Add unit tests
  5. Open a pull request

License

Apache License 2.0. See LICENSE for details.