just-bash

A simulated bash environment with an in-memory virtual filesystem, written in TypeScript.

Designed for AI agents that need a secure, sandboxed bash environment.

Supports optional network access via curl with secure-by-default URL filtering.

Note: This is beta software. Use at your own risk and please provide feedback.

Table of Contents

• Security model
• Installation
• Usage
  • Basic API
  • Configuration
  • Custom Commands
  • Filesystem Options
  • AI SDK Tool
  • Vercel Sandbox Compatible API
  • CLI Binary
  • Interactive Shell
• Supported Commands
• Shell Features
• Default Layout
• Network Access
• Execution Protection
• AST Transform Plugins
• Development

Security model

• The shell only has access to the provided file system.
• Execution is protected against infinite loops or recursion. However, Bash is not fully robust against DOS from input. If you need to be robust against this, use process isolation at the OS level.
• Binaries or even WASM are inherently unsupported (Use Vercel Sandbox or a similar product if a full VM is needed).
• There is no network access by default.
• Network access can be enabled, but requests are checked against URL prefix allow-lists and HTTP-method allow-lists. See network access for details

Installation

npm install just-bash

Usage

Basic API

import { Bash } from "just-bash";

const env = new Bash();
await env.exec('echo "Hello" > greeting.txt');
const result = await env.exec("cat greeting.txt");
console.log(result.stdout); // "Hello\n"
console.log(result.exitCode); // 0
console.log(result.env); // Final environment after execution

Each exec() is isolated—env vars, functions, and cwd don't persist across calls (filesystem does).

Configuration

const env = new Bash({
  files: { "/data/file.txt": "content" }, // Initial files
  env: { MY_VAR: "value" }, // Initial environment
  cwd: "/app", // Starting directory (default: /home/user)
  executionLimits: { maxCallDepth: 50 }, // See "Execution Protection"
});

// Per-exec overrides
await env.exec("echo $TEMP", { env: { TEMP: "value" }, cwd: "/tmp" });

Lazy Files

File values can be functions (sync or async). The function is called on first read and the result is cached — if the file is written to before being read, the function is never called:

const env = new Bash({
  files: {
    "/data/config.json": () => JSON.stringify({ key: "value" }),
    "/data/remote.txt": async () => (await fetch("https://example.com")).text(),
    "/data/static.txt": "always loaded",
  },
});

This is useful for large or expensive-to-compute content that may not be needed.

Custom Commands

Extend just-bash with your own TypeScript commands using defineCommand:

import { Bash, defineCommand } from "just-bash";

const hello = defineCommand("hello", async (args, ctx) => {
  const name = args[0] || "world";
  return { stdout: `Hello, ${name}!\n`, stderr: "", exitCode: 0 };
});

const upper = defineCommand("upper", async (args, ctx) => {
  return { stdout: ctx.stdin.toUpperCase(), stderr: "", exitCode: 0 };
});

const bash = new Bash({ customCommands: [hello, upper] });

await bash.exec("hello Alice"); // "Hello, Alice!\n"
await bash.exec("echo 'test' | upper"); // "TEST\n"

Custom commands receive the full CommandContext with access to fs, cwd, env, stdin, and exec for running subcommands.

Filesystem Options

Four filesystem implementations are available:

InMemoryFs (default) - Pure in-memory filesystem, no disk access:

import { Bash } from "just-bash";
const env = new Bash(); // Uses InMemoryFs by default

OverlayFs - Copy-on-write over a real directory. Reads come from disk, writes stay in memory:

import { Bash } from "just-bash";
import { OverlayFs } from "just-bash/fs/overlay-fs";

const overlay = new OverlayFs({ root: "/path/to/project" });
const env = new Bash({ fs: overlay, cwd: overlay.getMountPoint() });

await env.exec("cat package.json"); // reads from disk
await env.exec('echo "modified" > package.json'); // stays in memory

ReadWriteFs - Direct read-write access to a real directory. Use this if you want the agent to be agle to write to your disk:

import { Bash } from "just-bash";
import { ReadWriteFs } from "just-bash/fs/read-write-fs";

const rwfs = new ReadWriteFs({ root: "/path/to/sandbox" });
const env = new Bash({ fs: rwfs });

await env.exec('echo "hello" > file.txt'); // writes to real filesystem

MountableFs - Mount multiple filesystems at different paths. Combines read-only and read-write filesystems into a unified namespace:

import { Bash, MountableFs, InMemoryFs } from "just-bash";
import { OverlayFs } from "just-bash/fs/overlay-fs";
import { ReadWriteFs } from "just-bash/fs/read-write-fs";

const fs = new MountableFs({ base: new InMemoryFs() });

// Mount read-only knowledge base
fs.mount("/mnt/knowledge", new OverlayFs({ root: "/path/to/knowledge", readOnly: true }));

// Mount read-write workspace
fs.mount("/home/agent", new ReadWriteFs({ root: "/path/to/workspace" }));

const bash = new Bash({ fs, cwd: "/home/agent" });

await bash.exec("ls /mnt/knowledge"); // reads from knowledge base
await bash.exec("cp /mnt/knowledge/doc.txt ./"); // cross-mount copy
await bash.exec('echo "notes" > notes.txt'); // writes to workspace

You can also configure mounts in the constructor:

import { MountableFs, InMemoryFs } from "just-bash";
import { OverlayFs } from "just-bash/fs/overlay-fs";
import { ReadWriteFs } from "just-bash/fs/read-write-fs";

const fs = new MountableFs({
  base: new InMemoryFs(),
  mounts: [
    { mountPoint: "/data", filesystem: new OverlayFs({ root: "/shared/data" }) },
    { mountPoint: "/workspace", filesystem: new ReadWriteFs({ root: "/tmp/work" }) },
  ],
});

AI SDK Tool

For AI agents, use bash-tool which is optimized for just-bash and provides a ready-to-use AI SDK tool:

npm install bash-tool

import { createBashTool } from "bash-tool";
import { generateText } from "ai";

const bashTool = createBashTool({
  files: { "/data/users.json": '[{"name": "Alice"}, {"name": "Bob"}]' },
});

const result = await generateText({
  model: "anthropic/claude-sonnet-4",
  tools: { bash: bashTool },
  prompt: "Count the users in /data/users.json",
});

See the bash-tool documentation for more details and examples.

Vercel Sandbox Compatible API

Bash provides a Sandbox class that's API-compatible with @vercel/sandbox, making it easy to swap implementations. You can start with Bash and switch to a real sandbox when you need the power of a full VM (e.g. to run node, python, or custom binaries).

import { Sandbox } from "just-bash";

// Create a sandbox instance
const sandbox = await Sandbox.create({ cwd: "/app" });

// Write files to the virtual filesystem
await sandbox.writeFiles({
  "/app/script.sh": 'echo "Hello World"',
  "/app/data.json": '{"key": "value"}',
});

// Run commands and get results
const cmd = await sandbox.runCommand("bash /app/script.sh");
const output = await cmd.stdout(); // "Hello World\n"
const exitCode = (await cmd.wait()).exitCode; // 0

// Read files back
const content = await sandbox.readFile("/app/data.json");

// Create directories
await sandbox.mkDir("/app/logs", { recursive: true });

// Clean up (no-op for Bash, but API-compatible)
await sandbox.stop();

CLI Binary

After installing globally (npm install -g just-bash), use the just-bash command as a secure alternative to bash for AI agents:

# Execute inline script
just-bash -c 'ls -la && cat package.json | head -5'

# Execute with specific project root
just-bash -c 'grep -r "TODO" src/' --root /path/to/project

# Pipe script from stdin
echo 'find . -name "*.ts" | wc -l' | just-bash

# Execute a script file
just-bash ./scripts/deploy.sh

# Get JSON output for programmatic use
just-bash -c 'echo hello' --json
# Output: {"stdout":"hello\n","stderr":"","exitCode":0}

The CLI uses OverlayFS - reads come from the real filesystem, but all writes stay in memory and are discarded after execution. The project root is mounted at /home/user/project.

Options:

• -c <script> - Execute script from argument
• --root <path> - Root directory (default: current directory)
• --cwd <path> - Working directory in sandbox
• -e, --errexit - Exit on first error
• --json - Output as JSON

Interactive Shell

pnpm shell

The interactive shell has full internet access enabled by default, allowing you to use curl to fetch data from any URL. Use --no-network to disable this:

pnpm shell --no-network

Supported Commands

File Operations

cat, cp, file, ln, ls, mkdir, mv, readlink, rm, rmdir, split, stat, touch, tree

Text Processing

awk, base64, column, comm, cut, diff, expand, fold, grep (+ egrep, fgrep), head, join, md5sum, nl, od, paste, printf, rev, rg, sed, sha1sum, sha256sum, sort, strings, tac, tail, tr, unexpand, uniq, wc, xargs

Data Processing

jq (JSON), python3/python (Python via Pyodide; required opt-in), sqlite3 (SQLite), xan (CSV), yq (YAML/XML/TOML/CSV)

Compression & Archives

gzip (+ gunzip, zcat), tar

Navigation & Environment

basename, cd, dirname, du, echo, env, export, find, hostname, printenv, pwd, tee

Shell Utilities

alias, bash, chmod, clear, date, expr, false, help, history, seq, sh, sleep, time, timeout, true, unalias, which, whoami

Network Commands

curl, html-to-markdown

All commands support --help for usage information.

Shell Features

• Pipes: cmd1 | cmd2
• Redirections: >, >>, 2>, 2>&1, <
• Command chaining: &&, ||, ;
• Variables: $VAR, ${VAR}, ${VAR:-default}
• Positional parameters: $1, $2, $@, $#
• Glob patterns: *, ?, [...]
• If statements: if COND; then CMD; elif COND; then CMD; else CMD; fi
• Functions: function name { ... } or name() { ... }
• Local variables: local VAR=value
• Loops: for, while, until
• Symbolic links: ln -s target link
• Hard links: ln target link

Default Layout

When created without options, Bash provides a Unix-like directory structure:

• /home/user - Default working directory (and $HOME)
• /bin - Contains stubs for all built-in commands
• /usr/bin - Additional binary directory
• /tmp - Temporary files directory

Commands can be invoked by path (e.g., /bin/ls) or by name.

Network Access

Network access (and the curl command) is disabled by default for security. To enable it, configure the network option:

// Allow specific URLs with GET/HEAD only (safest)
const env = new Bash({
  network: {
    allowedUrlPrefixes: [
      "https://api.github.com/repos/myorg/",
      "https://api.example.com",
    ],
  },
});

// Allow specific URLs with additional methods
const env = new Bash({
  network: {
    allowedUrlPrefixes: ["https://api.example.com"],
    allowedMethods: ["GET", "HEAD", "POST"], // Default: ["GET", "HEAD"]
  },
});

// Allow all URLs and methods (use with caution)
const env = new Bash({
  network: { dangerouslyAllowFullInternetAccess: true },
});

Note: The curl command only exists when network is configured. Without network configuration, curl returns "command not found".

Python Support

Python support via Pyodide is opt-in due to additional security surface. Enable it explicitly, but be aware of the risk:

const env = new Bash({
  python: true,
});

// Execute Python code
await env.exec('python3 -c "print(1 + 2)"');

// Run Python scripts
await env.exec('python3 script.py');

Note: The python3 and python commands only exist when python: true is configured. Python is not available in browser environments.

SQLite Support

The sqlite3 command uses sql.js (WASM-based SQLite) which is fully sandboxed and cannot access the real filesystem:

const env = new Bash();

// Query in-memory database
await env.exec('sqlite3 :memory: "SELECT 1 + 1"');

// Query file-based database
await env.exec('sqlite3 data.db "SELECT * FROM users"');

Note: SQLite is not available in browser environments. Queries run in a worker thread with a configurable timeout (default: 5 seconds) to prevent runaway queries from blocking execution.

Allow-List Security

The allow-list enforces:

• Origin matching: URLs must match the exact origin (scheme + host + port)
• Path prefix: Only paths starting with the specified prefix are allowed
• HTTP method restrictions: Only GET and HEAD by default (configure allowedMethods for more)
• Redirect protection: Redirects to non-allowed URLs are blocked

Using curl

# Fetch and process data
curl -s https://api.example.com/data | grep pattern

# Download and convert HTML to Markdown
curl -s https://example.com | html-to-markdown

# POST JSON data
curl -X POST -H "Content-Type: application/json" \
  -d '{"key":"value"}' https://api.example.com/endpoint

Execution Protection

Bash protects against infinite loops and deep recursion with configurable limits:

const env = new Bash({
  executionLimits: {
    maxCallDepth: 100, // Max function recursion depth
    maxCommandCount: 10000, // Max total commands executed
    maxLoopIterations: 10000, // Max iterations per loop
    maxAwkIterations: 10000, // Max iterations in awk programs
    maxSedIterations: 10000, // Max iterations in sed scripts
  },
});

All limits have sensible defaults. Error messages include hints on which limit to increase. Feel free to increase if your scripts intentionally go beyond them.

AST Transform Plugins

Parse bash scripts into an AST, run transform plugins, and serialize back to executable bash. Useful for instrumenting scripts (e.g., capturing per-command stdout/stderr) or analyzing them (e.g., extracting command names) before execution.

import { Bash, BashTransformPipeline, TeePlugin, CommandCollectorPlugin } from "just-bash";

// Standalone pipeline — output can be run by any shell
const pipeline = new BashTransformPipeline()
  .use(new TeePlugin({ outputDir: "/tmp/logs" }))
  .use(new CommandCollectorPlugin());
const result = pipeline.transform("echo hello | grep hello");
result.script;             // transformed bash string
result.metadata.commands;  // ["echo", "grep", "tee"]

// Integrated API — exec() auto-applies transforms and returns metadata
const bash = new Bash();
bash.registerTransformPlugin(new CommandCollectorPlugin());
const execResult = await bash.exec("echo hello | grep hello");
execResult.metadata?.commands; // ["echo", "grep"]

See src/transform/README.md for the full API, built-in plugins, and how to write custom plugins.

Development

pnpm test        # Run tests in watch mode
pnpm test:run    # Run tests once
pnpm typecheck   # Type check without emitting
pnpm build       # Build TypeScript
pnpm shell       # Run interactive shell

AI Agent Instructions

For AI agents, we recommend using bash-tool which is optimized for just-bash and provides additional guidance in its AGENTS.md:

cat node_modules/bash-tool/dist/AGENTS.md

License

Apache-2.0