just-bash

A virtual bash environment with an in-memory filesystem, written in TypeScript and designed for AI agents.

Broad support for standard unix commands and bash syntax with optional curl, Python, JS/TS, and sqlite support.

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

Quick Start

npm install just-bash

import { Bash } from "just-bash";

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

Each exec() call gets its own isolated shell state — environment variables, functions, and working directory reset between calls. The filesystem is shared across calls, so files written in one exec() are visible in the next.

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 a CommandContext with fs, cwd, env, stdin, and exec (for subcommands), and work with pipes, redirections, and all shell features.

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), sqlite3 (SQLite), xan (CSV), yq (YAML/XML/TOML/CSV)

Optional Runtimes

js-exec (JavaScript/TypeScript via QuickJS; requires javascript: true), python3/python (Python via CPython; requires python: true)

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

curl, html-to-markdown (require network configuration)

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

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"
  python: true, // Enable python3/python commands
  javascript: true, // Enable js-exec command
  // Or with bootstrap: javascript: { bootstrap: "globalThis.X = 1;" }
});

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

// Pass stdin to the script
await env.exec("cat", { stdin: "hello from stdin\n" });

// Start with a clean environment
await env.exec("env", { replaceEnv: true, env: { ONLY: "this" } });

// Pass arguments without shell escaping (like spawnSync)
await env.exec("grep", { args: ["-r", "TODO", "src/"] });

// Cancel long-running scripts
const controller = new AbortController();
setTimeout(() => controller.abort(), 5000);
await env.exec("while true; do sleep 1; done", { signal: controller.signal });

// Preserve leading whitespace (e.g., for heredocs)
await env.exec("cat <<EOF\n  indented\nEOF", { rawScript: true });

exec() options:

Option	Type	Description
`env`	`Record<string, string>`	Environment variables for this execution only
`cwd`	`string`	Working directory for this execution only
`stdin`	`string`	Standard input passed to the script
`args`	`string[]`	Additional argv passed directly to the first command (bypasses shell parsing; does not change `$1`, `$2`, ...)
`replaceEnv`	`boolean`	Start with empty env instead of merging (default: `false`)
`signal`	`AbortSignal`	Cooperative cancellation; stops at next statement boundary
`rawScript`	`boolean`	Skip leading-whitespace normalization (default: `false`)

Filesystem Options

Four filesystem implementations:

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

import { Bash } from "just-bash";

const env = new Bash({
  files: {
    "/data/config.json": '{"key": "value"}',
    // Lazy: called on first read, cached. Never called if written before read.
    "/data/large.csv": () => "col1,col2\na,b\n",
    "/data/remote.txt": async () => (await fetch("https://example.com")).text(),
  },
});

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 able 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

Keep ReadWriteFs pointed at a workspace directory, not at the installed just-bash package or any other trusted runtime code. Guest-writable roots should stay separate from trusted code.

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" }) },
  ],
});

Optional Capabilities

Network Access

Network access is disabled by default. Enable it with 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"]
  },
});

// Inject credentials via header transforms (secrets never enter the sandbox)
const env = new Bash({
  network: {
    allowedUrlPrefixes: [
      "https://public-api.com", // plain string — no transforms
      {
        url: "https://ai-gateway.vercel.sh",
        transform: [{ headers: { Authorization: "Bearer secret" } }],
      },
    ],
  },
});

// 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".

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
Header transforms: Firewall headers are injected at the fetch boundary and override any user-supplied headers with the same name, preventing credential substitution from inside the sandbox. Headers are re-evaluated on each redirect so credentials are never leaked to non-transform hosts

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

JavaScript Support

JavaScript and TypeScript execution via QuickJS is opt-in due to additional security surface. Enable with javascript: true:

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

// Execute JavaScript code
await env.exec('js-exec -c "console.log(1 + 2)"');

// Run script files (.js, .mjs, .ts, .mts)
await env.exec('js-exec script.js');

// ES module mode with imports
await env.exec('js-exec -m -c "import fs from \'fs\'; console.log(fs.readFileSync(\'/data/file.txt\', \'utf8\'))"');

Bootstrap Code

Run setup code before every js-exec invocation with the bootstrap option:

const env = new Bash({
  javascript: {
    bootstrap: `
      globalThis.API_BASE = "https://api.example.com";
      globalThis.formatDate = (d) => new Date(d).toISOString();
    `,
  },
});

await env.exec('js-exec -c "console.log(API_BASE)"');
// Output: https://api.example.com

Node.js Compatibility

js-exec supports require() and import with these Node.js modules:

fs: readFileSync, writeFileSync, readdirSync, statSync, existsSync, mkdirSync, rmSync, fs.promises.*
path: join, resolve, dirname, basename, extname, relative, normalize
child_process: execSync, spawnSync
process: argv, cwd(), exit(), env, platform, version
Other modules: os, url, assert, util, events, buffer, stream, string_decoder, querystring
Globals: console, fetch, Buffer, URL, URLSearchParams

fs.readFileSync() returns a Buffer by default (matching Node.js). Pass an encoding like 'utf8' to get a string.

Note: The js-exec command only exists when javascript is configured. It is not available in browser environments. Execution runs in a QuickJS WASM sandbox with a 64 MB memory limit and configurable timeout (default: 10s, 60s with network).

Python Support

Python (CPython compiled to WASM) is opt-in due to additional security surface. Enable with python: true:

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

sqlite3 uses sql.js (SQLite compiled to WASM), sandboxed from 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.

AST Transform Plugins

Parse bash scripts into an AST, transform them, and serialize back to bash. Good for instrumenting scripts (e.g., capturing per-command stdout/stderr) or extracting metadata 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.

Integrations

AI SDK Tool

bash-tool wraps just-bash as an 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 bash-tool for more.

Vercel Sandbox Compatible API

Sandbox is a drop-in replacement for @vercel/sandbox — same API, but runs entirely in-process with the virtual filesystem. Start with just-bash for development and testing, swap in a real sandbox when you need a full VM.

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

CLI Binary

Install globally (npm install -g just-bash) for a sandboxed CLI:

# 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.

Important: The project root is mounted at /home/user/project. Use this path (or relative paths from the default cwd) to access your files inside the sandbox.

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 by default. Disable with --no-network:

pnpm shell --no-network

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 defaults. Error messages tell you which limit was hit. Increase as needed for your workload.

Security Model

The shell only has access to the provided filesystem.
All execution happens without VM isolation. This does introduce additional risk. The code base was designed to be robust against prototype-pollution attacks and other break outs to the host JS engine and filesystem.
There is no network access by default. When enabled, requests are checked against URL prefix allow-lists and HTTP-method allow-lists.
Python and JavaScript execution are off by default as they represent additional security surface.
Execution is protected against infinite loops and deep recursion with configurable limits.
Use Vercel Sandbox if you need a full VM with arbitrary binary execution.

Browser Support

The core shell (parsing, execution, filesystem, and all built-in commands) works in browser environments. The following features require Node.js and are unavailable in browsers: python3/python, sqlite3, js-exec, and OverlayFs/ReadWriteFs (which access the real filesystem).

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.

AI Agent Instructions

For AI agents, bash-tool provides additional guidance in its AGENTS.md:

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

License

Apache-2.0