just-bash v3.6.0
A TypeScript bash interpreter with in-memory filesystem.
Designed for AI agents needing a secure, sandboxed environment.
Custom commands:
about About just-bash
install Installation instructions
github GitHub repository
Or try any bash command: ls, cat, echo, grep, awk, jq, sed, etc.
Type 'help' for a list of all built-in commands.
npm install just-bash
Usage:
import { Bash } from "just-bash";
const bash = new Bash();
const result = await bash.exec("echo hello");
https://github.com/vercel-labs/just-bash
# 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](#security-model).
## Quick Start
```bash
npm install just-bash
```
```typescript
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`:
```typescript
import { Bash, decodeBytesToUtf8, 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) => {
// ctx.stdin is a ByteString — decode to text before string ops.
return {
stdout: decodeBytesToUtf8(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 command callbacks receive a `ResolvedCommandContext` with `fs`, `cwd`,
`env`, `stdin`, resolved `limits`, and `exec` (for subcommands), and work with
pipes, redirections, and all shell features. The legacy `CommandContext` remains
available for standalone context inputs; use `createCommandContext({ fs })` when
calling a command directly with a fully resolved context.
Host-provided commands preserve the legacy trusted default whether supplied to
the `Bash` constructor, declared through `defineCommand`, loaded lazily, or
added later with `bash.registerCommand()`. Set `trusted: false` (or use
`defineCommand(name, execute, { trusted: false })`) to select the restricted
extension boundary. Trusted commands run in the embedding process and should
never execute guest-provided JavaScript.
Every invocation is bound by `maxExecutionTimeMs`. On cancellation, just-bash
revokes the command context immediately; `maxExtensionCleanupTimeMs` only
bounds how long it waits for the now-authority-free command promise to settle.
A command that is still resolving, such as one whose module is loading, is
never started and reports cancellation immediately instead.
A late continuation cannot use `ctx.fs`, `ctx.env`, `ctx.exec`, or other context
capabilities. Cleanup work that must run at scope closure can be registered with
`ctx.executionScope.registerCleanup()`. A cleanup failure is returned as a
generic exit-126 shell result rather than rejecting `Bash.exec()` or exposing
host error details. JavaScript cannot forcibly stop arbitrary host code, so
extensions requiring a hard guarantee against external side effects must run
in a terminable worker or process. Tests that invoke command objects directly
can use `createCommandContext({ fs })` to get a fully resolved context without
duplicating internal defaults.
<details>
<summary><h2>Supported Commands</h2></summary>
### File Operations
`cat`, `cp`, `file`, `ln`, `ls`, `mkdir`, `mktemp`, `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`, `yes`
### Network
`curl`, `html-to-markdown` (require [network configuration](#network-access))
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`
</details>
## Configuration
```typescript
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 $TEMP", { env: { TEMP: "value" }, cwd: "/tmp" });
// Pass stdin to the script
await env.exec("cat", { stdin: "hello from stdin\n" });
// Start with a clean environment
await env.exec("env", { replaceEnv: true, env: { ONLY: "this" } });
// Pass arguments without shell escaping (like spawnSync)
await env.exec("grep", { args: ["-r", "TODO", "src/"] });
// Cancel long-running scripts
const controller = new AbortController();
setTimeout(() => controller.abort(), 5000);
await env.exec("while true; do sleep 1; done", { signal: controller.signal });
// Preserve leading whitespace (e.g., for heredocs)
await env.exec("cat <<EOF\n indented\nEOF", { rawScript: true });
```
### Timezone
`date` defaults to UTC (`%Z=UTC`, `%z=+0000`) regardless of the host clock, so the sandbox does not leak the host timezone. To opt into a specific zone, pass `TZ` as an initial env var:
```typescript
const bash = new Bash({ env: { TZ: "America/New_York" } });
await bash.exec("date"); // Mon Jun 1 09:30:00 EDT 2026
```
`-u` always forces UTC; an unset or invalid `$TZ` falls back to UTC. Setting `TZ` exposes that timezone to scripts running in the sandbox, so only pass a value you are comfortable revealing — forwarding the host's real `$TZ` (e.g. `process.env.TZ`) reintroduces the disclosure that the UTC default exists to prevent.
`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:
```typescript
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:
```typescript
import { Bash } from "just-bash";
import { OverlayFs } from "just-bash/fs/overlay-fs";
const overlay = new OverlayFs({
root: "/path/to/project",
// Copy-on-write data is bounded independently from real-file reads.
maxMemoryBytes: 256 * 1024 * 1024,
});
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
```
`maxMemoryBytes` defaults to 1 GiB and covers aggregate files retained in the
copy-on-write layer, including append chunks. Set it to the deployment's memory
budget when an `OverlayFs` is reused across executions.
**ReadWriteFs** - Direct read-write access to a real directory. Use this if you want the agent to be able to write to your disk:
```typescript
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.
`ReadWriteFs` uses normal in-place filesystem operations for private regular
files. For multiply-linked regular files, it isolates append and metadata
changes by copying the file and replacing only the sandbox directory entry, so
a host-created hard link cannot carry those changes beyond the configured root.
Implicit copies are limited by `maxCopyOnWriteSize` (100 MB by default; set it
to `0` to disable the limit). Overwrite does not need to copy existing content.
Explicit `cp` copies can be limited with the opt-in `maxCopySize` option
(unlimited by default). The portable copy path may materialize sparse-file
holes, so embeddings that require a disk-allocation bound should configure
`maxCopySize`.
Shared-inode isolation has a few deliberate limitations:
- Append, `chmod`, and `utimes` on a multiply-linked regular file require read
access to the file and write access to its parent directory. They fail with
`EFBIG` when the file exceeds `maxCopyOnWriteSize`.
- Copies use `O_NOATIME` when the Node.js runtime exposes it and retry with
normal read semantics if the kernel returns `EPERM`. Runtimes and platforms
without `O_NOATIME` may update access-time metadata visible through another
hard link.
- Do not mutate a `ReadWriteFs` root concurrently through direct host filesystem
APIs. Node.js does not expose the descriptor-relative operations needed to
make pathname validation atomic against an external actor. A concurrent host
append to a multiply-linked file may be lost when the isolated entry is
replaced.
- Mutations in overlapping `ReadWriteFs` roots are serialized within the
process. Unrelated roots proceed independently. The queue is not cancellable
or bounded, so a large mutation can delay later operations in overlapping
roots even if the requesting script is subsequently aborted.
- Content writes and appends to FIFOs, sockets, devices, and other special files
are rejected. This avoids indefinitely occupying an overlapping-root mutation
slot on a blocking special-file open. Metadata operations remain supported
for single-link special files; multiply-linked special files are rejected
because they cannot be isolated without changing their file type.
- Private-file and single-link special-file metadata operations use pathname
APIs to preserve normal host permission semantics. They are not atomic
against a trusted host actor concurrently replacing that pathname with a
symlink. With `allowSymlinks: false`, symlinks present during normal path
validation are still rejected.
- Copying a symlink preserves whether its guest target is absolute or relative.
Only symlinks whose resolved targets remain inside the root are copied.
- Regular-file copies replace the destination entry to prevent writes through
hard links. Existing destinations must still be writable, and their parent
directory must be writable so the isolated entry can be committed. Thus a
writable destination in a non-writable directory cannot be copied over.
Copying over a FIFO, socket, device, or other special entry is rejected.
**MountableFs** - Mount multiple filesystems at different paths. Combines read-only and read-write filesystems into a unified namespace:
```typescript
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:
```typescript
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:
```typescript
// 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
```bash
# 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`:
```typescript
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:
```typescript
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 uses the `run` package's QuickJS sandbox with a 64 MB memory limit and configurable timeout (30 seconds in the default `normal` profile and 10 seconds in the opt-in `hardened` profile). Enabling network access does not extend the configured deadline.
#### Tool Invocation Hook
`js-exec` scripts can call host-defined tools through a global `tools` proxy
when `javascript.invokeTool` is provided:
```typescript
const bash = new Bash({
javascript: {
// path: "math.add" (dot-separated)
// argsJson: '{"a":1,"b":2}' (or "" for no args)
// return: JSON-stringified result, or "" for undefined
// throw: propagates as a sandbox exception
invokeTool: async (path, argsJson, abortSignal) => {
const args = argsJson ? JSON.parse(argsJson) : undefined;
if (path === "math.add") {
abortSignal.throwIfAborted();
return JSON.stringify({ sum: args.a + args.b });
}
throw new Error(`Unknown tool: ${path}`);
},
},
});
await bash.exec(`js-exec -c 'console.log((await tools.math.add({a:3,b:4})).sum)'`);
```
The `abortSignal` fires when the JavaScript execution is canceled or times
out. Tool implementations should forward it to network requests and other
cancelable work so effects do not outlive the sandbox execution.
The hook is generic — wire any tool framework through it (raw maps, MCP,
Anthropic tool-use, etc.). For full GraphQL / OpenAPI / MCP discovery via
`@executor-js/sdk`, plus auto-generated bash namespace commands, use the
companion package
[**`@just-bash/executor`**](../just-bash-executor/README.md).
### Python Support
Python (CPython compiled to WASM) is opt-in due to additional security surface. Enable with `python: true`:
```typescript
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:
```typescript
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 (30 seconds in the default `normal` profile and 5 seconds in the opt-in `hardened` profile) 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.
```typescript
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](src/transform/README.md) for the full API, built-in plugins, and how to write custom plugins.
## Integrations
### AI SDK Tool
[`bash-tool`](https://github.com/vercel-labs/bash-tool) wraps just-bash as an [AI SDK](https://ai-sdk.dev/) tool:
```bash
npm install bash-tool
```
```typescript
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](https://github.com/vercel-labs/bash-tool) for more.
### Vercel Sandbox Compatible API
`Sandbox` is a drop-in replacement for [`@vercel/sandbox`](https://vercel.com/docs/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.
```typescript
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:
```bash
# 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
- `--allow-write` - Allow write operations (in memory only; read-only by default)
- `-e, --errexit` - Exit on first error
- `--json` - Output as JSON
### Interactive Shell
```bash
pnpm shell
```
The interactive shell has network access disabled by default. Enable it with `--network`:
```bash
pnpm shell --network
```
## Execution Protection
Bash protects against infinite loops and deep recursion with configurable limits:
```typescript
const env = new Bash({
// `normal` is the liberal, compatibility-oriented default. Use `hardened`
// for tighter untrusted-workload policy, then override individual resources.
executionLimitProfile: "hardened",
executionLimits: {
maxCallDepth: 100, // Max function recursion depth
maxCommandCount: 20000, // Shared across nested execution
maxSourceBytes: 8 * 1024 * 1024, // Shell source before parsing
maxFileSystemBytes: 256 * 1024 * 1024, // Retained default-FS data
maxOutputSize: 32 * 1024 * 1024, // Aggregate stdout + stderr bytes
maxArchiveBytes: 256 * 1024 * 1024, // Expanded archive bytes
maxDatabaseBytes: 128 * 1024 * 1024, // SQLite image bytes
maxExecutionTimeMs: 30_000, // Whole execution wall-clock deadline
maxExtensionCleanupTimeMs: 25, // Cancellation acknowledgement grace
},
});
```
All resources remain bounded by default in both profiles. Explicit values
override the selected profile; non-negative safe integers and the legacy
`Infinity` spelling are accepted. Infinite deadlines omit the corresponding
platform timer rather than overflowing it; `js-exec` maps an infinite
JavaScript deadline to `run`'s longest timeout (about 24.9 days). Invalid
values are rejected when `Bash` is constructed. Error messages identify the
resource that was hit.
## Security Model
The Node.js package requires Node `>=20.19`.
- 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.
- `js-exec` guest code runs inside the `run` package's QuickJS/WASM realm. Its
primary isolation boundary is QuickJS plus the validated, bounded host
bridge; guest JavaScript does not execute in the Node worker realm.
- Execution is protected against infinite loops and deep recursion with configurable limits.
- Host-realm defense-in-depth uses the strongest scoped controls available on
each supported Node runtime. Where `node:module.registerHooks()` is present,
builtin ESM imports can also be denied only for the untrusted async context;
older runtimes retain best-effort scoped protection without failing existing
applications. It never installs a process-global deny-all loader. Query the
resolved capabilities with `DefenseInDepthBox.getInstance().getStatus()`.
Audit mode reports `level: "none"` because it records violations without
enforcing them.
- Scoped defense uses reversible proxies for `Reflect`, `JSON`, and `Math` and
restores their host descriptors on deactivation. This is reported as
`intrinsicProtection: "scoped-best-effort"`: same-realm JavaScript that
cached an intrinsic or a mutation function before activation cannot be fully
revoked (including the direct `delete` operator). The separately named
`processLifetimeIntrinsicHardening: true` option permanently freezes those
objects and locks selected well-known Symbol descriptors; use it only in a
disposable or process-lifetime realm. Use an isolated worker/process when
complete protection and reversible host state are both required.
- Node worker `resourceLimits` do not reliably cap the WebAssembly linear memory used by CPython or sql.js. Queue, deadline, file, database, bridge, and payload limits reduce exposure, but strong memory containment for these opt-in runtimes requires process/container isolation or a WASM build with a lower hard maximum.
- Use [Vercel Sandbox](https://vercel.com/docs/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`](https://github.com/vercel-labs/bash-tool) provides additional guidance in its `AGENTS.md`:
```bash
cat node_modules/bash-tool/dist/AGENTS.md
```
## License
Apache-2.0
Apache License
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http://www.apache.org/licenses/
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<!--
This file is distributed as dist/AGENTS.md in the npm package.
It provides instructions for AI agents using just-bash in their projects.
The build process copies this file to dist/AGENTS.md (removing this comment).
TypeScript and bash examples are validated by src/readme.test.ts.
-->
# AGENTS.md - just-bash
Instructions for AI agents using just-bash in projects.
## What is just-bash?
A sandboxed bash interpreter with an in-memory virtual filesystem. Use it when you need to:
- Execute shell commands without real filesystem access
- Run untrusted scripts safely
- Process text with standard Unix tools (grep, sed, awk, jq, etc.)
## For AI Agents
If you're building an AI agent that needs a bash tool, use [`bash-tool`](https://github.com/vercel-labs/bash-tool) which is optimized for just-bash:
```sh
npm install bash-tool
```
```typescript
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](https://github.com/vercel-labs/bash-tool) for more details.
## Quick Reference
```typescript
import { Bash } from "just-bash";
const bash = new Bash({
files: { "/data/input.txt": "content" }, // Initial files
cwd: "/data", // Working directory
});
const result = await bash.exec("cat input.txt | grep pattern");
// result.stdout - command output
// result.stderr - error output
// result.exitCode - 0 = success, non-zero = failure
```
## Key Behaviors
1. **Isolation**: Each `exec()` call is isolated. Environment variables, functions, and cwd changes don't persist between calls. Only filesystem changes persist.
2. **No real filesystem**: By default, commands only see the virtual filesystem. Use `OverlayFs` to read from a real directory (writes stay in memory).
3. **No network by default**: `curl` doesn't exist unless you configure `network` options with URL allowlists.
4. **No binaries/WASM**: Only built-in commands work. You cannot run node, python, or other binaries.
5. **ReadWriteFs root separation**: If you use `ReadWriteFs`, point it at a workspace directory, not at the installed `just-bash` package or other trusted runtime code.
6. **UTC by default**: `date` always shows UTC (`%Z=UTC`, `%z=+0000`) unless the host opts in by passing `TZ` as an env var (e.g. `new Bash({ env: { TZ: "America/New_York" } })`). `-u` always forces UTC; an invalid `$TZ` falls back to UTC.
## Available Commands
**Text processing**: `awk`, `cat`, `column`, `comm`, `cut`, `egrep`, `expand`, `fgrep`, `fold`, `grep`, `head`, `join`, `nl`, `paste`, `rev`, `rg`, `sed`, `sort`, `strings`, `tac`, `tail`, `tr`, `unexpand`, `uniq`, `wc`, `xargs`
**Data processing**: `jq` (JSON), `js-exec` (JavaScript/TypeScript via QuickJS), `python3`/`python` (Python via WASM/CPython), `sqlite3` (SQLite), `xan` (CSV), `yq` (YAML/XML/TOML/CSV)
**File operations**: `basename`, `chmod`, `cp`, `dirname`, `du`, `file`, `find`, `ln`, `ls`, `mkdir`, `mktemp`, `mv`, `od`, `pwd`, `readlink`, `rm`, `rmdir`, `split`, `stat`, `touch`, `tree`
**Utilities**: `alias`, `base64`, `bash`, `clear`, `curl`, `date`, `diff`, `echo`, `env`, `expr`, `false`, `gzip`, `gunzip`, `help`, `history`, `hostname`, `html-to-markdown`, `md5sum`, `printenv`, `printf`, `seq`, `sh`, `sha1sum`, `sha256sum`, `sleep`, `tar`, `tee`, `time`, `timeout`, `true`, `unalias`, `which`, `whoami`, `yes`, `zcat`
All commands support `--help` for usage details.
## Tools by File Format
### JSON - `jq`
```bash
# Extract field
jq '.name' data.json
# Filter array
jq '.users[] | select(.active == true)' data.json
# Transform structure
jq '[.items[] | {id, name}]' data.json
# From stdin
echo '{"x":1}' | jq '.x'
```
### YAML - `yq`
```bash
# Extract value
yq '.config.database.host' config.yaml
# Output as JSON
yq -o json '.' config.yaml
# Filter with jq syntax
yq '.users[] | select(.role == "admin")' users.yaml
# Modify file in-place
yq -i '.version = "2.0"' config.yaml
```
### TOML - `yq`
```bash
# Read TOML (auto-detected from .toml extension)
yq '.package.name' Cargo.toml
yq '.tool.poetry.version' pyproject.toml
# Convert TOML to JSON
yq -o json '.' config.toml
# Convert YAML to TOML
yq -o toml '.' config.yaml
```
### CSV/TSV - `yq -p csv`
```bash
# Read CSV (auto-detects from .csv/.tsv extension)
yq '.[0].name' data.csv
yq '.[0].name' data.tsv
# Filter rows
yq '[.[] | select(.status == "active")]' data.csv
# Convert CSV to JSON
yq -o json '.' data.csv
```
### Front-matter - `yq --front-matter`
```bash
# Extract YAML front-matter from markdown
yq --front-matter '.title' post.md
yq -f '.tags[]' blog-post.md
# Works with TOML front-matter (+++) too
yq -f '.date' hugo-post.md
```
### XML - `yq -p xml`
```bash
# Extract element
yq '.root.users.user[0].name' data.xml
# Access attributes (use +@ prefix)
yq '.root.item["+@id"]' data.xml
# Convert XML to JSON
yq -p xml -o json '.' data.xml
```
### INI - `yq -p ini`
```bash
# Read INI section value
yq '.database.host' config.ini
# Convert INI to JSON
yq -p ini -o json '.' config.ini
```
### HTML - `html-to-markdown`
```bash
# Convert HTML to markdown
html-to-markdown page.html
# From stdin
echo '<h1>Title</h1><p>Text</p>' | html-to-markdown
```
### Format Conversion with yq
```bash
# JSON to YAML
yq -p json '.' data.json
# YAML to JSON
yq -o json '.' data.yaml
# YAML to TOML
yq -o toml '.' config.yaml
# TOML to JSON
yq -o json '.' Cargo.toml
# CSV to JSON
yq -p csv -o json '.' data.csv
# XML to YAML
yq -p xml '.' data.xml
```
## Common Patterns
### Process JSON with jq
```bash
cat data.json | jq '.items[] | select(.active) | .name'
```
### Find and process files
```bash
find . -name "*.ts" -type f | xargs grep -l "TODO"
```
### Text transformation pipeline
```bash
cat input.txt | grep -v "^#" | sort | uniq -c | sort -rn | head -10
```
### AWK for columnar data
```bash
cat data.csv | awk -F',' '{sum += $3} END {print sum}'
```
## Limitations
- **32-bit integers only**: Arithmetic operations use 32-bit signed integers
- **No job control**: No `&`, `bg`, `fg`, or process suspension
- **No external binaries**: Only built-in commands are available
- **Execution limits**: Loops, recursion, command counts, and output sizes have configurable limits to prevent runaway execution (exit code 126 when exceeded)
## Error Handling
Always check `exitCode`:
```typescript
import { Bash } from "just-bash";
const bash = new Bash({ files: { "/file.txt": "some content" } });
const result = await bash.exec("grep pattern file.txt");
if (result.exitCode !== 0) {
// Command failed - check result.stderr for details
}
```
Common exit codes:
- `0` - Success
- `1` - General error or no matches (grep)
- `2` - Misuse of command (invalid options)
- `126` - Execution limit exceeded (loops, output size, string length)
- `127` - Command not found
## Debugging Tips
1. **Check stderr**: Error messages go to `result.stderr`
2. **Use --help**: All commands support `--help` for usage
3. **Test incrementally**: Build pipelines step by step
4. **Quote variables**: Use `"$var"` to handle spaces in values
## Security Model
- Virtual filesystem is isolated from the real system
- Network access requires explicit URL allowlists
- Execution limits prevent infinite loops and resource exhaustion
- No shell injection possible (commands are parsed, not eval'd)
### Execution Limits
All limits are configurable via `executionLimits` in `BashOptions`:
```typescript
const bash = new Bash({
executionLimits: {
maxCommandCount: 10000, // Max commands per exec()
maxLoopIterations: 10000, // Max bash loop iterations
maxCallDepth: 100, // Max function recursion depth
maxStringLength: 10485760, // Max string size (10MB)
maxArrayElements: 100000, // Max array elements
maxGlobOperations: 100000, // Max glob filesystem operations
maxAwkIterations: 10000, // Max AWK loop iterations
maxSedIterations: 10000, // Max sed branch loop iterations
maxJqIterations: 10000, // Max jq loop iterations
maxSubstitutionDepth: 50, // Max $() nesting depth
maxHeredocSize: 10485760, // Max heredoc size (10MB)
},
});
```
**Output size limits**: AWK, sed, jq, and printf enforce `maxStringLength` on their output buffers. Commands that exceed the limit exit with code 126.
**File read size limits**: `OverlayFs` and `ReadWriteFs` default to a 10MB max file read size. Override with `maxFileReadSize` in filesystem options (set to `0` to disable).
**Network response size**: `maxResponseSize` in `NetworkConfig` caps HTTP response bodies (default: 10MB).
## Discovering Types
TypeScript types are available in the `.d.ts` files. Use JSDoc-style exploration to understand the API:
```bash
# Find all type definition files
find node_modules/just-bash/dist -name "*.d.ts" | head -20
# View main exports and their types
cat node_modules/just-bash/dist/index.d.ts
# View Bash class options
grep -A 30 "interface BashOptions" node_modules/just-bash/dist/Bash.d.ts
# Search for specific types
grep -r "interface.*Options" node_modules/just-bash/dist/*.d.ts
```
Key types to explore:
- `BashOptions` - Constructor options for `new Bash()`
- `ExecResult` - Return type of `bash.exec()`
- `InitialFiles` - File specification format
## Bundling just-bash
just-bash ships pre-bundled (`dist/bundle/index.js`, `index.cjs`), but a handful
of dependencies are deliberately left **external** — the bundle imports them by
name instead of inlining them. Under plain Node this is invisible: they are
declared dependencies, so the package manager installs them and the imports
resolve at runtime.
It matters when you run just-bash through **another** bundler (Next.js,
webpack, esbuild, rollup) for a serverless or edge target. That bundler will try
to inline these, and some of them cannot be inlined. Mark all six as external:
| Package | What it is |
| --- | --- |
| `@mongodb-js/zstd` | native binding (`optionalDependencies`) |
| `node-liblzma` | native binding (`optionalDependencies`) |
| `sql.js` | ships a `.wasm` asset |
| `run` | embeds the QuickJS WASM runtime |
| `seek-bzip` | CommonJS-only bzip2 decoder |
| `guarded-fetch` | imports `node:dns/promises`, `node:net`, `node:dns` |
`guarded-fetch` is the one that bites bundlers targeting a non-Node context:
inlining it pulls in Node builtins, and a bundler whose chunking context has no
concept of them fails the build (Turbopack reports `the chunking context
(unknown) does not support external modules (request: node:dns/promises)`).
Next.js 15+:
```ts
// next.config.ts
const nextConfig = {
serverExternalPackages: [
"just-bash",
"@mongodb-js/zstd",
"node-liblzma",
"seek-bzip",
"sql.js",
"run",
"guarded-fetch",
],
};
```
Older Next.js uses `experimental.serverComponentsExternalPackages`. webpack:
add them to `externals` (or use `webpack-node-externals`). esbuild/rollup:
`--external:<name>` / `external: [...]`.
**Node version**: just-bash requires Node `>=20.19` (`guarded-fetch`'s floor).
**Optional dependencies**: `@mongodb-js/zstd` and `node-liblzma` are declared
in `optionalDependencies`, so an install that cannot build them still succeeds.
Only the compression they provide is lost: `tar -J` then exits non-zero with
`xz compression requires node-liblzma which failed to load`, rather than
crashing the interpreter. The other four are regular dependencies and are
required.
# just-bash website at https://justbash.dev/
This is an interactive demo of **just-bash** running entirely in your browser, with an AI agent that can explore the source code.
The paid-model `/api/agent` route is disabled by default in production. Set
`JUST_BASH_AGENT_API_TOKEN` and have an authenticated same-origin gateway add
`Authorization: Bearer <token>` to enable it. Never embed this server token in
browser JavaScript. The route also bounds request history, output tokens,
agent steps, retries, body-read and execution time, per-instance concurrency,
and admission rate. Production deployments should also add distributed provider/edge
per-principal rate and spend limits; an instance-local counter is not a global
quota in a horizontally scaled deployment.
## Architecture
```
+----------------------------------------------------------+
| BROWSER |
| +----------+ +----------+ +----------------+ |
| | xterm.js |--->| just-bash|--->| Virtual FS | |
| | Terminal | | (browser)| | (in-memory) | |
| +----------+ +----------+ +----------------+ |
| | |
| | `agent` command |
| v |
| +--------------------------------------------------+ |
| | SSE Stream (Server-Sent Events) | |
| +--------------------------------------------------+ |
+----------------------------|-----------------------------+
|
v
+----------------------------------------------------------+
| SERVER |
| +-------------+ +----------+ +----------------+ |
| |ToolLoopAgent|--->| bash-tool|--->| just-bash | |
| | (AI SDK) | | | | + OverlayFS | |
| |Claude Haiku | | - bash | | | |
| +-------------+ | - read | | Real files: | |
| | - write | | - just-bash/ | |
| +----------+ | - bash-tool/ | |
| +----------------+ |
+----------------------------------------------------------+
```
## Components
### 1. just-bash (Browser)
- Pure TypeScript bash interpreter
- Runs locally in browser for regular commands
- In-memory virtual filesystem with pre-loaded files
- No network calls for basic commands like `ls`, `cat`, `grep`
### 2. xterm.js (Browser Terminal)
- Renders a real terminal in the browser
- Handles keyboard input, cursor, colors, scrolling
- Supports ANSI escape codes for styling
### 3. `agent` Command
- Custom command that calls the server
- Sends conversation history to `/api/agent`
- Streams response via Server-Sent Events (SSE)
- Displays tool calls (bash commands, file reads) in real-time
### 4. ToolLoopAgent (Server - AI SDK)
- Uses Anthropic's Claude Haiku model
- Loops automatically: think -> tool call -> observe -> think -> ...
- Stops after 20 tool calls or when done
- Streams responses back to browser
### 5. bash-tool (Server)
- Provides tools for the AI agent:
- `bash` - Execute bash commands
- `readFile` - Read file contents
- `writeFile` - Write files (disabled in this demo)
- Integrates with just-bash sandbox
### 6. OverlayFS (Server)
- Overlays real filesystem (this source code) as read-only
- Agent can explore just-bash and bash-tool source
- Writes go to memory, not disk
## Data Flow
1. You type `agent "how does grep work?"`
2. Browser just-bash runs the `agent` command
3. Command POSTs to `/api/agent` with message history
4. Server creates ToolLoopAgent with bash-tool
5. Agent thinks, calls tools (bash, readFile), observes results
6. Each step streams back as SSE events
7. Browser displays tool calls and final response
8. Response added to conversation history for multi-turn chat
## Development
```bash
pnpm install
pnpm dev
```
Open [http://localhost:3000](http://localhost:3000) to see the terminal.
## Links
- **just-bash**: https://github.com/vercel-labs/just-bash
- **bash-tool**: https://github.com/vercel-labs/bash-tool
- **AI SDK**: https://ai-sdk.dev
- **xterm.js**: https://xtermjs.org
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