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just-bash
Advanced tools
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.
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.
Extend just-bash with your own TypeScript commands using defineCommand:
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 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.
cat, cp, file, ln, ls, mkdir, mv, readlink, rm, rmdir, split, stat, touch, tree
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
jq (JSON), sqlite3 (SQLite), xan (CSV), yq (YAML/XML/TOML/CSV)
js-exec (JavaScript/TypeScript via QuickJS; requires javascript: true), python3/python (Python via CPython; requires python: true)
gzip (+ gunzip, zcat), tar
basename, cd, dirname, du, echo, env, export, find, hostname, printenv, pwd, tee
alias, bash, chmod, clear, date, expr, false, help, history, seq, sh, sleep, time, timeout, true, unalias, which, whoami
curl, html-to-markdown (require network configuration)
All commands support --help for usage information.
cmd1 | cmd2>, >>, 2>, 2>&1, <&&, ||, ;$VAR, ${VAR}, ${VAR:-default}$1, $2, $@, $#*, ?, [...]if COND; then CMD; elif COND; then CMD; else CMD; fifunction name { ... } or name() { ... }local VAR=valuefor, while, untilln -s target linkln target linkconst 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 });
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:
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) |
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",
// 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:
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:
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.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.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.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.allowSymlinks: false, symlinks present during normal path
validation are still rejected.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" }) },
],
});
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".
The allow-list enforces:
allowedMethods for more)# 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 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\'))"');
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
js-exec supports require() and import with these Node.js modules:
readFileSync, writeFileSync, readdirSync, statSync, existsSync, mkdirSync, rmSync, fs.promises.*join, resolve, dirname, basename, extname, relative, normalizeexecSync, spawnSyncargv, cwd(), exit(), env, platform, versionos, url, assert, util, events, buffer, stream, string_decoder, querystringconsole, fetch, Buffer, URL, URLSearchParamsfs.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 (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.
js-exec scripts can call host-defined tools through a global tools proxy
when javascript.invokeTool is provided:
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) => {
const args = argsJson ? JSON.parse(argsJson) : undefined;
if (path === "math.add") {
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 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.
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.
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 (30 seconds in the default normal profile and 5 seconds in the opt-in hardened profile) to prevent runaway queries from blocking execution.
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.
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.
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();
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 JSONpnpm shell
The interactive shell has full internet access by default. Disable with --no-network:
pnpm shell --no-network
Bash protects against infinite loops and deep recursion with configurable limits:
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. Invalid values are rejected when
Bash is constructed. Error messages identify the resource that was hit.
The Node.js package requires Node >=20.18.1.
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.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.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.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).
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 directoryCommands can be invoked by path (e.g., /bin/ls) or by name.
For AI agents, bash-tool provides additional guidance in its AGENTS.md:
cat node_modules/bash-tool/dist/AGENTS.md
Apache-2.0
FAQs
A simulated bash environment with virtual filesystem
The npm package just-bash receives a total of 674,222 weekly downloads. As such, just-bash popularity was classified as popular.
We found that just-bash demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 1 open source maintainer collaborating on the project.

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