Bun Spawn

Spawn

Spawn child processes with Bun.spawn or Bun.spawnSync

Spawn a process (Bun.spawn())

Provide a command as an array of strings. The result of Bun.spawn() is a Bun.Subprocess object.

const proc = Bun.spawn(["bun", "--version"]);
console.log(await proc.exited); // 0

The second argument to Bun.spawn is a parameters object that can be used to configure the subprocess.

const proc = Bun.spawn(["bun", "--version"], {
  cwd: "./path/to/subdir", // specify a working directory
  env: { ...process.env, FOO: "bar" }, // specify environment variables
  onExit(proc, exitCode, signalCode, error) {
    // exit handler
  },
});

proc.pid; // process ID of subprocess

Input stream

By default, the input stream of the subprocess is undefined; it can be configured with the stdin parameter.

const proc = Bun.spawn(["cat"], {
  stdin: await fetch("https://raw.githubusercontent.com/oven-sh/bun/main/examples/hashing.js"),
});

const text = await proc.stdout.text();
console.log(text); // "const input = "hello world".repeat(400); ..."

The "pipe" option lets incrementally write to the subprocess's input stream from the parent process.

const proc = Bun.spawn(["cat"], {
  stdin: "pipe", // return a FileSink for writing
});

// enqueue string data
proc.stdin.write("hello");

// enqueue binary data
const enc = new TextEncoder();
proc.stdin.write(enc.encode(" world!"));

// send buffered data
proc.stdin.flush();

// close the input stream
proc.stdin.end();

Passing a ReadableStream to stdin lets you pipe data from a JavaScript ReadableStream directly to the subprocess's input:

const stream = new ReadableStream({
  start(controller) {
    controller.enqueue("Hello from ");
    controller.enqueue("ReadableStream!");
    controller.close();
  },
});

const proc = Bun.spawn(["cat"], {
  stdin: stream,
  stdout: "pipe",
});

const output = await proc.stdout.text();
console.log(output); // "Hello from ReadableStream!"

Output streams

You can read results from the subprocess via the stdout and stderr properties. By default these are instances of ReadableStream.

const proc = Bun.spawn(["bun", "--version"]);
const text = await proc.stdout.text();
console.log(text); // => "1.3.3\n"

Configure the output stream by passing one of the following values to stdout/stderr:

Exit handling

Use the onExit callback to listen for the process exiting or being killed.

const proc = Bun.spawn(["bun", "--version"], {
  onExit(proc, exitCode, signalCode, error) {
    // exit handler
  },
});

For convenience, the exited property is a Promise that resolves when the process exits.

const proc = Bun.spawn(["bun", "--version"]);

await proc.exited; // resolves when process exit
proc.killed; // boolean — was the process killed?
proc.exitCode; // null | number
proc.signalCode; // null | "SIGABRT" | "SIGALRM" | ...

To kill a process:

const proc = Bun.spawn(["bun", "--version"]);
proc.kill();
proc.killed; // true

proc.kill(15); // specify a signal code
proc.kill("SIGTERM"); // specify a signal name

The parent bun process will not terminate until all child processes have exited. Use proc.unref() to detach the child process from the parent.

const proc = Bun.spawn(["bun", "--version"]);
proc.unref();

Resource usage

You can get information about the process's resource usage after it has exited:

const proc = Bun.spawn(["bun", "--version"]);
await proc.exited;

const usage = proc.resourceUsage();
console.log(`Max memory used: ${usage.maxRSS} bytes`);
console.log(`CPU time (user): ${usage.cpuTime.user} µs`);
console.log(`CPU time (system): ${usage.cpuTime.system} µs`);

Using AbortSignal

You can abort a subprocess using an AbortSignal:

const controller = new AbortController();
const { signal } = controller;

const proc = Bun.spawn({
  cmd: ["sleep", "100"],
  signal,
});

// Later, to abort the process:
controller.abort();

Using timeout and killSignal

You can set a timeout for a subprocess to automatically terminate after a specific duration:

// Kill the process after 5 seconds
const proc = Bun.spawn({
  cmd: ["sleep", "10"],
  timeout: 5000, // 5 seconds in milliseconds
});

await proc.exited; // Will resolve after 5 seconds

By default, timed-out processes are killed with the SIGTERM signal. You can specify a different signal with the killSignal option:

// Kill the process with SIGKILL after 5 seconds
const proc = Bun.spawn({
  cmd: ["sleep", "10"],
  timeout: 5000,
  killSignal: "SIGKILL", // Can be string name or signal number
});

The killSignal option also controls which signal is sent when an AbortSignal is aborted.

Using maxBuffer

For spawnSync, you can limit the maximum number of bytes of output before the process is killed:

// Kill 'yes' after it emits over 100 bytes of output
const result = Bun.spawnSync({
  cmd: ["yes"], // or ["bun", "exec", "yes"] on Windows
  maxBuffer: 100,
});
// process exits

Inter-process communication (IPC)

Bun supports direct inter-process communication channel between two bun processes. To receive messages from a spawned Bun subprocess, specify an ipc handler.

const child = Bun.spawn(["bun", "child.ts"], {
  ipc(message) {
    /**
     * The message received from the sub process
     **/
  },
});

The parent process can send messages to the subprocess using the .send() method on the returned Subprocess instance. A reference to the sending subprocess is also available as the second argument in the ipc handler.

const childProc = Bun.spawn(["bun", "child.ts"], {
  ipc(message, childProc) {
    /**
     * The message received from the sub process
     **/
    childProc.send("Respond to child");
  },
});

childProc.send("I am your father"); // The parent can send messages to the child as well

Meanwhile the child process can send messages to its parent using with process.send() and receive messages with process.on("message"). This is the same API used for child_process.fork() in Node.js.

process.send("Hello from child as string");
process.send({ message: "Hello from child as object" });

process.on("message", message => {
  // print message from parent
  console.log(message);
});

// send a string
process.send("Hello from child as string");

// send an object
process.send({ message: "Hello from child as object" });

The serialization option controls the underlying communication format between the two processes:

• advanced: (default) Messages are serialized using the JSC serialize API, which supports cloning everything structuredClone supports. This does not support transferring ownership of objects.
• json: Messages are serialized using JSON.stringify and JSON.parse, which does not support as many object types as advanced does.

To disconnect the IPC channel from the parent process, call:

childProc.disconnect();

IPC between Bun & Node.js

To use IPC between a bun process and a Node.js process, set serialization: "json" in Bun.spawn. This is because Node.js and Bun use different JavaScript engines with different object serialization formats.

if (typeof Bun !== "undefined") {
  const prefix = `[bun ${process.versions.bun} 🐇]`;
  const node = Bun.spawn({
    cmd: ["node", __filename],
    ipc({ message }) {
      console.log(message);
      node.send({ message: `${prefix} 👋 hey node` });
      node.kill();
    },
    stdio: ["inherit", "inherit", "inherit"],
    serialization: "json",
  });

  node.send({ message: `${prefix} 👋 hey node` });
} else {
  const prefix = `[node ${process.version}]`;
  process.on("message", ({ message }) => {
    console.log(message);
    process.send({ message: `${prefix} 👋 hey bun` });
  });
}

Terminal (PTY) support

For interactive terminal applications, you can spawn a subprocess with a pseudo-terminal (PTY) attached using the terminal option. This makes the subprocess think it's running in a real terminal, enabling features like colored output, cursor movement, and interactive prompts.

const proc = Bun.spawn(["bash"], {
  terminal: {
    cols: 80,
    rows: 24,
    data(terminal, data) {
      // Called when data is received from the terminal
      process.stdout.write(data);
    },
  },
});

// Write to the terminal
proc.terminal.write("echo hello\n");

// Wait for the process to exit
await proc.exited;

// Close the terminal
proc.terminal.close();

When the terminal option is provided:

• The subprocess sees process.stdout.isTTY as true
• stdin, stdout, and stderr are all connected to the terminal
• proc.stdin, proc.stdout, and proc.stderr return null — use the terminal instead
• Access the terminal via proc.terminal

Terminal options

Terminal methods

The Terminal object returned by proc.terminal has the following methods:

// Write data to the terminal
proc.terminal.write("echo hello\n");

// Resize the terminal
proc.terminal.resize(120, 40);

// Set raw mode (disable line buffering and echo)
proc.terminal.setRawMode(true);

// Keep event loop alive while terminal is open
proc.terminal.ref();
proc.terminal.unref();

// Close the terminal
proc.terminal.close();

Reusable Terminal

You can create a terminal independently and reuse it across multiple subprocesses:

await using terminal = new Bun.Terminal({
  cols: 80,
  rows: 24,
  data(term, data) {
    process.stdout.write(data);
  },
});

// Spawn first process
const proc1 = Bun.spawn(["echo", "first"], { terminal });
await proc1.exited;

// Reuse terminal for another process
const proc2 = Bun.spawn(["echo", "second"], { terminal });
await proc2.exited;

// Terminal is closed automatically by `await using`

When passing an existing Terminal object:

• The terminal can be reused across multiple spawns
• You control when to close the terminal
• The exit callback fires when you call terminal.close(), not when each subprocess exits
• Use proc.exited to detect individual subprocess exits

This is useful for running multiple commands in sequence through the same terminal session.

Terminal support is only available on POSIX systems (Linux, macOS). It is not available on Windows.

Blocking API (Bun.spawnSync())

Bun provides a synchronous equivalent of Bun.spawn called Bun.spawnSync. This is a blocking API that supports the same inputs and parameters as Bun.spawn. It returns a SyncSubprocess object, which differs from Subprocess in a few ways.

1. It contains a success property that indicates whether the process exited with a zero exit code.
2. The stdout and stderr properties are instances of Buffer instead of ReadableStream.
3. There is no stdin property. Use Bun.spawn to incrementally write to the subprocess's input stream.

const proc = Bun.spawnSync(["echo", "hello"]);

console.log(proc.stdout.toString());
// => "hello\n"

As a rule of thumb, the asynchronous Bun.spawn API is better for HTTP servers and apps, and Bun.spawnSync is better for building command-line tools.

Benchmarks

Bun's spawnSync spawns processes 60% faster than the Node.js child_process module.

bun spawn.mjs

cpu: Apple M1 Max
runtime: bun 1.x (arm64-darwin)

benchmark              time (avg)             (min … max)       p75       p99      p995
--------------------------------------------------------- -----------------------------
spawnSync echo hi  888.14 µs/iter    (821.83 µs … 1.2 ms) 905.92 µs      1 ms   1.03 ms

node spawn.node.mjs

cpu: Apple M1 Max
runtime: node v18.9.1 (arm64-darwin)

benchmark              time (avg)             (min … max)       p75       p99      p995
--------------------------------------------------------- -----------------------------
spawnSync echo hi    1.47 ms/iter     (1.14 ms … 2.64 ms)   1.57 ms   2.37 ms   2.52 ms

Reference

A reference of the Spawn API and types are shown below. The real types have complex generics to strongly type the Subprocess streams with the options passed to Bun.spawn and Bun.spawnSync. For full details, find these types as defined bun.d.ts.

interface Bun {
  spawn(command: string[], options?: SpawnOptions.OptionsObject): Subprocess;
  spawnSync(command: string[], options?: SpawnOptions.OptionsObject): SyncSubprocess;

  spawn(options: { cmd: string[] } & SpawnOptions.OptionsObject): Subprocess;
  spawnSync(options: { cmd: string[] } & SpawnOptions.OptionsObject): SyncSubprocess;
}

namespace SpawnOptions {
  interface OptionsObject {
    cwd?: string;
    env?: Record<string, string | undefined>;
    stdio?: [Writable, Readable, Readable];
    stdin?: Writable;
    stdout?: Readable;
    stderr?: Readable;
    onExit?(
      subprocess: Subprocess,
      exitCode: number | null,
      signalCode: number | null,
      error?: ErrorLike,
    ): void | Promise<void>;
    ipc?(message: any, subprocess: Subprocess): void;
    serialization?: "json" | "advanced";
    windowsHide?: boolean;
    windowsVerbatimArguments?: boolean;
    argv0?: string;
    signal?: AbortSignal;
    timeout?: number;
    killSignal?: string | number;
    maxBuffer?: number;
    terminal?: TerminalOptions; // PTY support (POSIX only)
  }

  type Readable =
    | "pipe"
    | "inherit"
    | "ignore"
    | null // equivalent to "ignore"
    | undefined // to use default
    | BunFile
    | ArrayBufferView
    | number;

  type Writable =
    | "pipe"
    | "inherit"
    | "ignore"
    | null // equivalent to "ignore"
    | undefined // to use default
    | BunFile
    | ArrayBufferView
    | number
    | ReadableStream
    | Blob
    | Response
    | Request;
}

interface Subprocess extends AsyncDisposable {
  readonly stdin: FileSink | number | undefined | null;
  readonly stdout: ReadableStream<Uint8Array<ArrayBuffer>> | number | undefined | null;
  readonly stderr: ReadableStream<Uint8Array<ArrayBuffer>> | number | undefined | null;
  readonly readable: ReadableStream<Uint8Array<ArrayBuffer>> | number | undefined | null;
  readonly terminal: Terminal | undefined;
  readonly pid: number;
  readonly exited: Promise<number>;
  readonly exitCode: number | null;
  readonly signalCode: NodeJS.Signals | null;
  readonly killed: boolean;

  kill(exitCode?: number | NodeJS.Signals): void;
  ref(): void;
  unref(): void;

  send(message: any): void;
  disconnect(): void;
  resourceUsage(): ResourceUsage | undefined;
}

interface SyncSubprocess {
  stdout: Buffer | undefined;
  stderr: Buffer | undefined;
  exitCode: number;
  success: boolean;
  resourceUsage: ResourceUsage;
  signalCode?: string;
  exitedDueToTimeout?: true;
  pid: number;
}

interface TerminalOptions {
  cols?: number;
  rows?: number;
  name?: string;
  data?: (terminal: Terminal, data: Uint8Array<ArrayBuffer>) => void;
  /** Called when PTY stream closes (EOF or error). exitCode is PTY lifecycle status (0=EOF, 1=error), not subprocess exit code. */
  exit?: (terminal: Terminal, exitCode: number, signal: string | null) => void;
  drain?: (terminal: Terminal) => void;
}

interface Terminal extends AsyncDisposable {
  readonly stdin: number;
  readonly stdout: number;
  readonly closed: boolean;
  write(data: string | BufferSource): number;
  resize(cols: number, rows: number): void;
  setRawMode(enabled: boolean): void;
  ref(): void;
  unref(): void;
  close(): void;
}

interface ResourceUsage {
  contextSwitches: {
    voluntary: number;
    involuntary: number;
  };

  cpuTime: {
    user: number;
    system: number;
    total: number;
  };
  maxRSS: number;

  messages: {
    sent: number;
    received: number;
  };
  ops: {
    in: number;
    out: number;
  };
  shmSize: number;
  signalCount: number;
  swapCount: number;
}

type Signal =
  | "SIGABRT"
  | "SIGALRM"
  | "SIGBUS"
  | "SIGCHLD"
  | "SIGCONT"
  | "SIGFPE"
  | "SIGHUP"
  | "SIGILL"
  | "SIGINT"
  | "SIGIO"
  | "SIGIOT"
  | "SIGKILL"
  | "SIGPIPE"
  | "SIGPOLL"
  | "SIGPROF"
  | "SIGPWR"
  | "SIGQUIT"
  | "SIGSEGV"
  | "SIGSTKFLT"
  | "SIGSTOP"
  | "SIGSYS"
  | "SIGTERM"
  | "SIGTRAP"
  | "SIGTSTP"
  | "SIGTTIN"
  | "SIGTTOU"
  | "SIGUNUSED"
  | "SIGURG"
  | "SIGUSR1"
  | "SIGUSR2"
  | "SIGVTALRM"
  | "SIGWINCH"
  | "SIGXCPU"
  | "SIGXFSZ"
  | "SIGBREAK"
  | "SIGLOST"
  | "SIGINFO";