Spawn - Bun

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 configures 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; configure it 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); ..."

Value	Description
`null`	Default. Provide no input to the subprocess
`"pipe"`	Return a `FileSink` for fast incremental writing
`"inherit"`	Inherit the `stdin` of the parent process
`Bun.file()`	Read from the specified file
`TypedArray \| DataView`	Use a binary buffer as input
`Response`	Use the response `body` as input
`Request`	Use the request `body` as input
`ReadableStream`	Use a readable stream as input
`Blob`	Use a blob as input
`number`	Read from the file with a given file descriptor

With "pipe", the parent process can incrementally write to the subprocess’s input stream.

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 pipes its data 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

Read the subprocess’s output from 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:

Value	Description
`"pipe"`	Default for `stdout`. Pipe the output to a `ReadableStream` on the returned `Subprocess` object
`"inherit"`	Default for `stderr`. Inherit from the parent process
`"ignore"`	Discard the output
`Bun.file()`	Write to the specified file
`number`	Write to the file with the given file descriptor

Exit handling

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

index.ts

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

The exited property is a Promise that resolves when the process exits.

index.ts

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:

index.ts

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

index.ts

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

Resource usage

After the process exits, resourceUsage() reports its resource usage:

index.ts

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:

index.ts

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

Set timeout to terminate a subprocess after a duration in milliseconds:

index.ts

// 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, Bun kills timed-out processes with SIGTERM. Specify a different signal with the killSignal option:

index.ts

// 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 Bun.spawnSync, maxBuffer limits how many bytes of output the process can emit before Bun kills it:

index.ts

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

parent.ts

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

The parent process sends messages to the subprocess with the .send() method on the returned Subprocess instance. The ipc handler also receives the sending subprocess as its second argument.

parent.ts

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

The child process sends messages to its parent with process.send() and receives them with process.on("message"). This is the same API used for child_process.fork() in Node.js.

child.ts

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);
});

child.ts

// 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.

bun-node-ipc.js

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, use the terminal option to spawn a subprocess with a pseudo-terminal (PTY) attached. The subprocess sees a real terminal, which enables 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

Option	Description	Default
`cols`	Number of columns	`80`
`rows`	Number of rows	`24`
`name`	Terminal type for PTY configuration (set the `TERM` env var separately with the `env` option)	`"xterm-256color"`
`data`	Callback when data is received `(terminal, data) => void`	—
`exit`	Callback when PTY stream closes (EOF or error). `exitCode` is PTY lifecycle status (0=EOF, 1=error), not subprocess exit code. Use `proc.exited` for process exit.	—
`drain`	Callback when ready for more data `(terminal) => void`	—

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

To run multiple commands in sequence through the same terminal session, create a terminal independently and reuse it across 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

Platform differences

Bun.Terminal uses openpty() on Linux and macOS, and ConPTY (CreatePseudoConsole) on Windows. The core behavior — child sees a TTY, write() reaches the child’s stdin, child output reaches the data callback, resize() updates the child’s view — is the same on every platform. A few details differ:

No termios on Windows. inputFlags, outputFlags, localFlags, and controlFlags always read as 0 and setting them is a no-op. setRawMode() records the flag but has no effect on the child; the child controls its own console mode.
No echo without a child process on Windows. On POSIX, the kernel line discipline echoes write() input back to the data callback even with no process attached. ConPTY has no line discipline; input is buffered for the next reader. If you need echo, spawn a process that echoes.
ConPTY re-encodes output. ConPTY renders the child’s output to a virtual screen and emits whatever VT sequences describe the result, so the data callback receives semantically equivalent — but not byte-identical — escape sequences. Colors and text are preserved; cursor-positioning and reset sequences may be reordered or coalesced. ConPTY also emits a short VT init sequence (\x1b[?9001h\x1b[?1004h…) before any child output.
Input \r is not translated to \n on Windows. POSIX ICRNL maps carriage return to newline on input; ConPTY passes \r through unchanged.
process.on('SIGWINCH') in the child does not fire under ConPTY unless the child is reading stdin in raw mode. process.stdout.columns/rows do update after resize(). This is a libuv limitation that affects any libuv-based child (Node.js included).
On Windows before 11 24H2 (build 26100), terminal.close() may not terminate a still-running child promptly because ClosePseudoConsole blocks until conhost has flushed its output through the pipe on those versions. Kill the attached process first if you need to tear down with a running child.

Blocking API (`Bun.spawnSync()`)

Bun.spawnSync is the blocking equivalent of Bun.spawn. It supports the same inputs and parameters and returns a SyncSubprocess object, which differs from Subprocess in a few ways.

It contains a success property that indicates whether the process exited with a zero exit code.
The stdout and stderr properties are instances of Buffer instead of ReadableStream.
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.spawn and Bun.spawnSync use posix_spawn(3).

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

terminal

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

terminal

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

The following is a reference of the Spawn API and types. 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, see bun.d.ts.

See Typescript Definitions

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 (POSIX) / ConPTY (Windows) support
  }

  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";

​Spawn a process (Bun.spawn())

​Input stream

​Output streams

​Exit handling

​Resource usage

​Using AbortSignal

​Using timeout and killSignal

​Using maxBuffer

​Inter-process communication (IPC)

​IPC between Bun & Node.js

​Terminal (PTY) support

​Terminal options

​Terminal methods

​Reusable Terminal

​Platform differences

​Blocking API (Bun.spawnSync())

​Benchmarks

​Reference

Spawn a process (`Bun.spawn()`)

Input stream

Output streams

Exit handling

Resource usage

Using AbortSignal

Using timeout and killSignal

Using maxBuffer

Inter-process communication (IPC)

IPC between Bun & Node.js

Terminal (PTY) support

Terminal options

Terminal methods

Reusable Terminal

Platform differences

Blocking API (`Bun.spawnSync()`)

Benchmarks

Reference