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FspawnSync
Bun

function

spawnSync

function spawnSync<In extends Writable = 'ignore', Out extends Readable = 'pipe', Err extends Readable = 'pipe'>(
options: SpawnSyncOptions<In, Out, Err> & { cmd: string[]; onExit: undefined }
): SyncSubprocess<Out, Err>;

Spawn a new process

function spawnSync<In extends Writable = 'ignore', Out extends Readable = 'pipe', Err extends Readable = 'pipe'>(
cmds: string[],
options?: SpawnSyncOptions<In, Out, Err>
): SyncSubprocess<Out, Err>;

Synchronously spawn a new process

const {stdout} = Bun.spawnSync(["echo", "hello"]);
console.log(stdout.toString()); // "hello\n"

Internally, this uses posix_spawn(2)

@param cmds

The command to run

The first argument will be resolved to an absolute executable path. It must be a file, not a directory.

If you explicitly set PATH in env, that PATH will be used to resolve the executable instead of the default PATH.

To check if the command exists before running it, use Bun.which(bin).

Referenced types

interface SpawnSyncOptions<In extends Writable, Out extends Readable, Err extends Readable>

  • argv0?: string

    Path to the executable to run in the subprocess. This defaults to cmds[0].

    One use-case for this is for applications which wrap other applications or to simulate a symlink.

  • cwd?: string

    The current working directory of the process

    Defaults to process.cwd()

  • detached?: boolean

    Run the child in a separate process group, detached from the parent.

    • POSIX: calls setsid() so the child starts a new session and becomes the process group leader. It can outlive the parent and receive signals independently of the parent’s terminal/process group.
    • Windows: sets UV_PROCESS_DETACHED, allowing the child to outlive the parent and receive signals independently.

    Note: stdio may keep the parent process alive. Pass stdio: ["ignore", "ignore", "ignore"] to the spawn constructor to prevent this.

  • env?: Record<string, undefined | string>

    The environment variables of the process

    Defaults to process.env as it was when the current Bun process launched.

    Changes to process.env at runtime won't automatically be reflected in the default value. For that, you can pass process.env explicitly.

  • killSignal?: string | number

    The signal to use when killing the process after a timeout, when the AbortSignal is aborted, or when the process goes over the maxBuffer limit.

    // Kill the process with SIGKILL after 5 seconds
const subprocess = Bun.spawn({
  cmd: ["sleep", "10"],
  timeout: 5000,
  killSignal: "SIGKILL",
});
  • maxBuffer?: number

    The maximum number of bytes the process may output. If the process goes over this limit, it is killed with signal killSignal (defaults to SIGTERM).

  • serialization?: 'json' | 'advanced'

    The serialization format to use for IPC messages. Defaults to "advanced".

    To communicate with Node.js processes, use "json".

    When ipc is not specified, this is ignored.

  • signal?: AbortSignal

    An AbortSignal that can be used to abort the subprocess.

    This is useful for aborting a subprocess when some other part of the program is aborted, such as a fetch response.

    If the signal is aborted, the process will be killed with the signal specified by killSignal (defaults to SIGTERM).

    const controller = new AbortController();
const { signal } = controller;
const start = performance.now();
const subprocess = Bun.spawn({
 cmd: ["sleep", "100"],
 signal,
});
await Bun.sleep(1);
controller.abort();
await subprocess.exited;
const end = performance.now();
console.log(end - start); // 1ms instead of 101ms
  • stderr?: Err

    The file descriptor for the standard error. It may be:

    • "pipe", undefined: The process will have a ReadableStream for standard output/error
    • "ignore", null: The process will have no standard output/error
    • "inherit": The process will inherit the standard output/error of the current process
    • ArrayBufferView: The process write to the preallocated buffer. Not implemented.
    • number: The process will write to the file descriptor
  • stdin?: In

    The file descriptor for the standard input. It may be:

    • "ignore", null, undefined: The process will have no standard input
    • "pipe": The process will have a new FileSink for standard input
    • "inherit": The process will inherit the standard input of the current process
    • ArrayBufferView, Blob: The process will read from the buffer
    • number: The process will read from the file descriptor
  • stdio?: [In, Out, Err, ...Readable[]]

    The standard file descriptors of the process, in the form [stdin, stdout, stderr]. This overrides the stdin, stdout, and stderr properties.

    For stdin you may pass:

    • "ignore", null, undefined: The process will have no standard input (default)
    • "pipe": The process will have a new FileSink for standard input
    • "inherit": The process will inherit the standard input of the current process
    • ArrayBufferView, Blob, Bun.file(), Response, Request: The process will read from buffer/stream.
    • number: The process will read from the file descriptor

    For stdout and stdin you may pass:

    • "pipe", undefined: The process will have a ReadableStream for standard output/error
    • "ignore", null: The process will have no standard output/error
    • "inherit": The process will inherit the standard output/error of the current process
    • ArrayBufferView: The process write to the preallocated buffer. Not implemented.
    • number: The process will write to the file descriptor
  • stdout?: Out

    The file descriptor for the standard output. It may be:

    • "pipe", undefined: The process will have a ReadableStream for standard output/error
    • "ignore", null: The process will have no standard output/error
    • "inherit": The process will inherit the standard output/error of the current process
    • ArrayBufferView: The process write to the preallocated buffer. Not implemented.
    • number: The process will write to the file descriptor
  • timeout?: number

    The maximum amount of time the process is allowed to run in milliseconds.

    If the timeout is reached, the process will be killed with the signal specified by killSignal (defaults to SIGTERM).

    // Kill the process after 5 seconds
const subprocess = Bun.spawn({
  cmd: ["sleep", "10"],
  timeout: 5000,
});
await subprocess.exited; // Will resolve after 5 seconds
  • windowsHide?: boolean

    If true, the subprocess will have a hidden window.

  • windowsVerbatimArguments?: boolean

    If true, no quoting or escaping of arguments is done on Windows.

  • message: any,
    subprocess: Subprocess<In, Out, Err>,
    handle?: unknown
    ): void;

    When specified, Bun will open an IPC channel to the subprocess. The passed callback is called for incoming messages, and subprocess.send can send messages to the subprocess. Messages are serialized using the JSC serialize API, which allows for the same types that postMessage/structuredClone supports.

    The subprocess can send and receive messages by using process.send and process.on("message"), respectively. This is the same API as what Node.js exposes when child_process.fork() is used.

    Currently, this is only compatible with processes that are other bun instances.

    @param subprocess

    The Subprocess that received the message

  • onDisconnect(): void | Promise<void>;

    Called exactly once when the IPC channel between the parent and this subprocess is closed. After this runs, no further IPC messages will be delivered.

    When it fires:

    • The child called process.disconnect() or the parent called subprocess.disconnect().
    • The child exited for any reason (normal exit or due to a signal like SIGILL, SIGKILL, etc.).
    • The child replaced itself with a program that does not support Bun IPC.

    Notes:

    • This callback indicates that the pipe is closed; it is not an error by itself. Use onExit or Subprocess.exited to determine why the process ended.
    • It may occur before or after onExit depending on timing; do not rely on ordering. Typically, if you or the child call disconnect() first, this fires before onExit; if the process exits without an explicit disconnect, either may happen first.
    • Only runs when ipc is enabled and runs at most once per subprocess.
    • If the child becomes a zombie (exited but not yet reaped), the IPC is already closed, and this callback will fire (or may already have fired).
    const subprocess = spawn({
 cmd: ["echo", "hello"],
 ipc: (message) => console.log(message),
 onDisconnect: () => {
   console.log("IPC channel disconnected");
 },
});
  • subprocess: Subprocess<In, Out, Err>,
    exitCode: null | number,
    signalCode: null | number,
    error?: ErrorLike
    ): void | Promise<void>;

    Callback that runs when the Subprocess exits

    This is called even if the process exits with a non-zero exit code.

    Warning: this may run before the Bun.spawn function returns.

    A simple alternative is await subprocess.exited.

    @param error

    If an error occurred in the call to waitpid2, this will be the error.

    const subprocess = spawn({
 cmd: ["echo", "hello"],
 onExit: (subprocess, code) => {
   console.log(`Process exited with code ${code}`);
  },
});

interface SyncSubprocess<Out extends SpawnOptions.Readable = SpawnOptions.Readable, Err extends SpawnOptions.Readable = SpawnOptions.Readable>

A process created by Bun.spawnSync.

This type accepts 2 optional type parameters which correspond to the stdout and stderr options. Instead of specifying these, you should use one of the following utility types instead:

  • ReadableSyncSubprocess (pipe, pipe)
  • NullSyncSubprocess (ignore, ignore)