Bun.spawn function | API Reference | Bun

Spawn a new process

Spawn a new process

const proc = Bun.spawn(["echo", "hello"]);
const text = await proc.stdout.text();
console.log(text); // "hello\n"

Internally, this uses posix_spawn(2)

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).

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.

The current working directory of the process

Defaults to process.cwd()

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

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

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.

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",
});

If true, stdout and stderr pipes will not automatically start reading data. Reading will only begin when you access the stdout or stderr properties.

This can improve performance when you don't need to read output immediately.

const subprocess = Bun.spawn({
cmd: ["echo", "hello"],
lazy: true, // Don't start reading stdout until accessed
});
// stdout reading hasn't started yet
await subprocess.stdout.text(); // Now reading starts

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).

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.

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

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

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

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:

For stdout and stdin you may pass:

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

Spawn the subprocess with a pseudo-terminal (PTY) attached.

When this option is provided:

Only available on POSIX systems (Linux, macOS).

const proc = Bun.spawn(["bash"], {
terminal: {
cols: 80,
rows: 24,
data: (term, data) => console.log(data.toString()),
},
});

proc.terminal.write("echo hello\n");
await proc.exited;
proc.terminal.close();

You can also pass an existing Terminal object for reuse across multiple spawns:

const terminal = new Bun.Terminal({ ... });
const proc1 = Bun.spawn(["echo", "first"], { terminal });
await proc1.exited;
const proc2 = Bun.spawn(["echo", "second"], { terminal });
await proc2.exited;
terminal.close();

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

If true, the subprocess will have a hidden window.

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

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.

The Subprocess that received the message

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:

Notes:

const subprocess = spawn({
cmd: ["echo", "hello"],
ipc: (message) => console.log(message),
onDisconnect: () => {
console.log("IPC channel disconnected");
},
});

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.

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}`);
},
});

A process created by Bun.spawn.

This type accepts 3 optional type parameters which correspond to the stdio array from the options object. Instead of specifying these, you should use one of the following utility types instead:

Synchronously get the exit code of the process

If the process hasn't exited yet, this will return null

The exit code of the process

The promise will resolve when the process exits

Has the process exited?

The process ID of the child process

const { pid } = Bun.spawn({ cmd: ["echo", "hello"] });
console.log(pid); // 1234

This returns the same value as Subprocess.stdout

It exists for compatibility with ReadableStream.pipeThrough

Synchronously get the signal code of the process

If the process never sent a signal code, this will return null

To receive signal code changes, use the onExit callback.

If the signal code is unknown, it will return the original signal code number, but that case should essentially never happen.

Access extra file descriptors passed to the stdio option in the options object.

Entries beyond index 2 are number for "pipe" slots and, on POSIX, for slots where a raw file descriptor was supplied (the same fd is returned; it remains owned by the caller and is never closed by the subprocess). Other slots — including raw fds on Windows — are null.

The terminal attached to this subprocess, if spawned with the terminal option. Returns undefined if no terminal was attached.

When a terminal is attached, stdin, stdout, and stderr return null. Use terminal.write() and the data callback instead.

const proc = Bun.spawn(["bash"], {
terminal: { data: (term, data) => console.log(data.toString()) },
});

proc.terminal?.write("echo hello\n");

Disconnect the IPC channel to the subprocess. This is only supported if the subprocess was created with the ipc option.

Kill the process

The exitCode to send to the process

This method will tell Bun to wait for this process to exit after you already called unref().

Before shutting down, Bun will wait for all subprocesses to exit by default

Get the resource usage information of the process (max RSS, CPU time, etc)

Only available after the process has exited

If the process hasn't exited yet, this will return undefined

Send a message to the subprocess. This is only supported if the subprocess was created with the ipc option, and is another instance of bun.

Messages are serialized using the JSC serialize API, which allows for the same types that postMessage/structuredClone supports.

Before shutting down, Bun will wait for all subprocesses to exit by default

This method will tell Bun to not wait for this process to exit before shutting down.