Spawn child processes with Bun.spawn
or Bun.spawnSync
.
Spawn a process
Provide a command as an array of strings. The result of Bun.spawn()
is a Bun.Subprocess
object.
Bun.spawn(["echo", "hello"]);
The second argument to Bun.spawn
is a parameters object that can be used to configure the subprocess.
const proc = Bun.spawn(["echo", "hello"], {
cwd: "./path/to/subdir", // specify a working direcory
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 new Response(proc.stdout).text();
console.log(text); // "const input = "hello world".repeat(400); ..."
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. |
number | Read from the file with a given file descriptor. |
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();
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(["echo", "hello"]);
const text = await new Response(proc.stdout).text();
console.log(text); // => "hello"
Configure the output stream by passing one of the following values to stdout/stderr
:
"pipe" | Default for stdout . Pipe the output to a ReadableStream on the returned Subprocess object. |
"inherit" | Default for stderr . Inherit from the parent process. |
Bun.file() | Write to the specified file. |
null | Write to /dev/null . |
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.
const proc = Bun.spawn(["echo", "hello"], {
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(["echo", "hello"]);
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(["echo", "hello"]);
proc.kill();
proc.killed; // true
proc.kill(); // specify an exit code
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(["echo", "hello"]);
proc.unref();
Blocking API
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.
- It contains a
success
property that indicates whether the process exited with a zero exit code. - The
stdout
andstderr
properties are instances ofBuffer
instead ofReadableStream
. - There is no
stdin
property. UseBun.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
⚡️ Under the hood, Bun.spawn
and Bun.spawnSync
use posix_spawn(3)
.
Bun's spawnSync
spawns processes 60% faster than the Node.js child_process
module.
bun spawn.mjs
cpu: Apple M1 Max
runtime: bun 0.2.0 (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
interface Bun {
spawn(command: string[], options?: SpawnOptions): Subprocess;
spawnSync(command: string[], options?: SpawnOptions): SyncSubprocess;
}
interface SpawnOptions {
cwd?: string;
env?: Record<string, string>;
stdin?:
| "pipe"
| "inherit"
| "ignore"
| ReadableStream
| BunFile
| Blob
| Response
| Request
| number
| null;
stdout?:
| "pipe"
| "inherit"
| "ignore"
| BunFile
| TypedArray
| DataView
| null;
stderr?:
| "pipe"
| "inherit"
| "ignore"
| BunFile
| TypedArray
| DataView
| null;
onExit?: (
proc: Subprocess,
exitCode: number | null,
signalCode: string | null,
error: Error | null,
) => void;
}
interface Subprocess {
readonly pid: number;
readonly stdin?: number | ReadableStream | FileSink;
readonly stdout?: number | ReadableStream;
readonly stderr?: number | ReadableStream;
readonly exited: Promise<number>;
readonly exitCode: number | undefined;
readonly signalCode: Signal | null;
readonly killed: boolean;
ref(): void;
unref(): void;
kill(code?: number): void;
}
interface SyncSubprocess {
readonly pid: number;
readonly success: boolean;
readonly stdout: Buffer;
readonly stderr: Buffer;
}
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";