---
title: File I/O
description: Bun provides a set of optimized APIs for reading and writing files.
---

<Note>

The `Bun.file` and `Bun.write` APIs documented on this page are heavily optimized and are the recommended way to work with files in Bun. For operations they don't cover, such as `mkdir` or `readdir`, use Bun's [nearly complete](/runtime/nodejs-compat#node-fs) implementation of the [`node:fs`](https://nodejs.org/api/fs.html) module.

</Note>

---

## Reading files (`Bun.file()`)

`Bun.file(path): BunFile`

Create a `BunFile` instance with the `Bun.file(path)` function. A `BunFile` represents a lazily-loaded file; initializing it does not read the file from disk.

```ts
const foo = Bun.file("foo.txt"); // relative to cwd
foo.size; // number of bytes
foo.type; // MIME type
```

The reference conforms to the [`Blob`](https://developer.mozilla.org/en-US/docs/Web/API/Blob) interface, so the contents can be read in various formats.

```ts
const foo = Bun.file("foo.txt");

await foo.text(); // contents as a string
await foo.json(); // contents as a JSON object
foo.stream(); // contents as ReadableStream
await foo.arrayBuffer(); // contents as ArrayBuffer
await foo.bytes(); // contents as Uint8Array
```

You can also create file references from numerical [file descriptors](https://en.wikipedia.org/wiki/File_descriptor) or `file://` URLs.

```ts
Bun.file(1234);
Bun.file(new URL(import.meta.url)); // reference to the current file
```

A `BunFile` can point to a location on disk where a file does not exist.

```ts
const notreal = Bun.file("notreal.txt");
notreal.size; // 0
notreal.type; // "text/plain;charset=utf-8"
const exists = await notreal.exists(); // false
```

The default MIME type is `text/plain;charset=utf-8`. Override it by passing a second argument to `Bun.file`.

```ts
const notreal = Bun.file("notreal.json", { type: "application/json" });
notreal.type; // => "application/json;charset=utf-8"
```

For convenience, Bun exposes `stdin`, `stdout` and `stderr` as instances of `BunFile`.

```ts
Bun.stdin; // readonly
Bun.stdout;
Bun.stderr;
```

### Deleting files (`file.delete()`)

Call `.delete()` to delete a file.

```ts
await Bun.file("logs.json").delete();
```

---

## Writing files (`Bun.write()`)

`Bun.write(destination, data): Promise<number>`

The `Bun.write` function is a multi-tool for writing payloads of all kinds to disk.

The first argument is the `destination`, which can be any of the following:

- `string`: A path to a location on the file system. Use the `"path"` module to manipulate paths.
- `URL`: A `file://` descriptor.
- `BunFile`: A file reference.

The second argument is the data to write. It can be any of the following:

- `string`
- `Blob` (including `BunFile`)
- `ArrayBuffer` or `SharedArrayBuffer`
- `TypedArray` (`Uint8Array`, et. al.)
- `Response`

Bun handles each combination with the fastest available system call on the current platform.

<Accordion title="See syscalls">

| Output               | Input          | System call                   | Platform |
| -------------------- | -------------- | ----------------------------- | -------- |
| file                 | file           | copy_file_range               | Linux    |
| file                 | pipe           | sendfile                      | Linux    |
| pipe                 | pipe           | splice                        | Linux    |
| terminal             | file           | sendfile                      | Linux    |
| terminal             | terminal       | sendfile                      | Linux    |
| socket               | file or pipe   | sendfile (if http, not https) | Linux    |
| file (doesn't exist) | file (path)    | clonefile                     | macOS    |
| file (exists)        | file           | fcopyfile                     | macOS    |
| file                 | Blob or string | write                         | macOS    |
| file                 | Blob or string | write                         | Linux    |

</Accordion>

To write a string to disk:

```ts
const data = `It was the best of times, it was the worst of times.`;
await Bun.write("output.txt", data);
```

To copy a file to another location on disk:

```ts
const input = Bun.file("input.txt");
const output = Bun.file("output.txt"); // doesn't exist yet!
await Bun.write(output, input);
```

To write a byte array to disk:

```ts
const encoder = new TextEncoder();
const data = encoder.encode("datadatadata"); // Uint8Array
await Bun.write("output.txt", data);
```

To write a file to `stdout`:

```ts
const input = Bun.file("input.txt");
await Bun.write(Bun.stdout, input);
```

To write the body of an HTTP response to disk:

```ts
const response = await fetch("https://bun.com");
await Bun.write("index.html", response);
```

---

## Incremental writing with `FileSink`

Bun provides a native incremental file writing API called `FileSink`. To retrieve a `FileSink` instance from a `BunFile`:

```ts
const file = Bun.file("output.txt");
const writer = file.writer();
```

To incrementally write to the file, call `.write()`.

```ts
const file = Bun.file("output.txt");
const writer = file.writer();

writer.write("it was the best of times\n");
writer.write("it was the worst of times\n");
```

These chunks are buffered internally. To flush the buffer to disk, use `.flush()`. This returns the number of flushed bytes.

```ts
writer.flush(); // write buffer to disk
```

The buffer also auto-flushes when the `FileSink`'s _high water mark_ is reached; that is, when its internal buffer is full. You can configure this value.

```ts
const file = Bun.file("output.txt");
const writer = file.writer({ highWaterMark: 1024 * 1024 }); // 1MB
```

To flush the buffer and close the file:

```ts
writer.end();
```

By default, the `bun` process stays alive until this `FileSink` is explicitly closed with `.end()`. To opt out of this behavior, "unref" the instance.

```ts
writer.unref();

// to "re-ref" it later
writer.ref();
```

---

## Directories

Bun's implementation of `node:fs` is fast. Use `node:fs` for working with directories in Bun.

### Reading directories (readdir)

To read a directory in Bun, use `readdir` from `node:fs`.

```ts
import { readdir } from "node:fs/promises";

// read all the files in the current directory
const files = await readdir(import.meta.dir);
```

#### Reading directories recursively

To recursively read a directory in Bun, use `readdir` with `recursive: true`.

```ts
import { readdir } from "node:fs/promises";

// read all the files in the current directory, recursively
const files = await readdir("../", { recursive: true });
```

### Creating directories (mkdir)

To recursively create a directory, use `mkdir` in `node:fs`:

```ts
import { mkdir } from "node:fs/promises";

await mkdir("path/to/dir", { recursive: true });
```

---

## Benchmarks

The following is a 3-line implementation of the Linux `cat` command.

```ts cat.ts icon="/icons/typescript.svg"
// Usage
// bun ./cat.ts ./path-to-file

import { resolve } from "path";

const path = resolve(process.argv.at(-1));
await Bun.write(Bun.stdout, Bun.file(path));
```

To run the file:

```bash terminal icon="terminal"
bun ./cat.ts ./path-to-file
```

It runs 2x faster than GNU `cat` for large files on Linux.

<Frame>![Cat screenshot](/images/cat.jpg)</Frame>

---

## Reference

```ts
interface Bun {
  stdin: BunFile;
  stdout: BunFile;
  stderr: BunFile;

  file(path: string | number | URL, options?: { type?: string }): BunFile;

  write(
    destination: string | number | BunFile | URL,
    input: string | Blob | ArrayBuffer | SharedArrayBuffer | TypedArray | Response,
  ): Promise<number>;
}

interface BunFile {
  readonly size: number;
  readonly type: string;

  text(): Promise<string>;
  stream(): ReadableStream;
  arrayBuffer(): Promise<ArrayBuffer>;
  json(): Promise<any>;
  writer(params: { highWaterMark?: number }): FileSink;
  exists(): Promise<boolean>;
}

export interface FileSink {
  write(chunk: string | ArrayBufferView | ArrayBuffer | SharedArrayBuffer): number;
  flush(): number | Promise<number>;
  end(error?: Error): number | Promise<number>;
  start(options?: { highWaterMark?: number }): void;
  ref(): void;
  unref(): void;
}
```
