All packages downloaded from the registry are stored in a global cache at `~/.bun/install/cache`, or the path defined by the environment variable `BUN_INSTALL_CACHE_DIR`. They are stored in subdirectories named like `${name}@${version}`, so multiple versions of a package can be cached.

{% details summary="Configuring cache behavior (bunfig.toml)" %}

```toml
[install.cache]
# the directory to use for the cache
dir = "~/.bun/install/cache"

# when true, don't load from the global cache.
# Bun may still write to node_modules/.cache
disable = false

# when true, always resolve the latest versions from the registry
disableManifest = false
```

{% /details %}

## Minimizing re-downloads

Bun strives to avoid re-downloading packages multiple times. When installing a package, if the cache already contains a version in the range specified by `package.json`, Bun will use the cached package instead of downloading it again.

{% details summary="Installation details" %}
If the semver version has pre-release suffix (`1.0.0-beta.0`) or a build suffix (`1.0.0+20220101`), it is replaced with a hash of that value instead, to reduce the chances of errors associated with long file paths.

When the `node_modules` folder exists, before installing, Bun checks that `node_modules` contains all expected packages with appropriate versions. If so `bun install` completes. Bun uses a custom JSON parser which stops parsing as soon as it finds `"name"` and `"version"`.

If a package is missing or has a version incompatible with the `package.json`, Bun checks for a compatible module in the cache. If found, it is installed into `node_modules`. Otherwise, the package will be downloaded from the registry then installed.
{% /details %}

## Fast copying

Once a package is downloaded into the cache, Bun still needs to copy those files into `node_modules`. Bun uses the fastest syscalls available to perform this task. On Linux, it uses hardlinks; on macOS, it uses `clonefile`.

## Saving disk space

Since Bun uses hardlinks to "copy" a module into a project's `node_modules` directory on Linux and Windows, the contents of the package only exist in a single location on disk, greatly reducing the amount of disk space dedicated to `node_modules`.

This benefit also applies to macOS, but there are exceptions. It uses `clonefile` which is copy-on-write, meaning it will not occupy disk space, but it will count towards drive's limit. This behavior is useful if something attempts to patch `node_modules/*`, so it's impossible to affect other installations.

{% details summary="Installation strategies" %}
This behavior is configurable with the `--backend` flag, which is respected by all of Bun's package management commands.

- **`hardlink`**: Default on Linux and Windows.
- **`clonefile`** Default on macOS.
- **`clonefile_each_dir`**: Similar to `clonefile`, except it clones each file individually per directory. It is only available on macOS and tends to perform slower than `clonefile`.
- **`copyfile`**: The fallback used when any of the above fail. It is the slowest option. On macOS, it uses `fcopyfile()`; on Linux it uses `copy_file_range()`.
- **`symlink`**: Currently used only `file:` (and eventually `link:`) dependencies. To prevent infinite loops, it skips symlinking the `node_modules` folder.

If you install with `--backend=symlink`, Node.js won't resolve node_modules of dependencies unless each dependency has its own `node_modules` folder or you pass `--preserve-symlinks` to `node` or `bun`. See [Node.js documentation on `--preserve-symlinks`](https://nodejs.org/api/cli.html#--preserve-symlinks).

```bash
$ bun install --backend symlink
$ node --preserve-symlinks ./foo.js
$ bun --preserve-symlinks ./foo.js
```

{% /details %}
