Global cache

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.

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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.

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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.

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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.