hardlinks

A way of having two files that reference the same underlying data, such that modifying one modifies the other.

In the context of file system implementation, technically a hardlink is just a directory entry associating a name with an inode. Every addressable file has at least one, but in colloquial usage it’s more common to only talk about hardlinks when there are multiple for a single inode.

Most file systems support file hardlinks, associating multiple paths to a single inode. Using file hardlinks is fairly brittle.

Directory hardlinks usually aren’t supported because they create directory cycles, and most tools that traverse file systems assume a tree structure.

In macOS from OS 10.5 HFS+ supported directory hardlinks and used them for Time Machine. It was possible though risky to create directory hardlinks using the syscall api. APFS removed support for hardlinks in favor of APFS clones.