hard link

A hard link creates an additional directory entry that references the same inode (the data structure containing file metadata and disk block pointers) as the original file. Both the original and the hard link are equivalent—there is no primary or secondary file.

When you create a hard link with ln original_file link_name, you're adding a new name that points to the same inode. If you delete the original file, the data persists because the hard link still references it. The inode is only freed when all hard links to it are removed.

Hard links have important restrictions: they cannot span filesystems (since inodes are filesystem-specific) and cannot link to directories (to prevent circular references). For example, ln /home/user/document.txt /home/user/backup_link.txt creates a hard link; both names now refer to the same file content, and both have equal status.