Every file on a Unix filesystem has an inode — a small data structure storing the file’s metadata: permissions, ownership, timestamps, size, and pointers to the actual data blocks on disk. Everything except the filename, which lives in the directory instead. This separation means a single file can have multiple names (hard links) pointing at the same inode, and that deleting a “file” merely removes a directory entry — the data persists until the inode’s link count hits zero. It also means filesystems can run out of inodes before running out of disk space, which produces one of computing’s more confusing error messages: “No space left on device” on a filesystem that is demonstrably not full. Explaining inodes to a user who just wants to know why they can’t save a file is a rite of passage for every sysadmin.
