inode

An inode, short for index node, is a data structure used by many Unix-like file systems to store information about a file or directory, without including its name or its actual data. The inode is identified by an inode number, which uniquely identifies the file within the filesystem.

Inside an inode are metadata fields such as the file type, ownership (user and group IDs), access permissions, size in bytes, and timestamps including last access, modification, and change times. It also records the number of hard links to the file and various flags. Most importantly, the inode contains pointers to the disk blocks that contain the file's data, typically arranged as a set of direct pointers and indirect pointers for larger files.

Names are stored in directory entries that map filenames to inode numbers. This separation means that a file can have multiple hard links, all referencing the same inode. The file's data is kept on disk as blocks accessed through those pointers; when all links are removed and no process has the file open, the space can be freed and the inode reused.

In many file systems, inodes are allocated in a fixed inode table created at filesystem creation. The number and size of inodes affect how many files a filesystem can hold. Inodes sizes vary by filesystem; common sizes range from 128 to 256 bytes, with ext4 commonly using 256-byte inodes and ext2 often using 128-byte inodes depending on block size.

Access to a file’s metadata typically requires reading its inode from disk (e.g., via a stat operation). The inode number is often displayed by tools such as ls -li, and hard links share the same inode number while different filenames map to the same inode.