Linux must allocate an index node (inode) for every file and directory in the filesystem. Inodes do not store actual data. Instead, they store the metadata where you can find the storage blocks of each file’s data.
In other words, a “file” is actually composed of three different things:
- a PATH in the filesystem
- an inode with metadata
- data blocks pointed to by the inode
Metadata in an Inode
The following metadata exists in an inode:
- File type
- Permissions
- Owner ID
- Group ID
- Size of file
- Time last accessed
- Time last modified
- Soft/Hard Links
- Access Control List (ACLs)
Check the Inode Number in a Specific File
The command stat displays the file statistics, including the unique inode number:
stat
To see the metadata for a certain file, we can use the stat() or fstat() system calls. These calls take a pathname (or file descriptor) to a file and fill in a stat structure.
struct stat {
dev_t st_dev; // ID of device containing file
ino_t st_ino; // inode number
mode_t st_mode; // protection
nlink_t st_nlink; // number of hard links
uid_t st_uid; // user ID of owner
gid_t st_gid; // group ID of owner
dev_t st_rdev; // device ID (if special file)
off_t st_size; // total size, in bytes
blksize_t st_blksize; // blocksize for filesystem I/O
blkcnt_t st_blocks; // number of blocks allocated
time_t st_atime; // time of last access
time_t st_mtime; // time of last modification
time_t st_ctime; // time of last status change
};