What is the association of an inode with a file?

Question

I have learnt from some sources (such as: this), the following things:

• An inode is a data structure that stores relevant information about a file.
• An inode number points to an inode.
• There is a separate inode table which maintains the mapping information of inode number with corresponding inodes.
• On creation of a file, the inode number and filename are assigned to a file.
• When a file is accessed with a filename, internally, the name is first mapped with its inode number and the corresponding inode is accessed.

What I do not understand is mostly the last line. The inode accesses the inode, but the inode is a separate entity apart from the file. And the descriptions given do not say that an inode in any way is associated with the file, just that an inode is a data structure that contains the metadata related with the file. How is the file accessed then? Does the inode invoke the file?

Answer 1

The inode is the file, which is identified solely by its inode number. A file name is just metadata in the file system that refers to a file. A single file/inode can have multiple file names referring to it:

$ touch foo
$ ln foo bar
$ ls -li foo bar
28098391 -rw-r--r--  2 xxxxxxx  xxxxx  0 Jul  6 22:15 bar
28098391 -rw-r--r--  2 xxxxxxx  xxxxx  0 Jul  6 22:15 foo

The first column is the inode number; note that it is the same for both files. The first number after the permissions is the link count; both foo and bar have a link count of 2 because each of the two names refers to the same file.

$ rm foo
$ ls -li bar
28098391 -rw-r--r--  1 xxxxxxx  xxxxx  0 Jul  6 22:15 bar

rm foo only removes that specific link to the underlying file; bar still refers to it (note the inode number has not changed, but the link count is now 1 instead of 2). Also note that neither foo nor bar was the "real" name of the file; the fact that foo was created first didn't make it special in any way. A file is not actually removed until all links are removed (i.e., until the link count is reduced to 0). Even then, the file is not actually removed; the blocks allocated to that inode are simply marked as available for reuse by the file system.

Answer 2

Here is a simplified explanation for a regular file ...

Humans use filenames to refer to files. The kernel uses numbers called inode numbers to refer to files. A directory is a mapping between a filename and an inode number. An inode contains the metadata associated with the file and a pointer to the first data block of the file. The first data block contains a pointer to the second data block, and so on until the last data block in read.