Are files defined by their content blocks, inodes, both, or filenames?

Question

Are files defined by their content blocks, inodes, both, or filenames, or something else? Fundamentially, what is the definition of a file?

E.g.

two hard links can share the same inode and thus content blocks, but they have different filenames. Are they considered the same file or different files?

Are there some files which have different inodes, but the same content block? Are they considered the same file?

Thanks.

Answer 1

I'm assuming we're discussing "regular" files, not device files or unix-domain sockets or something not-so-regular like that.

I would say that files have names, metadata, and data.

This corresponds directly to filenames, inodes and blocks.

I believe that your two, hard-linked filenames are just two names for a single file.

I don't believe that under Unix or Linux or *BSD, you can have two inodes referencing the same data blocks without some kind of very unusual intervention. An suid program could potentially open a disk's device file, and manipulate two inodes to point to the same data blocks, but I sincerely doubt that situation could arise organically.

Answer 2

Filenames are used to lookup inodes. Nothing else. Inodes are the main point to reference a file. Afile may not have any data blocks at all if it is zero size or if it is small enough to fit in the blocklist portion of the inode and the filesystem has that optimization. if there are two hard links pointing at one inode it is still one file, it just has more than one name. This allows you to do such things as to rename and delete open files. This makes it much easier to upgrade software on a running system unlike windows where the primary identifier is the filename and you have to reboot after every upgrade to replace files in use.