UNIX for Dummies Questions & Answers

A directory is a type of file too. It has to be modified when you change its contents (i.e. by adding or removing a file or directory).

I see that creation of sub folder does changes its parent folder size.and so parent mtime must get changed and can be understood.
But creation of file in folder does not changes anything in its folder size but it does changes the mtime of its folder. I understand the corresponding file entry is getting added in folder.
What exactly might be getting changed in folder's entry?

File size is irrelevant. mtime is updated whenever a file is written to, whether or not its size is changed. As Scott pointed out, a directory is a file whose contents is a list of files. When a file is created or deleted, the parent directory's contents must be modified. This requires a write which requires an mtime update.

Also, in case it is relevant to your notion of size, the amount of storage consumed by a file and the length of the file's contents are not usually equal. Some tools report the former while others report the latter. Further, regardless of which value is reported, the default unit may be a very coarse block size which may not reflect minor changes.

Regards,
Alister

Thanks for your inputs.
I used du -b <folder name> to check folder size and there was not a single byte change but mtime was updated.
Is it that unix/linux behaves this way.