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More on empty files

Posted by jpluimers on 2021/10/07

TL;DR: Empty files are indeed of size zero, but there is some disk space involved for their meta-data (like name, permission, timestamps)

Some links (via [WayBack] create zero sized file – Google Search):

  • [WayBack] Zero-byte file – Wikipedia
  • [WayBack] filesystems – How can a file size be zero? – Super User (thanks [WayBack] phuclv):

    Filesystems store a lot of information about a file such as file name, file size, creation time, access time, modified time, created user, user and group permissions, fragments, pointer to clusters that store the file, hard/soft links, attributes… Those are called file metadata. Why do you count those metadata into file size when users do not (need to) care about them and don’t know about them? They only really care about the file content

    Moreover each filesystem stores different types of metadata which take different amounts of space on disk. For example POSIX permissions are very different from NTFS permission, and there are also inode numbers in POSIX which do not exist on Windows. Even POSIX filesystems vary a lot, like ext3 with 32-bit block address, ext4 with 48-bit, Btrfs with 64-bit and ZFS with 128-bit address. So how will you count those metadata into file size?

    Take another example with a 100-byte file whose metadata consumes 56 bytes on the current filesystem. We copy the file to another filesystem and now it takes 128 bytes of metadata. However the file contents are exactly the same, the number of bytes in the files are also the same. So displaying file size as 156 bytes on a system but 228 bytes on another is very confusing and counter-intuitive.

  • [WayBack] What is the concept of creating a file with zero bytes in Linux? – Unix & Linux Stack Exchange:

    touch will create an inode, and ls -i or stat will show info about the inode:

    $ touch test
    $ ls -i test
    28971114 test
    $ stat test
      File: ‘test’
      Size: 0           Blocks: 0          IO Block: 4096   regular empty file
    Device: fc01h/64513d    Inode: 28971114    Links: 1
    Access: (0664/-rw-rw-r--)  Uid: ( 1000/1000)   Gid: ( 1000/1000)
    Access: 2017-03-28 17:38:07.221131925 +0200
    Modify: 2017-03-28 17:38:07.221131925 +0200
    Change: 2017-03-28 17:38:07.221131925 +0200
     Birth: -

    Notice that test uses 0 blocks. To store the data displayed, the inode uses some bytes. Those bytes are stored in the inode table. Look at the ext2 page for an example of an inode structure [WayBack].

Oh and a nice NTFS thing (thanks [WayBack] Paweł Bulwan):

and in case of NTFS, the size of file reported by Windows and most tools is actually the size of the main stream of the file, which we perceive as the content of the file. The file stored on NTFS partition can additionaly have some data stored in alternative data streams, and still have the reported size of 0. It’s a nice filesystem feature to know if you want to have the full picture :)

Related: my really old post command line – create empty text file from a batch file (via: Stack Overflow)


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