finding size of a file using ls and du .what is difference

Question

There is a file named today.log in my server.

ls -l today.log showing 400GB.

du -sh today.log. showing 240GB

What is the difference between ls and du ...

Answer 1

du shows how much disk the file uses. ls shows how big the file is. These two values can be different. Files with holes can take up less space than their size. Most files do not completely fill the blocks of the filesystem, so they take up more space than their size. A file with a single byte still takes up at least one full block. (512 or 1024 bytes, typically.) As an examle, consider a file with a single byte at position 183738475 (randomly typed numbers). That file can be stored on disk using a single block (whenever the kernel queries the filesystem for bytes other than the single byte in the file, the filesystem reports them as being zero, and there is no need to store anything. Not all filesystems work this way.) But the size of the file is 183738475, so ls will report that and du will report how many blocks are used by the filesystem. du -h will report the number of blocks used times the block size converted to a human readable format. Keep in mind that the actual numbers will vary depending on your filesystem. For example:

$ echo > foo; ls -l foo |awk '{print $5}'; du foo; du -h foo
1
8   foo
4.0K    foo

This file is one byte in size but consumes 8 blocks on disk, and the block size is 512 so those 8 blocks consume 4k. (My filesystem has been optimized for large files, and small files waste a lot of space.)