Why is du giving me unexpected output?

Question

I'm new to Linux, so sorry for my simple question. I'm on Ubuntu.

I have this directory:

$ ls -al
total 16
drwxr-xr-x 2 alberto alberto 4096 2011-04-03 16:02 .
drwxr-xr-x 4 alberto alberto 4096 2011-04-03 16:01 ..
-rw-r--r-- 1 alberto alberto    7 2011-04-03 16:02 .hidden
-rw-r--r-- 1 alberto alberto    4 2011-04-03 15:51 testfile

When I run du, I'm expecting this:

$ du
4   testfile
4   .hidden

But I'm getting this:

$ du
12  .
$ du *
4   testfile
$ du .*
12  .
20  ../snippet
12  ../test
52  ..
4   .hidden

Why is this happening?

Answer 1

du summarizes the disk usage directory by directory. In your example, since the current directory has no subdirectory, the output is a single line.

If you pass the name of a file on the command line, its size is reported. If you pass the names of multiple files, this is equivalent¹ to running du multiple times; hence du .* is equivalent to du .; du ..; du .hidden.

Run du -a to see the space occupied by each file.

 $ du -a
 4   .hidden
 4   testfile
 12  .

¹ _{Almost: on some systems, if a regular file has multiple hard links, then it's only counted the first time it's reached in a particular du invocation.}