1. Name

du [man page] - estimate file space usage

2. Synopsis

du [OPTION]... [FILE]...
du [OPTION]... --files0-from=F
 

3. Frequently used options

-h, --human-readable print sizes in human readable 
format (e.g., 1K 234M 2G) 
-s, --summarize display only a total for 
each argument
--max-depth=N print the total for a directory 
(or file, with --all)
only if it is  N  or  fewer  levels
below  the  command   line   argument;
--max-depth=0 is the same as --summarize 

4. Examples

du is abbreviation of "disk usage". This command tool reports usage by given directory. Lets suppose that we have this directory structure in /tmp:

$ cd /tmp
$ mkdir -p dir1/dir2/dir3
$ tree dir1 

By default du command traverse whole directory structure:

$ du dir1 

by option -s we instruct du command to print total just for given argument. In our case the argument is dir1.

$ du -s dir1 

We can do the same by --max-depth=N:

$ du --max-depth=0 dir1 

Lets se how it works on /var directory:

# du -s /var 

# du --max-depth=1 /var 

If its hard for you to read size in bytes you can instruct du to display size in human readable format:

# du -h --max-depth=1 /var