How to empty a file(already created)

Shell Programming and Scripting
1

What is the command to empty an already existing file.

please provide me.i used Touch cmd to empty the file.but it changing the time only.

2 
> file

or

: > file
3

i need to empty a file which is already existing.

4

cat </dev/null >file

5

The command works also for an existing file :

$ ls -l my_file
-rw-r--r-- 1 Jean-Pierre Aucun 8943 Jul  4 18:55 my_file
$ > my_file
$ ls -l my_file
-rw-r--r-- 1 Jean-Pierre Aucun 0 Jul  4 18:56 my_file
$