when I open (etter.conf) to make some changes using the command :
sudo gedit /etc/etter.con I got an empty file
That is not the command to open a .conf
file, you must call a text editor and you have the location wrong.
To open it in a terminal
sudo nano /etc/ettercap/etter.conf
or to open it in a GUI application
gksudo gedit /etc/ettercap/etter.conf
You can swap gedit
for your favourite text editor.
Tested on Xubuntu 15.10
sudo /etc/etter.conf
should result in aPermission denied
error, because it's not supposed to be executable. What exactly are you doing and what's the error message?