If you are a little like me and you like to have everything under control on your PC, you probably don't like to see Skype starting every time you start your computer. Maybe you have tried to stop this from happening by going to start-up application only to realize that Skype isn't there.
If you have installed Skype from Ubuntu Software, you are about to solve your problem.
Open a terminal (cntr+alt+t) end enter the following command:
cd snap/skype/176/.config/autostart
If you check in this directory there should be only one file named skypeforlinux.desktop. If this is correct enter the next command:
cp skypeforlinux.desktop skypeforlinux.desktop.backup
Now you can remove the original file with this command
rm skypeforlinux.desktop
Reboot, and you will not see Skype bothering you again.
FINAL NOTE
The number 176 inside the path in the first command refers to the version of Skype and might be different, depending on your installation.
