You have two versions of mysql installed - one from Ubuntu repositories and one from some third party location. When you use the Ubuntu version, located in
/usr/bin/mysql it attempt to connect trough a socket in the default location,
/var/run/mysqld/mysqld.sock, which does not work, because the server running is not the Ubuntu stock mysql-server.
It works when you run
/opt/lampp/bin/mysql -uroot (no reason to run that as root, by the way), because this mysql binary has default path set to where the actual socket is.
You have three alternatives:
- Modify your path so that
/opt/lampp/bin/ is before/usr/bin/` in path. This will execute the lampp version of mysql before the Ubuntu version.
- Uninstall Ubuntu mysql client with
sudo apt-get remove mysql-client. This may break other packages.
- Uninstall lampp and install the required packages from Ubuntu.
#3 would be my preferred solution. Getting all the software from the repositories ensures that it will be kept up to date, be (mostly) compatible, and have a clear, well defined upgrade path to the next Ubuntu Release.
Apache, PHP, Mysql/MariaDB and a lot of apache modules are available in the Ubuntu repositories.