me@myhost$ mkdir public_html
http://localhost/~USER_NAME. We can improve upon this with fake domains, but first things first.
There are only two commands you need to enter to activate the User Directory feature, and then one command to reload the configuration files. The last command includes an absolute path, so it doesn't matter where you execute it from. The first two "ln" commands assume you are in the directory /etc/apache2/mods-enabled. What you need to do is create two symbolic links (soft links, symlinks) in the stated directory pointing to the corresponding module in /etc/apache2/mods-available. So, if "$" is your prompt,
me@myhost$ cd /etc/apache2/mods-enabled
me@myhost$ sudo ln -s ../mods-available/userdir.conf userdir.conf
me@myhost$ sudo ln -s ../mods-available/userdir.load userdir.load
me@myhost$ sudo /etc/init.d/apache2 restart
On your home system, restarting apache will give you a warning "apache2: