Ubuntu – What does “$$” mean in the shell

command line

What does the output of the command below mean? (result is positive integer)

echo $$

The $$ variable in bash and other shells contains the process id of the shell.

See this Stack overflow question and this one also has more detail about what it is more precisely, and commands similar to it.

~/ is shorthand for the current user's home folder. So if your user name is "foobar" it expands to /home/foobar/

In Unix/linux . means the current directory in your case /etc/mds-1.2-beta4/sana/config/etc/Gapache2. There are also many shortcuts like:

So to move all folders and files from /etc/mds-1.2-beta4/sana/config/etc/Gapache2 to /etc/apache2/sites-available, the command will be like this

mv ./* /etc/apache2/sites-available/

UPDATE: This link is a good resource for basic UNIX commands