Why did apt-get install Virtual Box instead of g++?

The command

sudo apt-get install g++ 5.0 

indicates you want to install two packages: g++ and 5.0. (Package names don't have spaces, and apt-get accepts multiple package names, separated by spaces.)

What probably happened is that it installed g++ as requested, then installed all packages (including version numbers) that match the regular expression 5.0 (since there's no package actually named 5.0). (thanks @edwinksl!)

To avoid this, make sure you have the correct package names, without spaces. You can also use the -s option to simulate an apt-get action before doing it for real:

sudo apt-get -s install g++ 5.0

will show you the actions that the command would perform, without actually installing anything. If it looks OK, you can remove the -s to perform the installation.

You could also consider using a more newbie-friendly graphical package manager, such as synaptic or muon.

The correct command to install g++ version 5.x is:

sudo apt-get install g++-5

This will install g++ version 5.3 on xenial, which is the current default (so apt-get install g++ installs it as well, but this will change in the future). In fact, there is no public 5.0 release of GCC. Other g++ releases are packaged, e.g. g++-4.9 or g++-6, which can be installed in the same way.

If you ever need to install a specific (existing) release of g++ which is not packaged for your system, you'll have to build it from sources.

Let's break it down.

First of all, few notes on how apt-get install (and most other apt-get arguments too) works:

• You can input multiple package names:

  sudo apt-get install foobar spamegg 
  

• The package names are actually Extended Regular Expression (ERE) (Check man 7 regex) patterns so a package name foo.bar means any package name that has a substring that starts with foo and ends with bar with any single character between foo and bar. Likely f.*r means any package name that contains a substring that has f and r with any number of characters i.e. anything in between. To do the whole package name matching, use start and end tokens e.g. ^foo.bar$. If you want any Regex token to be treat literally then you need to escape the token with \ e.g. for treating foo.bar literally, you need:

  sudo apt-get install 'foo\.bar'
  

  Here the single quoting is to prevent shell interpretation of the pattern as a globbing pattern, not necessary in this case but would be needed e.g. for pattern foo.*bar if you have a file in the current directory named e.g. foo.bar.

• There is a catch on the package name consideration. If there is any package matching the pattern, the pattern will be treated literally and no Regex interpretation will be done. For example, for a package pattern g++, it will match the package g++ literally irrespective of the Regex token +. If there were no package named g++ in the defined repositories, it will be treated as a ERE pattern.

Now, you have given the command:

sudo apt-get install g++ 5.0

This means:

• You want to install two patterns provided packages namely g++ and 5.0

• g++ matches the literal meta-package g++ as mentioned above

• The remaining portion, 5.0, has a Regex token, . i.e. any single character. So this will match any package name that contains 5<any_character>0. So all the packages that have the match has been selected to be installed and presumably virtualbox-5.0 has also been selected in the process.

Presumably you want to install g++ version 5, so doing the following would do(already mentioned in this answer); The meta-package, g++-5, will refer to the latest available minor released package of g++ version 5:

sudo apt-get install g++-5

To search for any packages, within the configured repositories, use apt-cache (uses ERE like apt-get):

apt-cache search 'g\+\+-[0-9]+'

If you do not want to Regex-ify it, use less to scroll down the rather larger list:

apt-cache search g++ | less

Also before installing anything you are not sure about, do not use -y (--assume-yes) option and test it first with -s (--simulate/--dry-run):

sudo apt-get install --dry-run foobar