Short Version:
Find the Exec command for your app by grepping the applications directory
grep 'Calculator' /usr/share/applications/*
Start the app
gcalctool
Kill it by hitting Ctrl+C
First, find out where your application is. A few directories are in an environment variable $PATH, so that you don't have to type the whole directory.
One way to find your app is to open the Menu Editor (right click on the Applications Menu and select Edit Menus) and find the app's command.
The calculator, for example is gcalctool at /usr/bin/gcalctool (use the whereis command to find the exact path of an app). Since /usr/bin is in the PATH variable (type echo $PATH), you can run it in your terminal by typing gcalctool.
It's now running in the foreground. You can pause it by pressing Ctrl+Z, resume it in the background by typing bg,or resume it in the foreground by typing fg. You can also use fg to raise an app to the foreground.
If you do this with multiple apps, you can use jobs to get a numbered list of all of them, and then use, for example, fg 3 to raise one of them.
An App that is in the foreground can be killed with Ctrl+C or Ctrl+\ (if it doesn't react to the former).
A universal way to find your app is to open the File Browser, go to Filesystem → usr → share → applications and to find your app there.
You'll see a bunch of .desktop files, which you can drag into a Text Editor (or use cat) to read. This is a piece of the Calculators .desktop file:
[Desktop Entry]
Name=Calculator
Comment=Perform arithmetic, scientific or financial calculations
Exec=gcalctool
The Exec entry is what you're interested in here. It's the same you would see if you went to the Menu Editor, just quicker. You can grep the files to search them for your app, if you can't find it.
If the app doesn't have a .desktop file in the applications directory, you have to know it's command of course. Use TAB to get suggestions from just a few letters. Press tab, tab, y to get a huge list of every application.
Best Answer
The best and simplest way is to use console command (this command should be available for all ubuntu-based distributions) :
fc-listExample output (stripped) :
From MAN :
To check all available options use MAN (manual):
man fc-list