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Managing Debian packages with apt
APT
, or 
Advanced Package Tool
, is a suite of tools that allows us to install, remove, 
and update Debian packages. There are various sub-commands that make up this 
suite, which we'll go over now. The most popular variation of the 
apt
 command is 
apt install
. If you've ever read instructions for how to do something in Ubuntu, 
chances are you've already run that command to install a package. And that's exactly 
what it does: it allows you to install packages for Ubuntu over the command line. For 
example, the following command will install the 
openssh-server
 package:
sudo apt install openssh-server
You can also install multiple packages at a time by separating each with a space, 
instead of installing each package one at a time. The following example will install 
three different packages:
sudo apt install
So, what actually happens when you install a package with 
apt
? If you've run 
through the process before, you're probably accustomed to this process already. 
But, typically, the process begins with 
apt
 calculating dependencies. The majority 
of packages require other packages to function, so 
apt
 will check to ensure that the 
package you're requesting is available and that its dependencies are available as well. 
First, you'll see a summary of the changes that 
apt
 wants to make to your server. In 
the case of installing the 
apache2
 package on an unconfigured Ubuntu Server, I enter 
the following command:
sudo apt install apache2
I see the following output on my system when it starts to install:
In some books, blogs, and articles, you may see longer versions 
of 
apt
 commands such as 
apt-get install
 instead of just 
apt install
. Being able to shorten commands such as 
apt-
get install
 to 
apt install
 is a relatively new feature of 
apt
 
in Debian and Ubuntu. Both methods are perfectly valid, but 
simplifying APT commands down to just 
apt
 is preferred going 
forward.

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Chapter 3