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This is safer than editing the 
/etc/apt/sources.list
 file directly, since there's always 
a chance you can make a typo and disrupt your ability to download packages from 
even the official repositories.
In addition, you may need to install a 
GNU Privacy Guard
 (
GnuPG
) key for a new 
repository, but this process differs from one application to another. Typically, the 
documentation will outline the entire process. This key basically protects you in that 
it makes sure that you're installing signed packages. Not all developers protect their 
applications this way, but it's definitely a good thing to do.
Once you have the repository (and possibly the key) installed on your server, you'll 
need to run the following command to update your package index:
sudo apt update
As mentioned earlier in this chapter, this command updates your local cache as to 
which packages are available on the remote server. APT is only aware of packages 
that are in its database, so you will need to sync this with that command before you'll 
be able to actually install the software contained within the repository.