How to add third-party apt / apt-get repository on Ubuntu

apt and apt-get are the easiest ways to install applications for Ubuntu. Official package repository for the tools is hosted and managed by Ubuntu. It contain most but not all packages that an average Ubuntu user would need.

Some users or corporations host packages in third-party repositories called PPA or Personal Package Archives. They host non-standard applications or even different versions of the applications that are hosted in the official repository. You can add the PPA repositories to your Ubuntu system using add-apt-repository tool and then install the packages using apt.

1. Search for PPA repository to install from Ubuntu's Personal Package Archives site.

   https://launchpad.net/ubuntu/+ppas

2. Launch terminal.
3. Add PPA apt repository using add-apt-repository.

   $ sudo add-apt-repository ppa:ondrej/php
   [sudo] password for user:
    Co-installable PHP versions: PHP 5.6, PHP 7.x and most requested extensions are included. Only Supported Versions of PHP (http://php.net/supported-versions.php) for Supported Ubuntu Releases (https://wiki.ubuntu.com/Releases) are provided. Don't ask for end-of-life PHP versions or Ubuntu release, they won't be provided.
   
   Debian oldstable and stable packages are provided as well: https://deb.sury.org/#debian-dpa
   
   You can get more information about the packages at https://deb.sury.org
   
   BUGS&FEATURES: This PPA now has a issue tracker:
   https://deb.sury.org/#bug-reporting
   
   CAVEATS:
   1. If you are using php-gearman, you need to add ppa:ondrej/pkg-gearman
   2. If you are using apache2, you are advised to add ppa:ondrej/apache2
   3. If you are using nginx, you are advised to add ppa:ondrej/nginx-mainline
      or ppa:ondrej/nginx
   
   PLEASE READ: If you like my work and want to give me a little motivation, please consider donating regularly: https://donate.sury.org/
   
   WARNING: add-apt-repository is broken with non-UTF-8 locales, see
   https://github.com/oerdnj/deb.sury.org/issues/56 for workaround:
   
   # LC_ALL=C.UTF-8 add-apt-repository ppa:ondrej/php
    More info: https://launchpad.net/~ondrej/+archive/ubuntu/php
   Press [ENTER] to continue or Ctrl-c to cancel adding it.
   ##### snipped

4. Update package list for apt.

   $ sudo apt update
   Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [107 kB]      
   Hit:2 http://ppa.launchpad.net/ondrej/php/ubuntu focal InRelease               
   Hit:3 http://jp.archive.ubuntu.com/ubuntu focal InRelease
   Get:4 http://jp.archive.ubuntu.com/ubuntu focal-updates InRelease [111 kB]
   Get:5 http://jp.archive.ubuntu.com/ubuntu focal-backports InRelease [98.3 kB]
   Fetched 317 kB in 3s (97.8 kB/s)    
   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   8 packages can be upgraded. Run 'apt list --upgradable' to see them.

5. Upgrade installed packages based on new package listing.

   $ sudo apt dist-upgrade --assume-yes
   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   Calculating upgrade... Done
   The following packages will be upgraded:
     libgd3 libidn2-0 libpcre2-32-0 libpcre2-8-0 libpcre3 libssl1.1 libxml2
     openssl
   8 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
   Need to get 3,894 kB of archives.
   After this operation, 100 kB of additional disk space will be used.
   Get:1 http://ppa.launchpad.net/ondrej/php/ubuntu focal/main amd64 libpcre3 amd64 2:8.44-1+ubuntu20.04.1+deb.sury.org+1 [240 kB]
   Get:2 http://ppa.launchpad.net/ondrej/php/ubuntu focal/main amd64 libidn2-0 amd64 2.3.0-1+ubuntu20.04.1+deb.sury.org+2 [68.7 kB]
   Get:3 http://ppa.launchpad.net/ondrej/php/ubuntu focal/main amd64 libpcre2-8-0 amd64 10.35-4+ubuntu20.04.1+deb.sury.org+1 [201 kB]
   Get:4 http://ppa.launchpad.net/ondrej/php/ubuntu focal/main amd64 libgd3 amd64 2.3.0-2+ubuntu20.04.1+deb.sury.org+1 [135 kB]
   ##### snipped

6. Install new packages made available from the newly added repository.

   $ sudo apt install --assume-yes php5.6
   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   The following additional packages will be installed:
     apache2 apache2-bin apache2-data apache2-utils libapache2-mod-php5.6 libapr1
     libaprutil1 libaprutil1-dbd-sqlite3 libaprutil1-ldap liblua5.2-0 php-common
     php5.6-cli php5.6-common php5.6-json php5.6-opcache php5.6-readline
   Suggested packages:
     apache2-doc apache2-suexec-pristine | apache2-suexec-custom php-pear
   The following NEW packages will be installed:
     apache2 apache2-bin apache2-data apache2-utils libapache2-mod-php5.6 libapr1
     libaprutil1 libaprutil1-dbd-sqlite3 libaprutil1-ldap liblua5.2-0 php-common
     php5.6 php5.6-cli php5.6-common php5.6-json php5.6-opcache php5.6-readline
   0 upgraded, 17 newly installed, 0 to remove and 0 not upgraded.
   Need to get 7,503 kB of archives.
   After this operation, 24.2 MB of additional disk space will be used.