How to install add-apt-repository in Ubuntu/Debian

The add-apt-repository command is essential for adding external repositories in Ubuntu and Debian. It resides in the software-properties-common package, which is not always installed by default.

When this package is missing, the system may display the following error:

$ sudo add-apt-repository ppa:ondrej/php
sudo: add-apt-repository: command not found

Installing software-properties-common provides tools that manage software sources and enable the addition of external repositories.

1. Launch the terminal.
2. Update package information for apt.

   $ sudo apt update
   [sudo] password for user: 
   Hit:1 http://ports.ubuntu.com/ubuntu-ports jammy InRelease
   Get:2 http://ports.ubuntu.com/ubuntu-ports jammy-updates InRelease [114 kB]
   Get:3 http://ports.ubuntu.com/ubuntu-ports jammy-backports InRelease [99.8 kB]
   Get:4 http://ports.ubuntu.com/ubuntu-ports jammy-security InRelease [110 kB]
   Get:5 http://ports.ubuntu.com/ubuntu-ports jammy-updates/main arm64 Packages [520 kB]
   Get:6 http://ports.ubuntu.com/ubuntu-ports jammy-updates/main arm64 DEP-11 Metadata [91.6 kB]
   Get:7 http://ports.ubuntu.com/ubuntu-ports jammy-updates/universe arm64 Packages [213 kB]
   Get:8 http://ports.ubuntu.com/ubuntu-ports jammy-updates/universe arm64 DEP-11 Metadata [146 kB]
   Get:9 http://ports.ubuntu.com/ubuntu-ports jammy-backports/universe arm64 DEP-11 Metadata [12.5 kB]
   Get:10 http://ports.ubuntu.com/ubuntu-ports jammy-security/main arm64 DEP-11 Metadata [11.4 kB]
   Get:11 http://ports.ubuntu.com/ubuntu-ports jammy-security/universe arm64 DEP-11 Metadata [10.1 kB]
   Fetched 1,329 kB in 10s (127 kB/s)                                           
   Reading package lists... Done
   Building dependency tree... Done
   Reading state information... Done
   8 packages can be upgraded. Run 'apt list --upgradable' to see them.

3. Upgrade installed packages (optional).

   $ sudo apt upgrade --assume-yes

4. Install software-properties-common using apt.

   $ sudo apt install --assume-yes software-properties-common
   Reading package lists... Done
   Building dependency tree... Done
   Reading state information... Done
   The following additional packages will be installed:
     python3-software-properties unattended-upgrades
   Suggested packages:
     bsd-mailx default-mta | mail-transport-agent needrestart
   The following NEW packages will be installed:
     python3-software-properties software-properties-common unattended-upgrades
   0 upgraded, 3 newly installed, 0 to remove and 8 not upgraded.
   Need to get 92.3 kB of archives.
   After this operation, 846 kB of additional disk space will be used.
   ##### snipped