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# apt-add-repository contrib
-bash: apt-add-repository: command not found

# apt-get install software-properties-common
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following additional packages will be installed:
  gir1.2-glib-2.0 gir1.2-packagekitglib-1.0 libappstream4 libdw1 libgirepository-1.0-1 libglib2.0-bin libgstreamer1.0-0 libpackagekit-glib2-18 libpolkit-agent-1-0 libpolkit-gobject-1-0
  libstemmer0d libunwind8 libyaml-0-2 packagekit packagekit-tools policykit-1 python3-dbus python3-distro-info python3-gi python3-software-properties unattended-upgrades
Suggested packages:
  gstreamer1.0-tools appstream python-dbus-doc python3-dbus-dbg bsd-mailx default-mta | mail-transport-agent needrestart powermgmt-base
The following NEW packages will be installed:
  gir1.2-glib-2.0 gir1.2-packagekitglib-1.0 libappstream4 libdw1 libgirepository-1.0-1 libglib2.0-bin libgstreamer1.0-0 libpackagekit-glib2-18 libpolkit-agent-1-0 libpolkit-gobject-1-0
  libstemmer0d libunwind8 libyaml-0-2 packagekit packagekit-tools policykit-1 python3-dbus python3-distro-info python3-gi python3-software-properties software-properties-common
  unattended-upgrades
0 upgraded, 22 newly installed, 0 to remove and 0 not upgraded.
Need to get 4,792 kB of archives.
After this operation, 15.6 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
 

# apt-add-repository contrib
'contrib' distribution component enabled for all sources.