Git is by far the most widely used distributed version control system while being free and open source. git client is not installed by default in the desktop variants of Ubuntu though despite the popularity.

git is available in Ubuntu's default apt repository, so it could easily be installed from the terminal using apt.

Step-by-step video guide:

Steps to install Git client on Ubuntu and Debian:

  1. Open terminal from the application launcher or via the <ctrl> + <alt> + <t> keyboard shortcut.
  2. Update apt's package list from the repository. 

    $ sudo apt update
[sudo] password for user:
Get:1 http://security.ubuntu.com/ubuntu disco-security InRelease [97.5 kB]
Hit:2 http://my.archive.ubuntu.com/ubuntu disco InRelease
Get:3 http://my.archive.ubuntu.com/ubuntu disco-updates InRelease [97.5 kB]
Get:4 http://my.archive.ubuntu.com/ubuntu disco-backports InRelease [88.8 kB]
Fetched 284 kB in 1s (198 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
  3. Install git package via apt. 

    $ sudo apt install --assume-yes git
[sudo] password for user:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
  git-man liberror-perl
Suggested packages:
  git-daemon-run | git-daemon-sysvinit git-doc git-el git-email git-gui gitk gitweb git-cvs git-mediawiki git-svn
The following NEW packages will be installed:
  git git-man liberror-perl
0 upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
##### snipped
  4. Test by running git from the command line. 

    $ git
usage: git [--version] [--help] [-C <path>] [-c <name>=<value>]
           [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
           [-p | --paginate | -P | --no-pager] [--no-replace-objects] [--bare]
           [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
           <command> [<args>]

These are common Git commands used in various situations:

start a working area (see also: git help tutorial)
   clone      Clone a repository into a new directory
   init       Create an empty Git repository or reinitialize an existing one
------ snipped ------
  5. Install GUI-based git client (optional).