Gnome

Restart Gnome

For situations like...

Press ALT + F2. In the Enter a Command box that should appear type r and press Enter

If that doesn't work (e.g. it sometimes seems to fail if you are running Gnome over VNC)...

killall -HUP gnome-shell

Later versions of Ubuntu use "Wayland". In this environment restarting using Alt+F2 and r is not possible.
However using Super (aka the Windows Key) +L will log you out without stopping running programs, which should have a similar effect

gnome-shell

Version

gnome-shell --version

gnome-tweak-tool

Check

apt list --installed gnome-tweak-tool

Install

sudo apt install gnome-tweak-tool

Run

gnome-tweaks

gnome-shell-extensions

Check

apt list --installed gnome-shell-extensions

Install

sudo apt install gnome-shell-extensions

Restart Gnome as described above. New Extensions should now appear in Tweak tool...

In Ubuntu 18.04 this also installs:
gir1.2-clutter-1.0 (GObject Introspection data for the Clutter 1.0 library)gir1.2-cogl-1.0 (GObject Introspection data for the Cogl 1.0 library)gir1.2-coglpango-1.0 (GObject Introspection for the GoglPango 1.0 library)

Add Extension (Manual Method)

Add Extension (Browser Add-On)

sudo apt-get install chrome-gnome-shell

Follow the link to install the browser extension. 
Install chrome-gnome-shell (note that this also works for Firefox and Opera).
The extensions.gmome.org website will now show a toggle switch to easily install/enable extensions.

Show Date in Menu Bar

Use gnome-tweak-tool or...

dconf write /org/gnome/desktop/interface/clock-show-date 'true'

Gnome File Indexer

Bibliography