How do I add minimize / maximize buttons to GNOME shell windows?

You need gnome tweak tool.

Click the Download button to install with Ubuntu Software Center:

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Or you can install in Terminal:

sudo apt-get update; sudo apt-get install gnome-tweak-tool

Then open Advanced Settings to change the minimize and maximize buttons.

Install dconf-editor:

sudo apt-get install dconf-tools
dconf-editor

Then navigate to:

org.gnome.shell.overrides >> button-layout

and set value to:

close,minimize,maximize: **OR** :maximize, minimize,close

Note that you might also perform this from console:

gsettings set org.gnome.shell.overrides button-layout close,minimize,maximize: 

In current versions of Gnome, you may need to use this instead:

gsettings set org.gnome.desktop.wm.preferences button-layout 'close,minimize,maximize:'

This is very simple guys, just type the following command in the terminal, it worked for me:

gsettings set  org.gnome.shell.overrides button-layout ':minimize,maximize,close'

In Ubuntu 17.10 (and Debian 9 Stretch) with gnome 3.26.1 command is:

gsettings set org.gnome.desktop.wm.preferences button-layout ':minimize,maximize,close'

(thanks to Ohto Nordberg's reply).