If you have wmctrl installed, the first line of output from command

wmctrl -m

will show you the name of the window manager. If you don't have wmctrl installed, use the following command to install it:

sudo apt-get install wmctrl

Note, man wmctrl (that is, the man page for wmctrl) says in part:

-m Display information about the window manager ...

On my Ubuntu 11.10 / Linux 3.0.0-14-generic #23-Ubuntu x86_64 / metacity 2.34.1 system, the first line of output from wmctrl -m was: Name: Metacity
while echo $DESKTOP_SESSION showed ubuntu-2d and echo $XDG_CURRENT_DESKTOP showed Unity.

On a fresh Ubuntu 15.10 install of Ubuntu desktop software on an old laptop, the first line of output from wmctrl -m was: Name: Compiz
while the DESKTOP_SESSION and XDG_CURRENT_DESKTOP values were ubuntu and Unity

On Linux systems I tested environment variables XDG_CURRENT_DESKTOP and GDMSESSION and got the following results.

You can use this line to get the output:

printf 'Desktop: %s\nSession: %s\n' "$XDG_CURRENT_DESKTOP" "$GDMSESSION"

Ubuntu 18.04 and 20.04 (Ubuntu on GNOME)

• XDG_CURRENT_DESKTOP=ubuntu:GNOME
• GDMSESSION=ubuntu

Ubuntu 18.04 (Ubuntu on Wayland)

• XDG_CURRENT_DESKTOP=ubuntu:GNOME
• GDMSESSION=ubuntu-wayland

Ubuntu 16.04

Unity (7)

• XDG_CURRENT_DESKTOP=Unity
• GDMSESSION=ubuntu

Ubuntu 14.04

Unity running with Mir

• XDG_CURRENT_DESKTOP=Unity
• GDMSESSION=ubuntu

Note MIR_SERVER_NAME is also set

• MIR_SERVER_NAME=session-$NUMBER

Unity running without Mir

• XDG_CURRENT_DESKTOP=Unity
• GDMSESSION=ubuntu

XFCE

• XDG_CURRENT_DESKTOP=XFCE
• GDMSESSION=xfce

Ubuntu 12.04

KDE

• XDG_CURRENT_DESKTOP=
• GDMSESSION=kde-plasma

Unity 3D

• XDG_CURRENT_DESKTOP=Unity
• GDMSESSION=ubuntu

Unity 2D

• XDG_CURRENT_DESKTOP=Unity
• GDMSESSION=ubuntu-2d

Gnome

• XDG_CURRENT_DESKTOP=GNOME
• GDMSESSION=gnome-shell

Gnome Classic

• XDG_CURRENT_DESKTOP=GNOME
• GDMSESSION=gnome-classic

Gnome Classic (no effects)

• XDG_CURRENT_DESKTOP=GNOME
• GDMSESSION=gnome-fallback

Other Ubuntu based distributions

Mint 13 (Cinnamon)

• XDG_CURRENT_DESKTOP=GNOME
• GDMSESSION=cinnamon

Mint 16 (KDE edition)

• XDG_CURRENT_DESKTOP=KDE
• GDMSESSION=default

Mint 17 (Cinnamon)

• XDG_CURRENT_DESKTOP=X-Cinnamon
• GDMSESSION=default

Lubuntu 12.04

• XDG_CURRENT_DESKTOP=LXDE
• GDMSESSION=Lubuntu

Wasta 14.04 / 18.04

• XDG_CURRENT_DESKTOP=X-Cinnamon
• GDMSESSION=cinnamon

Wasta 14.04 using Gnome desktop.

• XDG_CURRENT_DESKTOP=GNOME
• GDMSESSION=gnome

Ubuntu Mate 18.04

• XDG_CURRENT_DESKTOP=MATE
• GDMSESSION=mate

Xubuntu 18.04

• XDG_CURRENT_DESKTOP=XFCE
• GDMSESSION=xubuntu

Kali Linux kali-rolling 2020.2

• XDG_CURRENT_DESKTOP=GNOME
• GDMSESSION=lightdm-xsession

In the terminal type env to see all environment variables. Some of them are:

XDG_CURRENT_DESKTOP - Tells you what desktop environment you are using

GDMSESSION - Tells you what option you selected from the lightdm greeter to login.

To use them, go to the terminal and type:

echo $XDG_CURRENT_DESKTOP

(Will output for example 'unity' if you are using Unity)

or

echo $GDMSESSION  

(Will output for example 'ubuntu' if you selected ubuntu in the login or 'ubuntu-2d' if you selected that one in the login.)

You have others if you look closer at the output of env like DESKTOP_SESSION and COMPIZ_CONFIG_PROFILE

Since what you are looking for is the name of what Window Manager is in use, the only way I see how to get this is by looking in the process list. To do this there is a command called pgrep ( Wikipedia ). I did the following to get the name since the parameter -l adds the process ID:

pgrep -l compiz |cut -d " " -f2 since the process is running by the same user there is no need to prefix the sudo part. You can then use this to create a script that does an action based on the Window Manager.

To look for other types, just change the word compiz to another like mutter, kwin, etc..

I found pstree to be a big help.

Installation

sudo apt-get install pstree

output

This is what I got.

 ├─lightdm─┬─Xorg───2*[{Xorg}]
 │         ├─lightdm─┬─gnome-session─┬─bluetooth-apple───2*[{bluetooth-apple}]
 │         │         │               ├─compiz─┬─sh───gtk-window-deco───2*[{gtk-window-deco}]
 │         │         │               │        └─5*[{compiz}]
 │         │         │               ├─deja-dup-monito───2*[{deja-dup-monito}]
 │         │         │               ├─gnome-fallback-───2*[{gnome-fallback-}]
 │         │         │               ├─gnome-screensav───2*[{gnome-screensav}]
 │         │         │               ├─gnome-settings-───2*[{gnome-settings-}]
 │         │         │               ├─nautilus───2*[{nautilus}]
 │         │         │               ├─nm-applet───2*[{nm-applet}]
 │         │         │               ├─polkit-gnome-au───2*[{polkit-gnome-au}]
 │         │         │               ├─ssh-agent
 │         │         │               ├─telepathy-indic───2*[{telepathy-indic}]
 │         │         │               ├─update-notifier───2*[{update-notifier}]
 │         │         │               ├─vino-server───2*[{vino-server}]
 │         │         │               ├─zeitgeist-datah───{zeitgeist-datah}
 │         │         │               └─3*[{gnome-session}]
 │         │         └─{lightdm}
 │         └─2*[{lightdm}]

xprop -id $(xprop -root -notype | awk '$1=="_NET_SUPPORTING_WM_CHECK:"{print $5}') -notype -f _NET_WM_NAME 8t 

Just two cents.

$ sudo apt install neofetch
$ neofetch

#! /bin/bash


windowManagerName () {
    local window=$(
        xprop -root -notype
    )

    local identifier=$(
        echo "${window}" |
        awk '$1=="_NET_SUPPORTING_WM_CHECK:"{print $5}'
    )

    local attributes=$(
        xprop -id "${identifier}" -notype -f _NET_WM_NAME 8t
    )

    local name=$(
        echo "${attributes}" |
        grep "_NET_WM_NAME = " |
        cut --delimiter=' ' --fields=3 |
        cut --delimiter='"' --fields=2
    )

    echo "${name}"
}


windowManagerName

Or in one line:

id=$(xprop -root -notype | awk '$1=="_NET_SUPPORTING_WM_CHECK:"{print $5}'); xprop -id "${id}" -notype -f _NET_WM_NAME 8t | grep "_NET_WM_NAME = " | cut --delimiter=' ' --fields=3 | cut --delimiter='"' --fields=2

I've been testing also with KDE and my conclusion is:

a) Graphical way, with HardInfo: the answer is normally in "Operating System" > "Desktop Environment", but if not you can look to "Environment variables". HardInfo is ready with all the tested distros, except the one with KDE, but it can be easily and quickly installed (only 2 packages in Linux Mint 13).

b) Command line, with this command: ps -A | egrep -i "gnome|kde|mate|cinnamon|lx|xfce|jwm". The item that appears in more lines should be the answer (if there is a draw the item with "session" should be the solution).

None of the above actually worked for me, I just wanted to know whether I had Gnome running as my current desktop session.

This is what worked, open a terminal

lsb_release -a

(to check which flavor or version I had installed on my UBUNTU​ machine)

ls /usr/bin/*session

(launches which desktop session/desktop environment currently in use)

gnome-shell --version 

(to find out which version of gnome is installed)

This question is 9 years old, but still topical. On my Ubuntu 20.04.2 LTS, I get :
wmctrl -m: GNOME Shell
inxi -Sxx: Desktop = Gnome 3.36.4; wm = gnome-shell; dm = GDM3 3.36.3
gnome-shell -- version: 3.36.4
env:GDM_SESSION : ubuntu ; DESKTOP_SESSION: ubuntu ; XDG_CURRENT_DESKTOP: ubuntu:GNOME
BUT
xprop... (from ruario): _NET_WM_NAME : GNOME Shell ; _GNOME_WM_KEYBINDINGS : Mutter, GNOME Shell ; _MUTTER_VERSION : 3.36.7
and neofetch: de = GNONE ; wm = Mutter
I deduce from this (but not very sure): GNOME is the desktop manager and GNOME Shell the window manager (Mutter must be a trace of my previous LTS versions 16.04 et 18.04).