Add xmonad-xfce to gdm

It is easy to find instructions of how to make xfce to use xmonad as window manager. However there is no article I aware that adding a xmonad-xfce to GDM, yet keeping the original XFce.

Step by step:

As root:

1. cd /usr/share/xsessions; cp xfce.desktop xmonad-xfce.desktop
2. Edit xmonad-xfce.desktop and make following to change
   a. Search Xfce and replace with Xmonad-Xfce (case sensitive)
   b. Change the line Exec=…. to Exec=PREF_WM=xmonad startxfce4

As normal user:

1. Having a a following script file, assume its name is ifvar

   VAR_NAME=$1
   VAR=$(eval echo \$${VAR_NAME})
   THEN_CMD="$2"
   ELSE_CMD="$3"
   
   if [ -n "${VAR}" ]; then
   	eval ${THEN_CMD}
   else
   	eval ${ELSE_CMD}
   fi

2. Edit ${HOME}/.config/autostart/WM_Selector.desktop as following

   Encoding=UTF-8
   Version=0.9.4
   Type=Application
   Name=WM selector
   Comment=Window manager selector
   Exec=ifvar PREF_WM "exec ${PREF_WM} &" "exec /usr/bin/xfwm4 &"
   StartupNotify=false
   Terminal=false
   Hidden=false

3. Logout, you should see the xmonad-xfce in your GDM menu