Cannot select unity in greetor (GDM/LightDM)

It has been caused by a bug in ubuntu-session package in Trisquel repository.

The content of ubuntu-session in Trisquel 7 (ubuntu-session_3.9.90-0ubuntu12+7.0trisquel2_all):
ubuntu-session_3.9.90-0ubuntu12+7.0trisquel2_all
├── DEBIAN
│ ├── control
│ └── md5sums
└── usr
└── share
└── doc
└── ubuntu-session
├── AUTHORS
├── buildinfo_all.gz
├── changelog.Debian.gz
├── changelog.gz
├── copyright
├── NEWS.gz
└── README

5 directories, 9 files

The content of ubuntu-session in Ubuntu 14.04 (ubuntu-session_3.9.90-0ubuntu12_all):
ubuntu-session_3.9.90-0ubuntu12_all
├── DEBIAN
│ ├── control
│ └── md5sums
└── usr
└── share
├── doc
│ └── ubuntu-session
│ ├── AUTHORS -> ../gnome-session-common/AUTHORS
│ ├── changelog.Debian.gz -> ../gnome-session-common/changelog.Debian.gz
│ ├── copyright
│ ├── NEWS.gz -> ../gnome-session-common/NEWS.gz
│ └── README -> ../gnome-session-common/README
├── gnome-session
│ └── sessions
│ └── ubuntu.session
├── lightdm
│ └── lightdm.conf.d
│ └── 50-ubuntu.conf
└── xsessions
└── ubuntu.desktop

10 directories, 10 files

An can be seen above, the gnome-session, lightdm, and xsessions directories are missing in the modified package in Trisquel.

WORKAROUND:
You can manually add Unity to the greeter by running the attached file using sudo.

This is the second time I am trying to attach the file.

This is the third time I am trying to attach the file.

Anyway, this is the content of add-unity-to-greeter.sh


echo '[GNOME Session]
Name=Ubuntu
RequiredComponents=unity-settings-daemon;compiz;
DesktopName=Unity' > /usr/share/gnome-session/sessions/ubuntu.session

echo '[Desktop Entry]
Name=Ubuntu
Comment=This session logs you into Ubuntu
Exec=gnome-session --session=ubuntu
TryExec=unity
Icon=
Type=Application
X-LightDM-DesktopName=Unity
X-Ubuntu-Gettext-Domain=gnome-session-3.0' > /usr/share/xsessions/ubuntu.desktop