Most Linux distributions package some version of CGAL. The demos are usually packaged separately.
# Debian or Linux Mint sudo apt-get install libcgal-dev # install the CGAL library sudo apt-get install libcgal-demo # install the CGAL demos
# ArchLinux sudo pacman -S cgal
To build CGAL from source on a UNIX-Like System you have to download a release as a tarball or you can clone the current development version of CGAL from GitHub. The build process is documented in the Installation Manual.
Please note that the directory layout and build process of the development version on GitHub are different from the release version.