gotchas.github.com

instal gcc-7 from ppa:ubuntu-toolchain-r/test on ubuntu version earlier then 17.04

sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
sudo cat <<- LEADING_TABS_IGNORED  | sudo tee -a /etc/apt/preferences
Package: gcc-7
Pin: release n=zesty
Pin-Priority: 990
LEADING_TABS_IGNORED
sudo cat /etc/apt/preferences
sudo apt-get update
sudo apt-get install gcc-7

note how here document can be redirected to a file with root priviledges: pipe to a tee -a command run by root

preparation on ubuntu to build gcc from source code tarball

sudo apt-get install build-essential checkinstall
sudo apt-get install cvs subversion git-core mercurial
sudo chown $USER /usr/local/src
sudo chmod u+rwx /usr/local/src