Open $HOME/.bashrc in a text editor and add the path to the PATH environment variable by adding this line to the file export PATH="/usr/local/src/sage-5.4.1:$PATH"

Save the file and either open a new terminal, or type exec bash in the current terminal. Just sage should work now. You will need to edit the above line whenever you have a new Sage installation. This is a standard unix way of making things work without giving the absolute path to the commands. Search Google for unix PATH to learn more.