Drush not working after restart
The use of $username in the path looks a bit suspect. You should use .bash_profile for mac as as .bashrc seems not to be auto loaded.
There usually isn't a /username/ directory, its usually /home/username/ so your path should probably be:

export PATH="$HOME/.composer/vendor/bin:$PATH"


then execute this to load the file:

source .bash_profile


or open a new terminal