Here are some commands for installing drush from the command line
sudo apt-get install php-pear
pear channel-discover pear.drush.org
pear install drush/drush
drush version
pear upgrade drush/drush
But this is the new preferred way using Composer on *nix
# install composer
curl -sS https://getcomposer.org/installer | php -- --install-dir=bin --filename=composer
# You may need to mkdir ~/bin first if you don't have one.