Its a combinations of two words : Drupal + Shell = DruSh
Drush is a shell-based application used to control, manipulate, and administer Drupal sites. On the surface, drush is an excellent tool for updating site modules, but here is a more comprehensive list of features for drush.
Below Command will install Drush on system...
$ sudo apt-get install drush
To see all the commands about Drush on system :
$ drush help [command]
Core drush commands: