force install el capitan terminal command code example
Example: bypass this copy of the install os x el capitan
installer -pkg /Volumes/Mac\ OS\ X\ Install\ DVD/Packages/OSInstall.mpkg -target /Volumes/Macintosh\ HD
installer -pkg /Volumes/Mac\ OS\ X\ Install\ DVD/Packages/OSInstall.mpkg -target /Volumes/Macintosh\ HD