"media change: please insert the disc labeled" when trying to install Ruby on Rails [duplicate]

When I type sudo apt-get install rails I get the message:

media change: please insert the disc labeled
'Ubuntu 12.04.3 LTS _Precise Pangolin_ - Release amd64 (20130820.1)
in the drive '/media/cdrom/' and press enter

I am new to Linux and I have the installation CD in the drive, but I have no idea what disc it is asking for and I'm not sure what /media/cdrom/ means. All I have is the installation CD and it is in the only optical drive I have.

Solution 1:

Just remove the cdrom entry from the sources.list file. This can be done easily:

sudo sed -i '/cdrom/d' /etc/apt/sources.list

This should take care of the problem. The message is because somehow you still have the cdrom entry in your sources.list file, you can check the content of the file using:

grep -v '#' /etc/apt/sources.list

This will show you all the repositories you have activated.