Create an ISO image of a data cd in linux either with a graphical user interface application or via the command line interface.
Distro Common GUI
Most Linux distributions have a file manager that will allow you to create an ISO file using the right-click menu as well.
Ubuntu uses the Brasero disc-burning utility.
CLI - DD
dd if=/dev/cdrom of=~/iso-name.iso
Looking at Linux Mint or Ubuntu this is going to look something like the following
Mkisofs
Not for copying a cdrom to an iso, but for creating an iso from a portion of your file system.
The mkisofs command creates a hybrid ISO9660/JOLIET/HFS filesystem with optional Rock Ridge attributes. mkisofs takes a snapshot of a given directory tree, and generates a binary image which will correspond to an ISO9660 or HFS filesystem when written to a block device. Limitations include 8 character filenames with no more than 3 character extensions with a limited directory depth of 8 levels. All of the typical old character limitations apply. Basically, this is crap.