If Linux is like Solaris, which I believe they are similar. I know the Linux kernel is different - but this is not a discussion on kernels.
So, to resolve the question. You must start runInstaller from outside the cd-rom directory. In other words, change directory(cd) to a directory that is not on cd-rom (try ORACLE_HOME). Then run the runInstall app from that location.
oracle> cd $ORACLE_HOME
oracle> . /cdrom/cdrom0/runInstaller
Notice there is a space between the dot (.) and the command (/cdrom/cdrom0/runInstaller). The dot(.) is not current directory dot. The dot represents starting the application is a separate shell.\