How to create ISO image from your Harddisk, CD or DVD

Sometimes we want to do back up from our CD/DVD even files from our harddrive

Here are the how-to's:
dd if=/dev/dvd of=dvd.iso (if you want to back up from DVD)
dd if=/dev/cdrom of=cd.iso (if you back up from CDROM)
dd if=/dev/scd0 of=cd.iso (if you back up from SCSI CDROM)
mkisofs -o /tmp/cd.iso /tmp/backup/ (to make an ISO image from files on your hard drive)