Hi,
How should i use dd command to create an image file of a CD?

This will create an image of the /dev/dvd device named image.iso in your home directory. You may need to replace /dev/dvd with the actual device name of your DVD player.

Note that multi-layer DVDs can't easily be burnt from an image. Also, the filesystem may not necessarily be ISO9660.

dd if=/dev/cdrom of=cd.img

in the simplest case. 'man dd' for details.

-- Oops. Sorry about that. I went to let the cat in between typing and posting.

can i use the following command to copy an image file to usb device?
$ dd if=boot.iso of=/dev/sda1

Do i have to specify anything else? My intension is to create a bootable usb frm the image.

Mmm. As far as I know. I just got my first USB device a few days ago... well, two actually, counting today (early Christmas ) - so I don't know but, so far, my USB hard drive is just another hard drive. Not sure about other devices, though. Maybe somebody more familiar with them could say for sure.

Most USB devices I've seen come with a VFAT image, and you seem to be proposing to install an ISO9660 (.iso) filesystem. It could work; it depends on which filesystems the bootloader supports.

Just a thought: you should be warned that it's not a good idea to run formatting tools directly onto a USB flash device (as you might be tempted to do); these perform many more reads/writes than is strictly needed, which wears the device out. Use a blank file of appropriate length (dd if=/dev/zero of=fs.img) then format that file as an image, then copy the image across with dd as you propose.