The usual utilities for creating filesystems on hard-disk partitions write an empty filesystem onto them, which is then mounted and filled with files by the users as they need it. A writable CD is only writable once so if we wrote an empty filesystem to it, it would get formatted and remain completely empty forever. This is also true for re-writable media as you cannot change arbitrary sectors yet; you must erase the whole disk. The tool to create the filesystem is called mkisofs. A sample usage looks like this:

	$ mkisofs  -r   -o cd_image   private_collection/
	

The option -r sets the permissions of all files on the CD to be public readable and enables Rockridge extensions. You probably want to use this option unless you really know what you're doing (hint: without -r the mount point gets the permissions of private_collection!).

mkisofs will try to map all filenames to the 8.3 format used by DOS to ensure the highest possible compatibility. In case of naming conflicts (different files have the same 8.3 name), numbers are used in the filenames and information about the chosen filename is printed via STDERR (usually the screen). Don't panic: Under Linux you will never see these odd 8.3 filenames because Linux makes use of the Rockridge extensions which contain the original file information (permissions, filename, etc.). Use the option -J (MS Joliet extensions) or use mkhybrid if you want to generate a more Windows-friendly CD-ROM. You can also use mkhybrid to create HFS CD-ROMS Read the man-page for details on the various options. Another extention is El Torito, which is used to create bootable CD-ROM filesystems.

Reasons why the output of mkisofs is not directly sent to the writer device:

One could also think of creating an extra partition and writing the image to that partition instead to a file. This is possible, but has a few drawbacks. If you write to the wrong partition due to a typo, you could lose your complete Linux system. Furthermore, it is a waste of disk space because the CD-image is temporary data that can be deleted after writing the CD. However, using raw partitions saves you the time of deleting 650 MB of files.

Linux has the ability to mount files as if they were disk partitions. This feature is useful to check that the directory layout and file-access permissions of the CD image matches your wishes. Although media is very cheap today, the writing process is still time consuming, and you may at least want to save some time by doing a quick test.

To mount the file cd_image created above on the directory /cdrom, give the command

	$ mount -t iso9660 -o ro,loop=/dev/loop0 cd_image /cdrom
	

Now you can inspect the files under /cdrom -- they appear exactly as they were on a real CD. To unmount the CD-image, just say umount /cdrom.

The command cdrecord is used to write images to a SCSI CD-burner. Non-SCSI writers require compatibility drivers, which make them appear as if they were real SCSI devices. For a short explanation see the section called “Configuring IDE CD burners”

Before showing you the last command, a warning that CD-writers want to be fed a constant stream of data. So, the process of writing the image to the CD must not be interrupted or a corrupt CD will result. It's easy to interrupt the data stream by deleting a very large file. Example: if you delete an old CD-image of 650 Mb, the kernel must update its information on 650,000 blocks of the hard disk (assuming you have a block size of 1 Kb for your filesystem). That takes some time and is very likely to slow down disk activity long enough for the data stream to pause for a few seconds. However, reading mail, browsing the web, or even compiling a kernel generally will not affect the writing process on modern machines.

Please note that no writer can re-position its laser and continue at the original spot on the CD when it gets disturbed. Therefore any strong vibrations or other mechanical shocks will probably destroy the CD you are writing.

Now, find the SCSI-BUS, -ID and -LUN number with cdrecord -scanbus and use these to write the CD:

	shell> SCSI_BUS=0   #
	shell> SCSI_ID=6    # taken from cdrecord -scanbus
	shell> SCSI_LUN=0   #
	shell> cdrecord -v speed=2 dev=$SCSI_BUS,$SCSI_ID,$SCSI_LUN \
                    -data  cd_image

	# same as above, but shorter:
	shell> cdrecord -v speed=2 dev=0,6,0  -data  cd_image
	

For better readability, the coordinates of the writer are stored in three environment variables with natural names: SCSI_BUS, SCSI_ID, SCSI_LUN.

If you use cdrecord to overwrite a CD-RW, you must add the option blank=... to erase the old content. Please read the man page to learn more about the various methods of clearing the CD-RW.

If the machine is fast enough, you can feed the output of mkisofs directly into cdrecord:

	shell> IMG_SIZE=`mkisofs -R -q -print-size private_collection/ 2>&1 \
	| sed -e "s/.* = //"`
	
	shell> echo $IMG_SIZE
	
	shell> [ "0$IMG_SIZE" -ne 0 ] && mkisofs -r
	private_collection/ \
	| cdrecord speed=2 dev=0,6,0 tsize=${IMG_SIZE}s -data -
	
	#                        don't forget the s --^       ^-- read data 	from STDIN
	

The first command is an empty run to determine the size of the image (you need the mkisofs from the cdrecord distribution for this to work). You need to specify all parameters you will use on the final run (e.g. -J or -hfs). If your writer does not need to know the size of the image to be written, you can leave this dry run out. The printed size must be passed as a tsize-parameter to cdrecord (it is stored in the environment variable IMG_SIZE). The second command is a sequence of mkisofs and cdrecord, coupled via a pipe.

It is possible to make a 1:1 copy of a data CD. But you should be aware of the fact that any errors while reading the original (due to dust or scratches) will result in a defective copy. Please note that both methods will fail on audio CDs!

First case: you have a CD-writer and a separate CD-ROM drive. By issuing the command

	cdrecord -v dev=0,6,0 speed=2 -isosize /dev/scd0
	

you read the data stream from the CD-ROM drive attached as /dev/scd0 and write it directly to the CD-writer.

Second case: you don't have a separate CD-ROM drive. In this case you have to use the CD-writer to read out the CD-ROM first:

	dd if=/dev/scd0 of=cdimage
	

This command reads the content of the CD-ROM from the device /dev/scd0 and writes it into the file cdimage. The content of this file is equivalent to what mkisofs produces, so you can proceed as described earlier in this document (which is to take the file cdimage as input for cdrecord).