Mounting and Unmounting the CD-ROM

This document discusses how to this CD-ROM under Linux (or any Unix-like operating system) or under Microsoft Windows. There are two issues to consider in particular: how to mount and unmount the CD-ROM, and how to access files on the CD-ROM.

In this document, “$” will indicate the ordinary user’s shell prompt or the Cygwin shell prompt (under Microsoft Windows), and “#” will indicate the root superuser’s shell prompt. Anything in a bold monospaced font is what you are expected to type in; an ordinary monospaced font is used for the computer’s response. Please note that this document doesn’t show every response from the computer! Something like “Start » Settings » Control Panel” means that you click on the Start button, select the Settings sub-menu and finally click on Control Panel.

Under Linux

In order to use a CD-ROM under Linux (or any Unix-like operating system), you must first mount it. Mounting a CD-ROM (or any other physical media, such as a floppy disk, a ZIP disk or a hard drive) tells the operating system which block device to use and where that device is to appear within the directory tree — the mount point. Once you have finished using the CD-ROM, you must unmount it.

Mounting the CD-ROM

The canonical command to mount the CD-ROM under Linux is:

The block device device specifies the physical device or bus the CD-ROM is attached to. On most Linux systems, you can use /dev/cdrom, which would be a symbolic link to something like /dev/hdc for an ATAPI CD-ROM, selected as a master device, on the secondary IDE cable.

The mount point dir specifies where the contents of the CD-ROM are to appear. In Linux and other Unix-like operating systems, every file and directory that can be accessed appears in a single directory tree starting from “/”, the root directory. The mount point dir, then, is simply a directory within that directory tree.

You can use any directory that you like for the mount point dir, as long as that directory exists and is empty. The convention under Linux is to use either /mnt/cdrom or /cdrom.

So, for a typical Linux system, the command you would use to mount a CD-ROM would be:

Remember that only the root superuser can run this version of the mount command, so you need to log in as root to do this. For the safety and security of your own computer, you should log out as soon as you have entered this command successfully. Better yet, learn how to use the su command. You should never do your ordinary work as root! To do so invites the whole world to crack into your computer, destroy your data and use your system to attack others.

Accessing the CD-ROM

Once you have mounted the CD-ROM, you can access files and directories on that CD-ROM as if they were part of your directory tree — as, in fact, they now are. For example, if you mounted the CD-ROM on /mnt/cdrom, you can view the file you are now reading (stored on the CD-ROM as unsw/common/cdrom-mounting.html) by typing something like:

Of course, if you used a different mount point than /mnt/cdrom, substitute that mount point as appropriate. For example, if you used /cdrom, you would type:

To free you from the hassles of mounting and unmounting CD-ROMs in the Laboratory, you can access the contents of this Companion CD-ROM by using ~course/cdrom instead of /mnt/cdrom. For example, if you are studying ELEC2041, you can use ~elec2041/cdrom on the Laboratory computers — and you don’t have to mount or unmount any CD-ROMs:

Unmounting the CD-ROM

Once you have finished using the CD-ROM, you need to unmount it. The operating system makes you do this so that it can check that nothing is, in fact, using the CD-ROM when you try to eject it.

To unmount the CD-ROM, you need to enter the following command as the root superuser:

The directory dir is the mount point that you used for mounting the CD-ROM in the first place (eg, /mnt/cdrom). And please note that the command is umount, not unmount: the first “n” is missing!

If you get an error message stating that “device is busy”, it means that something is still using files and/or directories on the CD-ROM. Even something as simple as having your current directory set to some directory on the CD-ROM is enough to get you this message.

Advanced Topics

If you are new to Linux and Unix in general, you should take the time to read a book like Learning Red Hat Linux by Bill McCarty (O’Reilly and Associates, ISBN 0-596-00071-5) or Running Linux by Matt Welsh et. al. (O’Reilly and Associates, ISBN 0-596-00272-6).

If you would like the full technical details of the mount command, you should refer to mount(8). This cryptic notation simply means you look up the page for mount in section 8 of the on-line Linux Reference Manual. You do this by typing:

If you want to be able to mount and unmount CD-ROMs without being logged in as the root superuser, you need to modify the file /etc/fstab to include the following line:

Please note that you may need to replace /dev/cdrom with whatever you used as the block device for the mount command, and /mnt/cdrom with whatever you used for the mount point. In addition, do check that you don’t already have a line similar to this one in /etc/fstab!

Once you have modified /etc/fstab, you can type the following commands as an ordinary user:

For more information, see fstab(5), mount(8) and umount(8).

Under Microsoft Windows

You do not need to mount and unmount CD-ROMs under Microsoft Windows. This is because there is no Unix-like concept of a single directory tree. Instead, multiple directory trees exist, each starting with a drive letter and colon.

This lack of mounting and unmounting sounds like a benefit to users — and it often is! However, it does mean that you are the one responsible for figuring out which drive letter belongs to the CD-ROM. In many systems it is “D:”, but this may not be the case on yours. If it is not, you need to replace every occurrence of “D:” on this CD-ROM with whatever is appropriate for your system.

Another major difference between Microsoft Windows and Linux (and other Unix-like operating systems) is the fact that Windows uses “\” to separate directory and filename elements, whereas Unix uses “/”. This CD-ROM uses the Unix format, not the Windows format, for filenames. For example, the document you are currently reading can be found as unsw/common/cdrom-mounting.html on this CD-ROM. Under Windows, you would use D:\unsw\common\cdrom-mounting.html instead (assuming, of course, that your CD-ROM appears as drive “D:”). In other words, replace every “/” with a “\” and prepend the drive letter and colon.

If this is not enough to confuse you, the Cygwin Unix emulator, which you may need to use to run the GNU Tools, is a Unix-like system. In other words, it does use “/”! But since it is only an emulator and not a real operating system of its own, it does not have the same semantics for the mount command that a real Linux or Unix-like system does. Instead, the CD-ROM “magically” appears in the directory /cygdrive/d, where the “d” is the same as the drive letter for your CD-ROM.

What all this means is that, once you have installed Cygwin under Microsoft Windows, you can access this document by opening a Cygwin command line (usually by selecting Start » Programs » Cygwin » Cygwin Bash Shell) and typing:

In the Digital Systems Laboratory, you can access the contents of this CD-ROM by using ~course/cdrom instead of /cygdrive/d. For example, if you are studying ELEC2041, you can use ~elec2041/cdrom on the Laboratory computers — and you don’t have to bring in your own copy of this CD-ROM:

Advanced Topics

The relationship between directory paths under Microsoft Windows and under the Cygwin Unix emulator can be quite complex. As already mentioned, Cygwin uses Unix-style paths with “/” path separators; Windows uses “\”. Furthermore, Cygwin presents you with a virtual root directory: when you are in the Cygwin Bash Shell window, it is as if you are in a real Unix environment with / as your root directory — with no drive letters.

This virtual root directory is just an illusion, however. Each directory in Cygwin is actually mapped to a directory in the underlying Windows file system. You use the cygpath(1) utility program to map between the two. For example:

By the way, please note that the results your computer gives might be different to what is shown above. In particular, the path ~elec2041/cdrom will only work in the Laboratory!

If you would like the full technical details about how the Cygwin Unix emulator works, you should definitely consult the  Cygwin User’s Guide on this CD-ROM. You should also refer to cygpath(1) and mount(1). This cryptic notation simply means you look up the page for cygpath or mount in the on-line Cygwin Reference Manual. You do this by typing: