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44.6. Mounting and Unmounting Removable Filesystems

Removable disks are prevalent in Unix machines; CD-ROMs, DVD-ROMs, Zip disks, and floppies are all removable disks. When a Unix system boots, normal filesystems are all mounted automatically. By definition, removable filesystems may not even be in the machine at boot time, and you certainly don't want to have to reboot your machine just to change CDs.

To do this, you use mount and umount. The -t option allows you to specify the type of filesystem. On my FreeBSD machine, I can mount a FAT-formatted Zip disk with:

# mount -t msdos /dev/afd0s4 /zip

If I've formatted the Zip disk with a BSD ufs filesystem instead, I don't need the -t option, since ufs is the default on FreeBSD, and I would use the BSD partitioning scheme (/dev/afd0c) instead of the BIOS partitions (/dev/afd0s4).

If you use your removable disk regularly, you can add it to your fstab and make this simpler:

/dev/acd0c              /cdrom          cd9660  ro,noauto       0       0
/dev/afd0c              /zip            ufs     rw,noauto       0       0
/dev/afd0s4             /mszip          msdos   rw,noauto       0       0

Note that I've set up my fstab for both ufs-formatted and FAT-formatted Zip disks, and that the Zip drive and the CD-ROM are both set noauto to keep them from being automatically mounted. Having these in my fstab means I can just type mount /zip or mount /cdrom to mount a Zip disk or CD-ROM. Don't forget to create the directories /cdrom, /zip, and /mszip!

Generally the mount and umount commands must be run as root. However, you'd often like normal users to be able to mount and unmount removable disks. Linux has an easy way to do this: just add user to the options field in /etc/fstab and normal users will be able to mount and unmount that device. (Incidentally, Linux also has an auto filesystem type, which is very handy for removable devices, because it does its best to dynamically figure out what filesystem is on the removable media.) On other platforms, it can be a little more complex. Generally, the trick is to set the permissions on the device file properly. On FreeBSD you also need to use sysctl to set vfs.usermount, which will allow users to mount properly chmoded devices on directories they own; similar tricks may be needed on other platforms. To set the floppy drive to allow anyone to mount it and the CD-ROM to allow anyone in the cdrom group to mount it, you'd do something like this:

# chmod 666 /dev/fd0

# chgrp cdrom /dev/acd0c
# chmod 640 /dev/acd0c

Then, as a normal user in group cdrom, you could:

% mkdir ~/cdrom
% mount -t cd9660 /dev/acd0c ~/cdrom

Solaris has a daemon, vold, which handles all of the messy details of removable media for you. At the time of this writing, very current versions of Linux have automount daemons and devfsd to handle such things; check your platform's current documentation.


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