One reason is that block level access is a bit lower level than ls would be able to work with. /dev/cdrom, or dev/sda1 may be your CD ROM drive and partition 1 of your hard drive, respectively, but they aren't implementing ISO 9660 / ext4 - they're just RAW pointers to those devices known as Device Files.
One of the things mount determines is HOW to use that raw access - what file system logic / driver / kernel modules are going to manage the reads/writes, or translate ls /mnt/cdrom into which blocks need to be read, and how to interpret the content of those blocks into things like file.txt.
Other times, this low level access can be good enough; I've just read from and written to serial ports, usb devices, tty terminals, and other relatively simple devices. I would never try to manually read/write from /dev/sda1 to, say, edit a text file, because I'd basically have to reimplement ext4 logic, which may include, among other things: look up the file inodes, find the storage blocks, read the full block, make my change(s), write the full blocks, then update the inode (perhaps), or instead write this all to the journal - much too difficult.
One way to see this for yourself is just to try it:
[root@ArchHP dev]# cd /dev/sda1
bash: cd: /dev/sda1: Not a directory
/dev is a directory, and you can cd and ls all you like. /dev/sda1 is not a directory; it's a special type of file that is what the kernel offers up as a 'handle' to that device.
See the wikipedia entry on Device Files for a more in depth treatment.
In Unix everything is a file.
These files are organized in a tree structure, beginning at the root /.
Your filesystem or filesystems will then be mounted at the appropriate places in your / according your /etc/fstab file. This file contains information about your filesystems, which device they belong to and to which point they will get mounted to - the mountpoint.
Thats the "mount concept".
It is not limited to disks and other blockdevices, here are some examples involving mount:
- Mount a representation of your running kernel under /proc
- Mount a special log partition (other device, "logfriendly" filesystem) under /var/log
- Install different systems and mount just one home directory
- Mount remote directories for example via NFS to your system
- Mount a image of a cd to a specific directory
More about this topic you can find at the following url:
- http://ultra.pr.erau.edu/~jaffem/tutorial/file_system_basics.htm
Best Answer
You can read or write /dev/cdrom (eg, using
ddorcat) but when you do that you are just reading or writing the raw bytes of the device. That can be useful in various circumstances (like cloning a partition), but generally we want to see the directories and files stored on the device.When you mount a device you're basically telling the kernel to use a layer of software (the filesystem driver) to translate those raw bytes into an actual filesystem. Thus mounting a device associates the filesystem on that device to the directory hierarchy.