Home » Why do we need to mount on Linux?

Why do we need to mount on Linux?


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.

Basically, and to put it easily, the operating system needs to know how to access the files on that device.

mount is not only “giving you access to the files”, it’s telling the OS the filesystem the drive has, if it’s read only or read/write access, etc.

/dev/cdrom is a low-level device, the operating system functions wouldn’t know how to access them… imagine you put a weirdly formatted cdrom in it (even an audio cd), how would ls tell which files (if any) are there on the cd-rom without “mounting” it first?

Note that this happens automatically in many OSes (even Linux on some distributions and graphic interfaces), but that doesn’t mean other OSes are not “mounting” the drives.

I’d call it historical reasons. Not that the other answers are wrong, but there’s a bit more to the story.

Compare Windows: Windows started as a single-computer, single-user OS. That single computer probably had one floppy drive and one hard drive, no network connection, no USB, no nothing. (Windows 3.11 had native networking capabilities; Windows 3.1 didn’t.)

The kind of setting Windows was born into was so simple that there was no need to be fancy: Just mount everything (all two devices) automatically every time, there aren’t (weren’t) many things that could go wrong.

In contrast, Unix was made to run on server networks with multiple users from the very start.

One of the Unix design decisions was that the file system should appear as a single uniform entity to the end users, no matter how many computers the physical disks were spread over, no matter what kind of disk, and no matter which of dozens of computer the user would access it from. The logical path to the user’s files would stay the same, even if the physical location of those files had changed overnight, e.g. due to server maintenance.

They were abstracting the logical file system, paths to files, from the physical devices that stored those files. Say server A is normally hosting /home, but server A needs maintenance: Just unmount server A and mount backup server B on /home instead, and noone apart from the administrators would even notice.

(Unlike the Windows convention of giving different names to different physical devices – C:, D:, etc. – which works against the transparency that Unix was striving for.)

In that kind of setting, you can’t just mount everything in sight willy-nilly,

In a large network, individual disks and computers are out of commission constantly. Administrators need the ability to say what is mounted where and when, e.g. to do a controlled shutdown of one computer while another computer transparently takes over hosting the same files.

So that’s why from a historical perspective: Windows and Unix came from different backgrounds. You could call it a cultural difference, if you like:

  • Unix was born in an environment where the administrator needed to control mounting; of the dozens of storage devices on the network the admin must decide what is mounted where and when.
  • Windows was born in a setting where there was no administrator and only two storage devices, and the user would probably know whether their file was on the floppy or the hard drive.
  • (Linux was born as a single-computer OS, of course, but it was also explicitly designed from the start to mimic Unix as closely as possible on a home computer.)

More recently, the OSes have been moving closer to each other:

  • Linux has added more single-computer, single-user stuff (like automounting); as it became frequently used in single-computer settings.
  • Windows has added more security, networking, support for multiple users etc.; as networking became more ubiquitous and Microsoft started making an OS for servers as well.

But it’s still easy to tell that the two are the result of different traditions.

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