Terry Carroll wrote:
I have a program that traverses the directory of a CDROM using os.walk.
I do most of my work on Windows, but some on (Ubuntu) Linux, and I'd
like this to work in both environments.
On Windows, I do something along the lines of this:
startpoint="D:/"
What if the user has two CD drives? What if they have a second hard disk
mounted on D:/, and a network drive on E:/, and use F:/ or A:/ or Z:/
for the CD drive?
If this program is for you, then it is fine to make assumptions about
where the CD drive will be mounted, but don't make the mistake of
thinking that they're not assumptions.
for (root, dirs, files) in os.walk(startpoint):
(stuff)
What would I use for startpoint in Linux? I've tried "/dev/sr0" and
"/dev/sr0/"; neither work. (If I recall correctly, the for-loop just
drops through; I'm not at my Linux box right now to confirm.)
As a general rule, don't touch anything in /dev unless you know what
you're doing. /dev/sr0, and it's more user-friendly name /dev/cdrom, are
devices, not folders. This is how low-level programs get access to the
raw bytes being read from the device (in this case the CDROM drive)
*before* it is turned into files.
A number of other mount points (like /dev/cdrom, I think) are links to
/dev/sr0; and don't work either.
No, of course not. If they're links, that will mean that they are
essentially nicknames or aliases. If Fred Smith doesn't respond when you
talk to him, then calling him Freddy, Mr Smith, Hey you! or Frederick
isn't going to work either.
It *does* work to start at "/media/VOLUMENAME", where VOLUMENAME is the
volume ID of the disc; but my program will not know that. I need to be
able to do this without knowing the volume name.
Yes, this is because you need to look at the mount point. The mount
point is where the CDROM disk is mounted as a file system, in other
words, where you can see the files on the disk *as files*. If you want
to read the raw bytes off the disk, you open /dev/cdrom as a file and
read from it.
On Unix and Linux systems, there are two conventions for mounting
external media. One is that if you, the user, mount something by hand
using the "mount" command, it gets placed in /mnt (old-school Unix sys
admins had keyboards without vowels *wink*). Often people would use
subdirectories under /mnt:
/mnt/cdrom
/mnt/floppy
are the two most common ones.
The other convention is that modern window/desktop managers like KDE and
Gnome will automatically mount devices by name under /media. This is
typically for CDs, DVDs, cameras and mobile phones with file storage,
USB sticks and portable hard drives, etc.
If you only have one CD drive, and no other devices mounted, you can
just look at /media and walk over that without caring what the CD drive
is called. In other words, just use /media as the starting point, and
let os.walk discover the name of the CD under it.
But if you might have an external hard drive plugged in, or a USB key,
or similar, then you need to find out what the volume name of the
mounted CD drive is. That's a good question and I don't have an answer
right now. Let me think about it and get back to you.
--
Steven
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