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This are some commonly asked linux questions that I've answered before. I figured it would be useful to have them listed on the web.

Sound card only works as root

Question:

Gary,

So I am using Linux and managed to install
a few packages myself and even mount my
drives.  But my soundcard does not work unless
I am logged in as root.  It flashes something
about permission denied when I login under my
user account.

Any suggestions on how to resolve this problem?

-Kevin

Answer:

There is an very well desinged way to solve this, in fact.

All accessing of hardware by programs in linux happens by programs reading
or writing to special files call "device nodes". All the device nodes are
stored in the /dev/ directory. For example, the mixer for your sound card
is a file called /dev/mixer.

If you do an ls -l on /dev/mixer, you will see something like:

gsteele@atlas:/dev$ ls -l /dev/mixer
crw-rw----    1 root     audio     14,   0 Aug  8  2001 /dev/mixer

What you can notice is that it is owned by root, with a group "audio", and
the permissions are owner RW, group RW.

To be able to use the mixer, you have to be part of the group "audio". You
can check what groups you are part of by running a command called
"groups":

gsteele@atlas:/dev$ groups
gsteele disk dialout cdrom floppy audio

You can see that on my machine, the user gsteele has permissions to read
and write to devices marked as groups disk (hard disks, /dev/hd*), dialout
(serial ports, /dev/ttyS*), cdrom (scsi cdroms, /dev/scd*), floppy (floppy
drives, /dev/fd*), and audio (audio devices, /dev/mixer, /dev/dsp, etc).

As root, you can add a user to a group using the "adduser" command:

gsteele@atlas:/dev$ adduser -h
adduser [--home DIR] [--shell SHELL] [--no-create-home] [--uid ID]
[--firstuid ID] [--lastuid ID] [--gecos GECOS] [--ingroup GROUP | --gid
ID]
[--disabled-password] [--disabled-login] user
   Add a normal user

adduser --system [--home DIR] [--shell SHELL] [--no-create-home] [--uid
ID]
[--gecos GECOS] [--group | --ingroup GROUP | --gid ID]
[--disabled-password]
[--disabled-login] user
   Add a system user

adduser --group [--gid ID] group
addgroup [--gid ID] group
   Add a system group

adduser user group
   Add an existing user to an existing group

For example, to add yourself to the audio group, you could run the command
"adduser kturner audio" as root.

I hope this has been enlightening :). You coming to the party tonight?
I'm gonna meet susan at central at 9:00.

Cheers,
Gary.

Only root can write to mounted a DOS drive

Question:

Squash at 7 tomorrow is good.

If you have time, I have a Linux question...

I have my windows drives mounted and the only
way I can write to them is if I am root.  So
I tried to remedy this by changing the group
they belong to and adding myself to that group.
I created a group called windrive and assigned
that as the group ID on the drive.  When I do
ls -l, I get this:

kturner@mems:/mnt$ ls -l
total 24
drwxr-xr-x   18 root     windrive     8192 Dec 31  1969 c
drwxr-xr-x   22 root     windrive    16384 Dec 31  1969 data

Which as I understand it, means root can read, write and
execute, but the group windrive can only read and exeute. So
I logged in as root and tried

chmod g+w ./c

which I thought should give the group permission to write on c. It
does not give errors, but it also doesn't change the permissions.
When I do ls -l again I still get the same permisssions

mems:/mnt# ls -l
total 24
drwxr-xr-x   18 root     windrive     8192 Dec 31  1969 c
drwxr-xr-x   22 root     windrive    16384 Dec 31  1969 data

Any suggestions?

-Kevin

Answer:

Yeah, this is a common problem. It is related to the fact that FAT
filesystems really don't have permissions.

The way that linux handles this is that when you mount a FAT drive, all
the files are given the _same_ file permissions at mount time.
Furthermore, once mounted, these permissions can't be modified.

The default permissions that mount will use for FAT partitions is
"rwxr-xr-x". To change this, you can add an option "umask=" in the fstab
file. For example, on my computer, I use:

/dev/hda1       /mnt/c          vfat    defaults,umask=0000 0 0

The argument to umask is and octal number. To understand what this number
means, you have to understand how file permissions actually work in unix.

The way file permissions are stored on the disk is as a 9-bit binary
number. The best way to show this is with examples. File permission are
commonly represented in three ways:

rwxr-xr-x  - human readable
111101101  - binary number
7  5  5    - 3 octal numbers

The last one is the most important. If you read the manpage for chmod, you
will notice that you can specify the desired permissions using and octal
number. Examples:

gsteele@atlas:~$ chmod 0755  foo
gsteele@atlas:~$ ls -l foo
-rwxr-xr-x    1 gsteele  gsteele   1123166 Mar 27 11:42 foo*
gsteele@atlas:~$ chmod 0766  foo
gsteele@atlas:~$ ls -l foo
-rwxrw-rw-    1 gsteele  gsteele   1123166 Mar 27 11:42 foo*
gsteele@atlas:~$ chmod 0777  foo
gsteele@atlas:~$ ls -l foo
-rwxrwxrwx    1 gsteele  gsteele   1123166 Mar 27 11:42 foo*

(Note: the first number is related to "special file permissions". You
should always leave this number as 0. You can read about these here:
http://www.lns.cornell.edu/public/COMP/info/fileutils/fileutils_3.html#SEC4)

Now, umask is like the binary NOT of the file permissions you want. For
example:

rwxr-xr-x  - desired file permissions
111101101  - binary file permissions
000010010  - umask
0  2  2    - octal umask

So, for example, in my fstab file, I use umask=0000, so that the
permissions on my dos drive are rwxrwxrwx. If you wanted the permissions
on the dos drive to be rwxrwxr-x, you would set umask=0002.

You can also change the group and the owner of the FAT partition files
using the options uid=value,gid=value in the fstab file, where the values
are the user id number and group id number.

The uid numbers for users are listed in the file /etc/passwd, and the gid
numbers are listed in the file /etc/group:

in /etc/passwd:

gsteele:x:1001:1001:Gary Steele,13-2033,3-4810,:/home/gsteele:/bin/bash
           |    |
         uid    default gid   for user gsteele

in /etc/group:

gsteele:x:1001:
            |
          gid for group gsteele

This should sort out any problems.

Cheers,
Gary.

Linux video player, upgrading debian distribution

Question:

Hi Gary

How's things? I've a couple of questions about Linux for you. Is there a
Linux equivalent of Windows Media Player, that can play .avi, mpegs etc?
Also is it possible to simply update the version of Debian that I'm
running or will that lead me to having to play with some of the
configuration files? What about the GNome window manager - how can I
update this?

How is everything going at MIT these days? What have you lot been upto
recently? Have fun and keep in touch.

All the best

Peter

Answer:

Hi Peter,

Not been upto much lately here. I'm gonna go to Conor's beer party
tonight, we'll see how that goes :)

About playing movies, I'd recommend you try either Xine (there are debian
packages) or mplayer.

I really like mplayer. There are no packages, but it is pretty easy to
compile, and works very well with almost _all_ avi files, as well as any
mpeg. You can download it from:

http://www.mplayerhq.hu/homepage/

About debian, one thing you could try is "apt-get dist-upgrade". If you
have edited you /etc/apt/sources.list file to use "woody" (or "testing"
which is the same thing), you can just type "apt-get dist-upgrade" and
with any luck, everything will work.

Warning: this will replace a hundred or more packages, so there could be
problems. If you do have problems, I'd recommend typing "apt-get -f
install", and then "apt-get dist-upgrade" again. You can continue doing
this until it doesn't have anything left to do.

Once you've dist-upgraded, I'd recommend running "tasksel", which will
allow you to pick big groups of packages to install from the new
distribution.

Cheers,
Gary.