CS98/198 Intermediate Systems Administration DeCal
Lab #2 SOLUTIONS

I. Filesystems upon Filesystems

1. df -h tells you about how much free space you have. The -h option specifies
   a human readable format, i.e. it will use KB, MB, and GB to specify sizes
   instead of bytes.

   Home directories on the institutional cluster are stored on a network
   fileserver called "ping". They are not stored individually on the client
   machines so that your home directory is consistent every time you log in.

2. In this particular case, if the mount starts with a machine name, e.g.
   ping:/vol/vol0/home, then it is a network share. Locally available mounts
   come from /dev.

3. a. /dev/sda1 (Linux kernel image)
   b. /dev/mapper/VolGroup00-LogVol00 (Red Hat package manager data)
   c. /dev/mapper/VolGroup00-LogVol00 (X.org server, for graphical environment)
   d. project:/inst/aa (for me; it may be different for you)

II. The /proc filesystem

1. If you have a process with ID $PID, you can find information about it
   in the directory /proc/$PID. For example, the command line the program
   was invoked with is in /proc/$PID/cmdline.

2. ps -ef or ps ax will let you see other people's processes. You can't
   see much information about it because the /proc/$PID directories are owned
   by the person running the program.

3. Here's an example:
   dr-xr-xr-x  7 cs186-dp  cs186             0 2008-09-27 11:53 32225

   in this case, the directory is owned by cs186-dp, so we can deduce that PID
   32225 is being run by cs186-dp.

4. /proc/kcore is the physical memory image of the computer. If you could read
   it, you could potentially sieve passwords and other sensitive information
   out of it, and that is insecure. So it's good that only root may read it.

5. /proc/net/dev tells you about bandwidth statistics for each network device
   on the machine. With its raw data you can calculate how many bytes per
   second each device is pumping through.

6. /proc/self represents the directory of the current process, that is, the one
   accessing /proc/self. It is accomplished by using a symlink to your process'
   /proc/$PID directory.

   lrwxrwxrwx  1 root      root             64 2008-10-16 17:26 self -> 28143

III. Device Files - The /dev directory

1. program > /dev/stderr (equivalent to: program >&2, useful hint!)

2. /dev/zero can be used like /dev/null, except if you read out of /dev/zero
   it returns an infinite stream of NULL characters (ASCII character 0, not
   the number 0.) /dev/full acts like a full storage device, so if you want
   to trick a program into thinking your disk is full, tell it to write to
   /dev/full!

3. df -h shows this as the root partition on icom1:

   /dev/mapper/VolGroup00-LogVol00      28G  22G  4.6G  83% /

   If you try to cat the dev file, you will get permission denied. This is good
   for the same reason that you can't read /proc/kcore. Actually, if you were
   able to read it, this would undermine all sense of filesystem-level
   permissions that you've learned about all semester!

4. Device files are actual files on disk, but they don't have sizes because
   they point to devices like hard disks, terminals, and serial ports. They
   use a numbering system called "major/minor" to distinguish them. All
   device files with the same major number are generally related (e.g. different
   terminals, partitions on a hard disk, sound cards.)

   You can use the mknod command to make a device file anywhere, but you
   must be root to do it. That way, you could make a file that behaves
   like /dev/null but it isn't in /dev at all, just by copying its major
   and minor numbers.