The sixth case study in the greatest class I will ever take.

Understand the use of device files within the Unix environment.

Shown below, I have navigated to /dev and listed the files. I then continued to discover what types of files were in the directory.

lab46:~$ cd /dev
lab46:/dev$ ls
MAKEDEV          hvc2     mem                 root      tty10  tty20  tty30  tty40  tty50  tty60  urandom      zero
block            hvc3     net                 shm       tty11  tty21  tty31  tty41  tty51  tty61  vcs
char             hvc4     network_latency     snapshot  tty12  tty22  tty32  tty42  tty52  tty62  vcs1
console          hvc5     network_throughput  snd       tty13  tty23  tty33  tty43  tty53  tty63  vcsa
core             hvc6     null                sndstat   tty14  tty24  tty34  tty44  tty54  tty7   vcsa1
cpu_dma_latency  hvc7     port                stderr    tty15  tty25  tty35  tty45  tty55  tty8   vga_arbiter
disk             initctl  ppp                 stdin     tty16  tty26  tty36  tty46  tty56  tty9   xconsole
fd               input    psaux               stdout    tty17  tty27  tty37  tty47  tty57  ttyS0  xen
full             kmsg     ptmx                tty       tty18  tty28  tty38  tty48  tty58  ttyS1  xvda1
hvc0             log      pts                 tty0      tty19  tty29  tty39  tty49  tty59  ttyS2  xvda2
hvc1             loop0    random              tty1      tty2   tty3   tty4   tty5   tty6   ttyS3  xvda3
lab46:/dev$ file full
full: character special
lab46:/dev$ file net
net: directory
lab46:/dev$ file loop0
loop0: block special
lab46:/dev$ file shm
shm: sticky directory
lab46:/dev$ file root
root: symbolic link to `xvda1'

Here are three block devices:

lab46:/dev$ ls block
202:1  202:2  202:3
lab46:/dev$ file block/202\:1
block/202:1: symbolic link to `../xvda1'
lab46:/dev$ file block/202\:2
block/202:2: symbolic link to `../xvda2'
lab46:/dev$ file block/202\:3
block/202:3: symbolic link to `../xvda3'

And three character devices:

lab46:/dev$ ls char/
10:1    10:63  1:1   1:8    229:4  4:10  4:16  4:21  4:27  4:32  4:38  4:43  4:49  4:54  4:6   4:65  5:0    7:129
10:231  116:2  1:11  1:9    229:5  4:11  4:17  4:22  4:28  4:33  4:39  4:44  4:5   4:55  4:60  4:66  5:1
10:59   13:32  1:3   229:0  229:6  4:12  4:18  4:23  4:29  4:34  4:4   4:45  4:50  4:56  4:61  4:67  5:2
10:60   13:63  1:4   229:1  229:7  4:13  4:19  4:24  4:3   4:35  4:40  4:46  4:51  4:57  4:62  4:7   7:0
10:61   13:64  1:5   229:2  4:0    4:14  4:2   4:25  4:30  4:36  4:41  4:47  4:52  4:58  4:63  4:8   7:1
10:62   13:65  1:7   229:3  4:1    4:15  4:20  4:26  4:31  4:37  4:42  4:48  4:53  4:59  4:64  4:9   7:128
lab46:/dev$ file char/10:1
char/10:1: symbolic link to `../psaux'
lab46:/dev$ file char/10:62
char/10:62: symbolic link to `../xen/gntdev'
lab46:/dev$ file char/4:51
char/4:51: symbolic link to `../tty51'

After using the df command, I can see:

• The root directory is mounted on /dev/xvda1
• /home is mounted on nfs:/home (which I assume exists on a separate machine? hence nfs)
• /tmp is mounted on /dev/xvda2

lab46:~$ df
Filesystem            Size  Used Avail Use% Mounted on
/dev/xvda1            4.0G  2.8G  993M  75% /
tmpfs                 766M     0  766M   0% /lib/init/rw
udev                  738M   36K  738M   1% /dev
tmpfs                 766M  4.0K  766M   1% /dev/shm
/dev/xvda2            248M   11M  225M   5% /tmp
nfs:/home             2.8T  1.4T  1.5T  49% /home
nfs:/lib/mail         2.8T  1.4T  1.5T  49% /var/mail

I have no stinking clue where the system swap is located. But thanks to the df command, I can see that Lab46 uses Xen Virtual Disks (since the server is virtual while running on top of Xen Server) and NFS for the storage of our home directories and mail! NFS… Pretty sure that's located on a different system and it's been mounted as a drive on this system. Meow.

The permissions on my two TTY files are shown below:

lab46:/dev/pts$ tty
/dev/pts/0
lab46:/dev/pts$ file 0
0: character special
lab46:/dev/pts$ ls -la | grep adahmane
crw--w----  1 adahmane tty  136,  0 Dec 12 00:09 0
crw--w----  1 adahmane tty  136, 59 Dec 12 00:09 59

I personally have read/write access and the tty group has write access. Here's what I get with the who command…

lab46:/dev/pts$ who | grep adahmane
adahmane + pts/0        2013-12-11 22:33   .         10880 (cpe-67-252-123-191.stny.res.rr.com)
lab46:/dev/pts$ mesg
is y

My current messaging status is y (which means my terminal is available). Changing that to n ought to remove the write access from the tty group. That's my assumption… Let's see!

lab46:/dev/pts$ mesg n
lab46:/dev/pts$ mesg
is n
lab46:/dev/pts$ who | grep adahmane
adahmane - pts/0        2013-12-11 22:33   .         10880 (cpe-67-252-123-191.stny.res.rr.com)
lab46:/dev/pts$ ls -la | grep adahmane
crw-------  1 adahmane tty  136,  0 Dec 12 00:15 0
crw--w----  1 adahmane tty  136, 59 Dec 12 00:14 59

WOO! Correct. Since we are all part of the tty group, we are given the ability to print to another person's terminal. Taking away the write access from the tty group (with the use of “mesg n”) will prevent anyone from printing to my terminal!

Mini-mind-blow #7428307940 - Since everything is a file and /dev/pts/0 is my current terminal session, I could theoretically redirect output to that file and it would be the same as using the echo command! Again, let's see…

lab46:/dev/pts$ echo meow
meow
lab46:/dev/pts$ echo meow > /dev/pts/0
meow
lab46:/dev/pts$ echo meow > /dev/pts/
-bash: /dev/pts/: Is a directory
lab46:/dev/pts$ who
NAME       LINE         TIME             IDLE          PID COMMENT
adahmane + pts/0        2013-12-11 22:33   .         10880 (cpe-67-252-123-191.stny.res.rr.com)
smeas    + pts/2        2013-05-10 16:24 01:05        1516 (li583-226:S.0)
alius    + pts/13       2013-05-13 16:26 08:05       14720 (cpe-74-65-109-183:S.0)
bh011695 + pts/22       2013-05-15 10:42  old        32271 (cpe-74-67-87-78:S.0)
smeas    + pts/24       2013-12-08 15:12 05:12       16386 (li583-226.members.linode.com)
jmendoza + pts/30       2013-12-11 22:55   .         11292 (pool-96-238-196-119.bing.east.verizon.net)
jvanzil4 + pts/38       2013-12-11 08:35 00:05       20621 (cpe-69-205-156-203.stny.res.rr.com)
cclay    + pts/54       2013-12-11 23:54   .         12088 (pool-96-238-196-119.bing.east.verizon.net)
vcordes1 + pts/53       2013-12-11 22:09   .         10512 (cpe-67-252-122-189.stny.res.rr.com)
jkosty6  + pts/55       2013-12-11 23:25 00:03       11470 (cpe-69-204-219-21.stny.res.rr.com)
jlapham1 + pts/57       2013-12-11 22:56 00:47       11315 (cpe-69-204-215-105.stny.res.rr.com)
jkosty6  + pts/58       2013-12-11 23:38   .         11575 (cpe-69-204-219-21.stny.res.rr.com)
jvanzil4 - pts/46       2013-12-11 22:18   .         10620 (cpe-69-205-156-203.stny.res.rr.com)
jmendoza + pts/8        2013-12-11 23:17 00:23       11439 (pool-96-238-196-119.bing.east.verizon.net)
lab46:/dev/pts$ echo wow.very_shell.much_command.
wow.very_shell.much_command.
lab46:/dev/pts$ echo wow.very_shell.much_command. > /dev/pts/58
-bash: /dev/pts/58: Permission denied

Well, I think that shows my understanding of the pts stuff. Just another way to make text appear on my screen! Really, any command could be redirected to /dev/pts/0 (or whatever port I'm on) to display text on my terminal! As you can see above, I attempted to harass John Kosty but it didn't work out as well as I had hoped…

lab46:~$ cat /etc/motd
 __         _     _ _   __   . . . . . . . . . . . . . . . . . . . . . . . . .
|  |   __ _| |__ / | |_/ /   . Basic System Usage:  Type 'usage' at prompt   .
|  |__/ _` | '_ \\_  _/ _ \  . Events and News:     Type 'news' at prompt    .
|_____\__,_|_.__/  |_|\___/  . Broken E-mail?       Type 'fixmail' at prompt .
---------------------------  . Check Lab46 Mail:    Type 'alpine' at prompt  .
c o r n i n g - c c . e d u  . . . . . . . . . . . . . . . . . . . . . . . . .

 Lab46 is the Computer & Information Science Department's Student Development
 Server for Computer-related coursework, projects, and exploration.  For more
 information, please check out:
  .. .  .    .        .                .                .        .    .  . ..
 .  Lab46 Web Page:       http://lab46.corning-cc.edu/                       .
 .  Lab46 Help Form:      http://lab46.corning-cc.edu/help_request           .
 .  Help E-mail:          haas@corning-cc.edu or wedge@lab46.corning-cc.edu  .
  .. .  .    .        .                .                .        .    .  . ..
      _
  .-./*)
_/___/   `.
  U U      'You can filter data with the 'grep' command.'
lab46:~$ cat /etc/motd > /dev/tty
 __         _     _ _   __   . . . . . . . . . . . . . . . . . . . . . . . . .
|  |   __ _| |__ / | |_/ /   . Basic System Usage:  Type 'usage' at prompt   .
|  |__/ _` | '_ \\_  _/ _ \  . Events and News:     Type 'news' at prompt    .
|_____\__,_|_.__/  |_|\___/  . Broken E-mail?       Type 'fixmail' at prompt .
---------------------------  . Check Lab46 Mail:    Type 'alpine' at prompt  .
c o r n i n g - c c . e d u  . . . . . . . . . . . . . . . . . . . . . . . . .

 Lab46 is the Computer & Information Science Department's Student Development
 Server for Computer-related coursework, projects, and exploration.  For more
 information, please check out:
  .. .  .    .        .                .                .        .    .  . ..
 .  Lab46 Web Page:       http://lab46.corning-cc.edu/                       .
 .  Lab46 Help Form:      http://lab46.corning-cc.edu/help_request           .
 .  Help E-mail:          haas@corning-cc.edu or wedge@lab46.corning-cc.edu  .
  .. .  .    .        .                .                .        .    .  . ..
      _
  .-./*)
_/___/   `.
  U U      'You can filter data with the 'grep' command.'

Apparently I just did something very similar above. Kinda. This method allowed me to redirect the output of the cat to my terminal session. Nifty!

I'm not even going to type this one into the terminal… I know that redirecting to null won't show me anything. We learned this in class one day. It could be useful for cronjobs so as not to receive any output from the completion of the task! That's just one of a million or more examples.