CPS 346 & 444/544 Lecture notes: Terminals


Coverage: [UPE] Chapter 2 (pp. 52, 65-70) and [USP] §6.5 (pp. 203-213)

crypt program

  • encrypts data
  • reads from standard input and writes to standard output
  • requires a key
  • your password is encrypted in the password file; encryption algorithm has the property that it is easy to go from the unencrypted to the encrypted form, but very hard to go in the other direction

Terminals

  • tty0, tty1, ..., ptty00
    • user terminals
    • owned by user
    • names vary
  • tty
    • synonym for your login terminal
    • terminal associated with this process
    • sometimes referred to as controlling terminal
    • fixed name
    • $ date >/dev/tty
Sun Feb 19 18:07:16 EST 2006
  • controlling terminals cannot be redirected from the command line; consider users storing passwords in a file!

  • how to find out which terminal you are using 
    $ tty
/dev/pts/2
$ ls -l /dev/pts/2
lrwxrwxrwx   1 root     root          28 Aug  1  2005 /dev/pts/2 -> ../../devices/pseudo/pts@0:2
$
$ ls -l /devices/pseudo/pts@0:2
crw--w----   1 cps445-n1.03 tty       24,  2 Feb 19 18:02 /devices/pseudo/pts@0:2
    • notice that cps445-n1.03 owns it and has read and write permission on it
    • notice that those in the tty group (i.e., everybody) can write to it
    • chmod 600 /devices/pseudo/pts@0:2 (same as mesg n)

  • use /dev/tty when program needs to interact with a user even when standard input and output are redirected
      (ref. [UPE]) 
      $ crypt <cleartext >crypted text
Enter key:
$

Echo off

  • notice that in addition to explicitly reading from /dev/tty, crypt also turns off automatic character echoing
  • illustrated and experimented with code to turn echo off; necessary for all sorts of authentication services

Communication

  • getting write to work right
  • what about talk? the precursor to instant messaging
  • mesg [y/n]
  • dmesg (system message buffer)
  • biff [y/n]
  • mail << HERE

References

    [UPE] B.W. Kernighan and R. Pike. The UNIX Programming Environment. Prentice Hall, Upper Saddle River, NJ, Second edition, 1984.
    [USP] K.A. Robbins and S. Robbins. UNIX Systems Programming: Concurrency, Communication, and Threads. Prentice Hall, Upper Saddle River, NJ, Second edition, 2003
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