Homework / Exam

1
Homework / Exam

• HW7 is due next class
• Starting Glass chapter 4 and parts of 7
• Exam 3 Class 26
• Open Book / Open Notes
• Up through End of KR Chapter 7
• and Appendix B Standard Library
• Plus UNIX Shells / Processes / Shell Scripts

2
Remaining Course Schedule

• Exam 3 is class 26
• HW8 is the last one - Due class 28
• Last Week of Classes - Mostly Review
• Bring your questions to class
• Email me any specific questions that you want me
  to be prepared to go over for you
• Final Exam for CS240
• - See posted schedule and location

3
UNIX Shells

• A picture of the relationship between UNIX shells

T Shell
Korn Shell
C Shell
Bourne Shell
Common Core
Common Core
4
UNIX Shells

• At UMB, we use tcsh, the T shell which is based
  on the C shell
• Note "common core" for both shell families
• Figure shows division of core features
• Many important features are there and we'll cover
  them first, Glass Ch 4
• Then we'll go on to the C shell, Glass Ch 7

5
UNIX Processes

• Basic to UNIX is the idea of a process
• Each process contains a program in execution (It
  might be stopped, but it is in the system
  anyway).
• Each process has own memory space with code,
  data, and program stack
• Most programs we use daily are written in C and
  have "main (int argc, char argv)" through
  which they access the arguments on their command
  line
• Even if not written in C, they have similar access

6
UNIX Shells

• Shells are just programs that provide a user with
  a command line interface
• Each shell runs in its own process for you as a
  user and you interact with it via commands
• Typically, have shell running in parent process
  handling command interface sometimes with a
  program running under it (e.g, a command) in its
  own child process

7
UNIX Shells

• The shell is a program that is basically an
  initialization and then a loop processing user
  commands
• Shell interprets a user command input, does
  whatever that command requires, and waits for
  another command
• Shell terminates when user types control-D at the
  beginning of a new line or enters the shell
  command to exit, typically exit or logout.
• UMB standard .login files disable the control-D
  option
• The "logout" command causes this shell and all
  other programs running in your UNIX session to
  go away

8
UNIX Shells

• echo and cd are built-in shell commands--instead
  of running a program, the shell program detects
  these in the input line from the user and
  performs the right action itself
• blade64(2) which cd ? find cd program path name
• cd shell built-in command.
• Note that cd needs to be built-in to change the
  current dir in the shell process
• Doing that action in a program run from the shell
  would only change the directory for the child
  process not for the parent shell process itself

9
UNIX Shells

• Non built-in shell commands are programs
• ls or lpr or vi commands
• myprog
• How to see this UNIX which command
• blade64(5) which ls ? find ls program path name
• /usr/ucb/ls ? path name to the ls executable
• These are all programs in system directories (or
  "myprog" which is in your own current dir)
• UNIX shell simply runs the program in a child
  process, passing its arguments to it via
  argc/argv and waits for the child to exit before
  next prompt

10
Hidden Files

• There are hidden files on your home directory
  not normally displayed by ls
• Names of these files begin with ., e.g. .login
• You can see them if you use ls with A option
• blade64(3) ls -A
• .cshrc .plan cs240.f02 mbox
  private_stuff
• .login cs105 cs240.old
  playpen public_html
• .msgsrc cs240 cs241
  playpen2 student
• .cshrc is a script file executed when shell is
  started
• .login file is a script file executed at time of
  login

11
Alias

• Alias defines a new command name or overrides an
  existing command name
• blade64(36) alias dir "pwdls -lg"
• blade64(37) alias dir
• dir pwdls -lg
• blade64(38) dir
• /home/bobw
• total 20
• drwxr-s--- 8 bobw faculty 512 Nov 19
  2057 cs105
• . . .
• To remove an alias
• unalias dir

12
Shell Variables

• A shell variable or local variable is a name with
  a value in the current shell process space
• set x5
• set hwdirbobw/cs240/hw4
• We access shell variable value via name
• echo cking variables x hwdir
• cking variables 5 /home/bobw/cs240/hw4
• cd hwdir
• To delete the definition for a shell variable
• unset x

13
Display Shell Variables

• blade64(3) set
• _ cat .cshrc
• addsuffix
• argv ()
• autologout 60
• cwd /home/bobw
• dirstack /home/bobw
• echo_style bsd
• edit
• exec_prefix /tools/modules-2.2b1
• filecomp
• gid 12
• group faculty

14
Environment Variables

• An environment variable is a name with a value
  that gets communicated from shell to programs
  running under the shell including other shells
• To define an environment variable of your own
  using the C shell (NOTE NO sign)
• setenv y 10
• setenv printer lw_office

15
Environment Variables

• Values are accessed the same way as shell vars
• echo y printer
• 10 lw_office
• lpr -Pprinter .c
• To delete definition for an environment variable
• unsetenv y

16
Display Environment Variables

• blade64(4) setenv
• USERbobw
• LOGNAMEbobw
• HOME/home/bobw
• PATH/tools/req/bin/groups/ulab/bin/groups/ulab/
  pcdev/bin/home/bobw/bin/usr/local/bin/usr/local
  /hosts/tools/netscape-4.75/tools/netscape-4.75/j
  ava/classes/java40.jar/tools/xv/bin/usr/ucb/usr
  /bin/bin/usr/local/gnu/bin/usr/openwin/bin/usr
  /dt/bin/usr/ccs/bin.
• MAIL/var/mail//bobw
• SHELL/bin/tcsh

17
Environment Variables

• Accessing environment variables in C programs
• To set an environment variable
• include ltstdlib.hgt
• int return putenv(MY_EVfoobar)
• / if not error, return 0,
• otherwise return ! 0 /
• To read an environment variable
• include ltstdlib.hgt
• char path getenv(MY_EV)

18
Forking a child process

• Processes can give birth to other processes using
  the fork() system call (KR, Chap 8)
• Then, there are both a parent and a child process
• Typically, the parent keeps track of the child
  but not vice versa
• A common thing for a parent to do is just wait
  until the child finishes its work and exits by
• returning at level of main ( )
• or
• executing exit ( )

19
Forking a child process

• Which branch of the if statement executes?
• include ltstdio.hgt
• include ltunistd.hgt
• int main ( )
• int pid
• pid fork()
• if (pid)
• printf("parent process d forked child
  d\n", getpid(), pid)
• else
• printf("child process d from parent d\n",
  getpid(), getppid())
• return 0

20
Forking a child process

• Answer Both!!
• blade64(5) fork
• child process 29098 from parent 29097
• parent process 29097 forked child 29098
• blade64(6)
• One branch executes in the parent process and the
  other executes in the child process

21
Forking a child shell process

• A child shell inherits parents environment
  variables and gets a new clean copy of shell
  variables (Glass, Figure 3.5)

Parent Shell
Child Shell
Environment
Copied from parent
Environment
Local
Local
Clean, initialized
22
Forking a child shell process and program

• Fork and execute a new program in child
• if(fork() 0) / child /
• execvp(argv1, argv1)
• / call should not return /
• printf("s\n, execvp failed)
• else / parent /
• wait(NULL)

23
Background Processing

• Launch a command in a background (child shell)
  process using at the end of the command line
• blade64(54) grep ju junk.c
• / junk.c/
• blade64(55) grep ju junk.c gtjunk.g
• 1 19913 ? PID for child process
• 1 Done grep ju
  junk.c gt junk.g
• blade64(56) cat junk.g
• / junk.c/
• blade64(57)