Explain what the return value of fork-shell programming, Programming Languages

Assignment Help:

Simple Shell

In this LAB, you will explore and extend a simple Unix shell interpreter. In doing so, you will learn the basics of system calls for creating and managing processes.
STEP 1: Compile the shell
chmod +x b.sh
make
make test # Use in Step 5 to test your changes to the Program
./shell

STEP 2: Try using the shell

Note: You need to specify the absolute paths of commands. Some commands to try:
/bin/ls
/bin/ls ..
cd /
/bin/pwd
/bin/bash
exit
./shell (Note: You need to be in the smp1 directory.)
./shell& (Note: You need to be in the smp1 directory.)
./b.sh (Note: You need to be in the smp1 directory.)
/bin/kill -s KILL nnnn (Where nnnn is a process ID.)
"./" means the current directory

STEP 3: Study the implementation of the shell

In preparation for the questions in Step 4, please explore the source code for the shell contained in 'shell.c'. You needn't understand every detail of the implementation, but try to familiarize yourself with the structure of the code, what it's doing, and the various library functions involved. Please use the 'man' command or Internet to browse the manual pages describing functions with which you are unfamiliar.
STEP 4: Questions
1. Why is it necessary to implement a change directory 'cd' command in the shell? Could it be implemented by an external program instead?
2. Explain how our sample shell implements the change directory command.
3. What would happen if this program did not use the fork function, but just used execv directly? (Try it!)

Try temporarily changing the code 'pid_from_fork = fork();'
to 'pid_from_fork = 0;'
4. Explain what the return value of fork() means and how this program uses it.
5. What would happen if fork() were called prior to chdir(), and chdir() invoked within the forked child process? (Try it!)

Try temporarily changing the code for 'cd' to use fork:
if (fork() == 0) {
if (chdir(exec_argv[1]))
/* Error: change directory failed */
fprintf(stderr, "cd: failed to chdir %s\n", exec_argv[1]);
exit(EXIT_SUCCESS);
}
6. Can you run multiple versions of ./b.sh in the background? What happens to their output?
7. Can you execute a second instance of our shell from within our shell program (use './shell')? Which shell receives your input?
8. What happens if you type CTRL-C while the countdown script ./b.sh is running? What if ./b.sh is running in the background?
9. Can a shell kill itself? Can a shell within a shell kill the parent shell?

./shell
./shell
/bin/kill -s KILL NNN (Where NNN is the parent's PID.)
10. What happens to background processes when you exit from the shell? Do they continue to run? Can you see them with the 'ps' command?

./shell
./b.sh&
exit
ps

STEP 5: Modify the Program

Please make the following modifications to the given file shell.c. As in previous lab, In this assignment there are some built-in test cases, which are described along with the feature requests below.
In addition to running the tests as listed individually, you can run "make test" to attempt all tests on your modified code.
1. Modify this Program so that you can use 'ls' instead of '/bin/ls' (i.e. the shell searches the path for the command to execute.)

Test: ./shell -test path
2. Modify this Program so that the command prompt includes a counter that increments for each command executed (starting with 1). Your program should use the following prompt format:

"Shell(pid=%1)%2> " %1=process pid %2=counter
(You will need to change this into a correct printf format)
Do not increment the counter if no command is supplied to execute.
Test: ./shell -test counter
3. Modify this Program so that '!NN' re-executes the n'th command entered. You can assume that NN will only be tested with values 1 through 9, no more than 9 values will be entered.

Shell(...)1> ls
Shell(...)2> !1 # re-executes ls
Shell(...)3> !2 # re-executes ls
Shell(...)4> !4 # prints "Not valid" to stderr
Test: ./shell -test rerun
4. Modify the Program so that it uses waitpid instead of wait.
5. Create a new builtin command 'sub' that forks the program to create a new subshell. The parent shell should run the imtheparent( ) function just as if we were running an external command (like 'ls').

./shell
Shell(.n1..)1> sub
Shell(.n2..)1> exit # Exits sub shell
Shell(.n1..)1> exit # Exits back to 'real' shell
6. Create a new global variable to prevent a subshell from invoking a subshell invoking a subshell (i.e., more than 3 levels deep):

./shell
Shell(.n1..)1> sub
Shell(.n2..)1> sub
Shell(.n3..)1> sub # prints "Too deep!" to stderr
Test: ./shell -test sub


Related Discussions:- Explain what the return value of fork-shell programming

X86 Assembly, You are to Use ASCII operations to design an IA-32 assembly ...

You are to Use ASCII operations to design an IA-32 assembly language program which will do addition and subtraction on positive integers of any length represented in characters. T

C++, fow to I write a emplate that reads a set of numbers typed at the keyb...

fow to I write a emplate that reads a set of numbers typed at the keyboard into an array of T

Shell script, find value of nCr using function

find value of nCr using function

Mvc framework, MVC Framework The ASP.NET MVC Structure is a web program fra...

MVC Framework The ASP.NET MVC Structure is a web program framework that tools the model-view-controller (MVC) design. According to ASP.NET, it allows application designers to devel

Program to built book recommendation system, 1 Problem Description The...

1 Problem Description The task of the assignment is to build a book recommendation system for Auer Libraries. The system is to be implemented in Prolog and as the owners of t

Fortran source code, For this programming assignment, you are to develop a ...

For this programming assignment, you are to develop a Fortran 90/95/2003 program to automate a useful task. The aim of the assignment is for you to reveal your competence in the Fo

Enter no of hours days months years contains pattern , Normal 0 ...

Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4

Calculate the sum of negative elements of specified column, Write a program...

Write a program that will generate 25 DWORD values in the range from +/-50 representing a two dimensional array of size 5 x 5. It should then display the array as a table (5 x 5) b

Ajax and php, would you like to see some of my code. I am trying to do an a...

would you like to see some of my code. I am trying to do an add friend request. when I view my friends page I click on the add button. with ajax I want the script to send over the

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd