(C) Operating Systems Project Assistance /*

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?

5) What would happen if fork() were called prior to chdir(), and chdir() invoked within the forked child process?

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 is running in the background?

9) can a shell kill itself? Can a shell within a shell kill the parent shell?

10) What happens to background process when you exit from the shell? Do they continue to run? Can you see them with the 'ps' command?

--MODIFICATIONS TO THE PROGRAM--

1) Modify this MP so that you can use 'ls' instead of '/bin/ls' (i.e the shell searches the path for the command to execute.)

2) Modify this MP so that the command prompt includes a counter that increments for each command executed (starting with 1). Your program should use the following prompt form: "Shell(pid=%1)%2>" %1=process pid %2=counter (you will need to change this into a correct printf format) Do not increment the counterif no command is supplied to execute.

3) Modify this MP 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.

4) Modify the MP so that it uses waitpid instead of wait.

5) Create a new bulletin 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').

6) Create a new global variable to prevent a subshell from invoking a subshell invoking a subshell (i.e more than 3 levels deep).

*/

/* SMP1: Simple Shell */

/* LIBRARY SECTION */

#include /* Character types */

#include /* Standard buffered input/output */

#include /* Standard library functions */

#include /* String operations */

#include /* Data types */

#include /* Declarations for waiting */

#include /* Standard symbolic constants and types */

#include "smp1_tests.h" /* Built-in test system */

/* DEFINE SECTION */

#define SHELL_BUFFER_SIZE 256 /* Size of the Shell input buffer */

#define SHELL_MAX_ARGS 8 /* Maximum number of arguments parsed */

/* VARIABLE SECTION */

enum { STATE_SPACE, STATE_NON_SPACE }; /* Parser states */

int imthechild(const char *path_to_exec, char *const args[])

{

return execv(path_to_exec, args) ? -1 : 0;

}

void imtheparent(pid_t child_pid, int run_in_background)

{

int child_return_val, child_error_code;

/* fork returned a positive pid so we are the parent */

fprintf(stderr,

" Parent says 'child process has been forked with pid=%d' ",

child_pid);

if (run_in_background) {

fprintf(stderr,

" Parent says 'run_in_background=1 ... so we're not waiting for the child' ");

return;

}

wait(&child_return_val);

/* Use the WEXITSTATUS to extract the status code from the return value */

child_error_code = WEXITSTATUS(child_return_val);

fprintf(stderr,

" Parent says 'wait() returned so the child with pid=%d is finished.' ",

child_pid);

if (child_error_code != 0) {

/* Error: Child process failed. Most likely a failed exec */

fprintf(stderr,

" Parent says 'Child process %d failed with code %d' ",

child_pid, child_error_code);

}

}

/* MAIN PROCEDURE SECTION */

int main(int argc, char **argv)

{

pid_t shell_pid, pid_from_fork;

int n_read, i, exec_argc, parser_state, run_in_background;

/* buffer: The Shell's input buffer. */

char buffer[SHELL_BUFFER_SIZE];

/* exec_argv: Arguments passed to exec call including NULL terminator. */

char *exec_argv[SHELL_MAX_ARGS + 1];

/* Entrypoint for the testrunner program */

if (argc > 1 && !strcmp(argv[1], "-test")) {

return run_smp1_tests(argc - 1, argv + 1);

}

/* Allow the Shell prompt to display the pid of this process */

shell_pid = getpid();

while (1) {

/* The Shell runs in an infinite loop, processing input. */

fprintf(stdout, "Shell(pid=%d)> ", shell_pid);

fflush(stdout);

/* Read a line of input. */

if (fgets(buffer, SHELL_BUFFER_SIZE, stdin) == NULL)

return EXIT_SUCCESS;

n_read = strlen(buffer);

run_in_background = n_read > 2 && buffer[n_read - 2] == '&';

buffer[n_read - run_in_background - 1] = ' ';

/* Parse the arguments: the first argument is the file or command *

* we want to run. */

parser_state = STATE_SPACE;

for (exec_argc = 0, i = 0;

(buffer[i] != ' ') && (exec_argc < SHELL_MAX_ARGS); i++) {

if (!isspace(buffer[i])) {

if (parser_state == STATE_SPACE)

exec_argv[exec_argc++] = &buffer[i];

parser_state = STATE_NON_SPACE;

} else {

buffer[i] = '\0';

parser_state = STATE_SPACE;

}

}

/* run_in_background is 1 if the input line's last character *

* is an ampersand (indicating background execution). */

buffer[i] = '\0'; /* Terminate input, overwriting the '&' if it exists */

/* If no command was given (empty line) the Shell just prints the prompt again */

if (!exec_argc)

continue;

/* Terminate the list of exec parameters with NULL */

exec_argv[exec_argc] = NULL;

/* If Shell runs 'exit' it exits the program. */

if (!strcmp(exec_argv[0], "exit")) {

printf("Exiting process %d ", shell_pid);

return EXIT_SUCCESS; /* End Shell program */

} else if (!strcmp(exec_argv[0], "cd") && exec_argc > 1) {

/* Running 'cd' changes the Shell's working directory. */

/* Alternative: try chdir inside a forked child: if(fork() == 0) { */

if (chdir(exec_argv[1]))

/* Error: change directory failed */

fprintf(stderr, "cd: failed to chdir %s ", exec_argv[1]);

/* End alternative: exit(EXIT_SUCCESS);} */

} else {

/* Execute Commands */

/* Try replacing 'fork()' with '0'. What happens? */

pid_from_fork = fork();

if (pid_from_fork < 0) {

/* Error: fork() failed. Unlikely, but possible (e.g. OS *

* kernel runs out of memory or process descriptors). */

fprintf(stderr, "fork failed ");

continue;

}

if (pid_from_fork == 0) {

return imthechild(exec_argv[0], &exec_argv[0]);

/* Exit from main. */

} else {

imtheparent(pid_from_fork, run_in_background);

/* Parent will continue around the loop. */

}

} /* end if */

} /* end while loop */

return EXIT_SUCCESS;

} /* end main() */