Question

I am new to C and I am trying to create a simple C shell that will allow the user to perform various functions like chdir, cd, exit, mkdir.

I’ve posted my code below. Can anyone look through it and see what I am doing wrong? I am not sure if I am using fork and execcv correctly. Thanks!

include stdio.h
include stdlib.h
include unistd.h
include <string.h>
include sys/types.h

main() {
    //char *user;

    //if ((user = getlogin()) == NULL)
    //    perror("__getlogin1() error");
    //else printf("__getlogin1() returned %s\n", user);
    int j, status;
    int pid, c_pid;
    int i = 0;
    char *tmp, **ap;
    char instring[80]; // store one line of input
    char *argv[10]; // store parameters in the format for execv()

    promptstart:

    printf("Please enter a commcand:\n");

    // read a char at a time and put it in instring[]
    // put a '\0' at the end
    instring[i] = getc(stdin); // stdin is the keyboard
    while (instring[i] != '\n') {
        i++;
        instring[i] = getc(stdin);
    }
    instring[i] = '\0'; // replace '\n' with '\0'

    tmp = instring;
    i = 0;
    argv[i] = strsep(&tmp, " \t"); // put first word int argv[0]
    while ((i < 10) && (argv[i] != '\0')) {
        i++;
        argv[i] = strsep(&tmp, " \t");
    }

    // print out the command and options.
    i = 0;
    while (argv[i] != '\0') {
        printf("your entered: %s\n", argv[i++]);
    }

    //PLACE ERROR HERE

    if ((c_pid = fork()) == 0) {
        for (j = 0; j < 10; j++)
            printf("child (%d) prints %d\n", getpid(), j);
        exit(0);
    } else if (c_pid > 0) {
        c_pid = wait(&status);
        printf("child %d exited with status %d\n", c_pid, status);
    } else {
        execvp(argv[0], argv);

    }
    goto promptstart;
}

Answer 1

At least IMO, you’re putting far too much into main. I’d start with something like:

int main() { 
    char input[128];

    do { 
        fgets(stdin, input, sizeof(input));
        dispatch(input);
    } while (strcmp(input, "exit"));
    return 0;
}

Then dispatch will look for internal commands, and only do an exec when/if it’s given a command it doesn’t recognize. To keep things simple to start with, you might consider using popen to execute external commands, and leave switching to a “raw” fork/exec for later, when the limitations of popen start to cause you problems.