Question

I’m trying to write a shell in c and it mostly works except for grep. Whenever I give a grep command in the shell, it just doesn’t output anything. Here is the part of the code I use to create a new child process and then run execvp() in it.

The file descriptors(fd_in and fd_out) in dup2 are passed as arguments to the function which has this code. And most interestingly, when I give ‘grep’ or ‘grep –help’ it displays as usual. Am I missing anything? Or something special has to be done with grep?

This is what happens with my shell: The last command outputs when run from bash.

--> grep
Usage: grep [OPTION]... PATTERN [FILE]...
Try `grep --help' for more information.
--> wc /etc/hosts
 11  33 314 /etc/hosts
--> grep -i "perror" shell.c
--> 

Here is the code :

void 
create_process(char *cmd_argv[], int fd_in, int fd_out, char *buffer_copy) {

    /*Flag bit for Background processes*/
    int FLAG = 0;

    pid_t cpid;
    int status;
    int i = 0,j = 0;

    /*Find the no. of arguments*/
    while(cmd_argv[j] != NULL)
        j++;

    /*Set the flag bit*/
    if(strcmp("&", cmd_argv[j-1]) == 0) {
        FLAG = 1;
        cmd_argv[j-1] = NULL;
    }

    //Create a child process
    cpid = fork();

    if (cpid == -1) {
        perror("fork");
        exit(EXIT_FAILURE);
    }

    //In the child...
    if (cpid == 0) {

    /*Checking if the file descriptors are already assigned*/

        /*For stdin*/
        if (fd_in != STDIN_FILENO) {
            dup2(fd_in, STDIN_FILENO);
            close(fd_in);
        }

        /*For stdout*/
        if (fd_out != STDOUT_FILENO) {
            dup2(fd_out, STDOUT_FILENO);
            close(fd_out);
        }

        /*Run the cmd specified*/
        status = execvp(cmd_argv[0], cmd_argv);

        /*In case of errors*/
        if(status < 0) {
            perror("execvp ");
            exit(1);
        }
    }

    //In the parent...
    else {

        if(FLAG == 1) {
            /*Find where the new bg process can be inserted*/
            while(1) {
                if (bgprocess[i].pid == 0) {
                    bgprocess[i].pid = cpid;
                    strcpy(bgprocess[i].cmd, buffer_copy);
                    break;
                }
                i++;
            }
            printf("[%d] : %s\n", cpid, cmd_argv[0] );
        }

        /*If not bg, wait for the process to exit*/
        else 
            waitpid(cpid, NULL, 0);
    }
}

Answer 1

The problem is using the quotes in your shell. Bash does a lot of things in background.
grep -i perror shell.c should give you output on your shell, whatever is anticipated when run from bash.