Unix Shell Implementing Cat in C - File Descriptor Issue

Question

I've about got my practice implementation of a Unix shell done, except I'm having an issue with implementing cat when its output is to a file; IE: cat foo.txt > bar.txt - outputting foo 's contents to bar .

Let's start from the main function & then I'll define the submethods:

int main(int argc, char **argv)
{           
    printf("[MYSHELL] $ ");

    while (TRUE) {
        user_input = getchar();
        switch (user_input) {

            case EOF:
                exit(-1);

            case '\n':
                printf("[MYSHELL] $ ");
                break;

            default:
                // parse input into cmd_argv - store # commands in cmd_argc
                handle_user_input();

                //determine input and execute foreground/background process
                execute_command();
        }
        background = 0;
    }
    printf("\n[MYSHELL] $ ");
    return 0;    
}

---

handle_user_input just populates the cmd_argv array to execute the user_input , and removes the > and sets an output flag if the user wishes to output to a file. This is the meat of that method:

while (buffer_pointer != NULL) { 
        cmd_argv[cmd_argc] = buffer_pointer;
        buffer_pointer = strtok(NULL, " ");

        if(strcmp(cmd_argv[cmd_argc], ">") == 0){
            printf("\nThere was a '>' in %s @ index: %d for buffer_pointer: %s \n", *cmd_argv,cmd_argc,buffer_pointer);
            cmd_argv[cmd_argc] = strtok(NULL, " ");
            output = 1;
        }

        cmd_argc++;

        if(output){
            filename = buffer_pointer;
            printf("The return of handling input for filename %s =  %s + %s \n", buffer_pointer, cmd_argv[0], cmd_argv[1]); 
            return;
        }        
}

---

execute_command is then called, interpreting the now populated cmd_argv . Just to give you an idea of the big picture. Obviously, none of these cases match and the create_process method is called:

int execute_command()
{
    if (strcmp("pwd", cmd_argv[0]) == 0){
        printf("%s\n",getenv("PATH"));  
        return 1;
    }
    else if(strcmp("cd", cmd_argv[0]) == 0){
        change_directory();
        return 1;        
    }
    else if (strcmp("jobs", cmd_argv[0]) == 0){
        display_job_list();
        return 1;   
    }
    else if (strcmp("kill", cmd_argv[0]) == 0){
        kill_job();
    }
    else if (strcmp("EOT", cmd_argv[0]) == 0){
        exit(1);
    }
    else if (strcmp("exit", cmd_argv[0]) == 0){
        exit(-1);
    }
    else{
        create_process();
        return;
    }
}

Pretty straight forward, right?

---

create_process is where I'm having issues.

void create_process()
{
    status = 0;
    int pid = fork();
    background = 0;

    if (pid == 0) {
        // child process
        if(output){
            printf("Output set in create process to %d\n",output);
            output = 0;
            int output_fd = open(filename, O_RDONLY);
            printf("Output desc = %d\n",output_fd);
            if (output_fd > -1) {
                dup2(output_fd, STDOUT_FILENO);
                close(output_fd);
            } else {
                perror("open");
            }
        }
        printf("Executing command, but STDOUT writing to COMMAND PROMPT instead of FILE - as I get the 'open' error above \n");
        execvp(*cmd_argv,cmd_argv);
        // If an error occurs, print error and exit
        fprintf (stderr, "unknown command: %s\n", cmd_argv[0]);
        exit(0);
    } else {
        // parent process, waiting on child process
            waitpid(pid, &status, 0);
        if (status != 0)
            fprintf  (stderr, "error: %s exited with status code %d\n", cmd_argv[0], status);
    }
    return;
}

My printed output_fd = -1 , and I manage to get the perror("open") inside the else stating: open: No such file or directory . It then prints that it's "writing to COMMAND PROMPT instead of FILE" , as I display to the console. Then executes execvp which handles cat foo.txt , but prints it to the console instead of the file.

I realize it shouldn't at this point, as having output_fd = -1 isnt desirable and should be returning another value; but I cant figure out how to use file descriptors correctly in order to open a new/existing file with cat foo.txt > bar.txt and write to it, as WELL AS GET BACK to the command line's stdin.

I have managed to output to the file, but then lose getting back the correct stdin. Could someone please direct me here? I feel like I'm going in circles over something silly I'm doing wrong or looking over.

Any help is greatly GREATLY appreciated.

Answer 1

Why do you use O_RDONLY if you want to write to the file? My guess is that you should use something like:

int output_fd = open(filename, O_WRONLY|O_CREAT, 0666);

(The 0666 is to set up the access rights when creating).

And obviously, if you can't open a redicted file, you shouldn't launch the command.

Answer 2

First, obvious thing I notice is that you've opened the file O_RDONLY. Not going to work so well for output!

Second, basic process for redirecting the output is:

• open file for writing
• dup stdout so you can keep a copy if needed. same with stderr if redirecting.
• fcntl your duplicate to CLOEXEC (alternatively, use dup3)
• dup2 file to stdout
• exec the command

and finally, are you really passing around command names as global variables? I think this will come back to haunt you once you try and implement cat foo | ( cat bar; echo hi; cat ) > baz cat foo | ( cat bar; echo hi; cat ) > baz or somesuch.