Thread: Writing a C Shell

Hello everyone! I am playing around with learning to write a shell in C, but I'm running into some problems. First of all, I'm also very new to C in general (I'm a Java guy), so any mistakes you see are probably from my lack of knowledge.

I have my code working with redirects (< and >) and appending (>>), but the background stuff is giving me trouble. I have it catching that there is an ampersand at the end of a line, but I don't know how to pass it to execvp, if that's even what I'm supposed to be doing. I'd like it to eventually distinguish between processes that can wait (vi) and those that can execute right away (ls).

Here's what I have:

Code:

/*
 * This code implements a simple shell program
 * It supports the internal shell command "exit", 
 * backgrounding processes with "&", input redirection
 * with "<" and output redirection with ">".
 * However, this is not complete.
 */


#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <errno.h>
#include <signal.h>


extern char **getline();


/*
 * Handle exit signals from child processes
 */
void sig_handler(int signal) {
  int status;
  int result = wait(&status);
  //UNCOMMENT
  //printf("Wait returned %d\n", result);
}


/*
 * The main shell function
 */ 
main() {
  int i;
  char **args; 
  int result;
  int block;
  int output;
  int a_output;
  int input;
  char *output_filename;
  char *input_filename;
  char *append_filename;


  // Set up the signal handler
  sigset(SIGCHLD, sig_handler);


  // Loop forever
  while(1) {


    // Print out the prompt and get the input
    printf("->");
    args = getline();
    
    for(i = 0; args[i] != NULL; i++) {
      printf("Argument %d: %s\n", i, args[i]);
    }
    printf("\n");
    
    // No input, continue
    if(args[0] == NULL)
      continue;


    // Check for internal shell commands, such as exit
    if(internal_command(args))
      continue;


    // Check for an ampersand
    block = (ampersand(args) == 0);


    // Check for redirected input
    input = redirect_input(args, &input_filename);


    switch(input) {
    case -1:
      printf("Syntax error!\n");
      continue;
      break;
    case 0:
      break;
    case 1:
      printf("Redirecting input from: %s\n", input_filename);
      break;
    }


    // Check for redirected output
    output = redirect_output(args, &output_filename);


    switch(output) {
    case -1:
      printf("Syntax error!\n");
      continue;
      break;
    case 0:
      break;
    case 1:
      printf("Redirecting output to: %s\n", output_filename);
      break;
    }
    
    // Check for appended output
    a_output = append_output(args, &append_filename);
    // a_output = 0;


    switch(a_output) {
    case -1:
      printf("Syntax error!\n");
      continue;
      break;
    case 0:
      break;
    case 1:
      printf("Appending output to: %s\n", append_filename);
      break;
    }
    
    // Do the command
    do_command(args, block, 
           input, input_filename, 
           output, output_filename,
           a_output, append_filename);
  }
}


/*
 * Check for ampersand as the last argument
 */
int ampersand(char **args) {
  int i;


  for(i = 1; args[i] != NULL; i++) ;


  if(args[i-1][0] == '&') {
    free(args[i-1]);
    args[i-1] = NULL;
    return 1;
  } else {
    return 0;
  }
  
  return 0;
}


/* 
 * Check for internal commands
 * Returns true if there is more to do, false otherwise 
 */
int internal_command(char **args) {
  if(strcmp(args[0], "exit") == 0) {
    exit(0);
  }


  return 0;
}


/* 
 * Do the command
 */
int do_command(char **args, int block,
           int input, char *input_filename,
           int output, char *output_filename,
           int a_output, char *append_filename) {
  
  int result;
  pid_t child_id;
  int status;


  // Fork the child process
  child_id = fork();


  // Check for errors in fork()
  switch(child_id) {
  case EAGAIN:
    perror("Error EAGAIN: ");
    return;
  case ENOMEM:
    perror("Error ENOMEM: ");
    return;
  }


  if(child_id == 0) {


    // Set up redirection in the child process
    if(input)
      freopen(input_filename, "r", stdin);


    if(output)
      freopen(output_filename, "w+", stdout);
      
    if(a_output)
      freopen(append_filename, "a+", stdout);  
    
    // Execute the command
    result = execvp(args[0], args);


    exit(-1);
  }


  // Wait for the child process to complete, if necessary
  if(block) {
    //UNCOMMENT
    //printf("Waiting for child, pid = %d\n", child_id);
    result = waitpid(child_id, &status, 0);
  }
}


/*
 * Check for input redirection
 */
int redirect_input(char **args, char **input_filename) {
  int i;
  int j;


  for(i = 0; args[i] != NULL; i++) {


    // Look for the <
    if(args[i][0] == '<') {
      free(args[i]);


      // Read the filename
      if(args[i+1] != NULL) {
    *input_filename = args[i+1];
      } else {
    return -1;
      }


      // Adjust the rest of the arguments in the array
      for(j = i; args[j-1] != NULL; j++) {
    args[j] = args[j+2];
      }


      return 1;
    }
  }


  return 0;
}


/*
 * Check for output redirection
 */
int redirect_output(char **args, char **output_filename) {
  int i;
  int j;


  for(i = 0; args[i] != NULL; i++) {
        // Look for the >
        if(args[i][0] == '>') {
            if(args[i+1][0] == '>'){
                return 0;
            }else{
              free(args[i]);


              // Get the filename 
              if(args[i+1] != NULL) {
                *output_filename = args[i+1];
              } else {
                return -1;
              }


              // Adjust the rest of the arguments in the array
              for(j = i; args[j-1] != NULL; j++) {
                args[j] = args[j+2];
              }


              return 1;
            }
        }
  }


  return 0;
}


/*
 * Check for append redirection
 */
int append_output(char **args, char **output_filename) {
  int i;
  int j;


  for(i = 0; args[i] != NULL; i++) {


    // Look for the >>
    if((args[i][0] == '>') && (args[i+1][0] == '>')) {
      free(args[i]);


      // Get the filename 
      if(args[i+2] != NULL) {
        *output_filename = args[i+2];
      } else {
        return -1;
      }


      // Adjust the rest of the arguments in the array
      for(j = i; args[j-1] != NULL; j++) {
        args[j] = args[j+3];
      }


      return 1;
    }
  }


  return 0;
}

Any advice would be helpful.

I'm a little confused about what you're getting at. My professor assigned us an assignment to make a C Shell. He gave us some base code, and we were to implement a variety of things, and I am stuck on the step where I need to execute a command in the background. I came here for help.

I didn't "find" this code. It's what my professor gave me. I assume that whoever posted in that link you showed must have had the same professor as I do now.

All I want is some direction as to what I'm doing wrong, or where I can start with this background stuff. I'm sorry if what I did wasn't correct, but I'm stressed beyond belief with this project. I've never coded anything in C before in my life. The curriculum at my university changed from C/C++ to Java the year I got here, and this OS class I'm in is the first experience I've had with anything that isn't Java.

I'm sorry if I offended you with what looked like a copy/paste job, but that wasn't the case. This is my code, altered from what the professor gave us.

You still want us to do something that you have been assigned. If you don't understand what you are doing wrong, perhaps you should ask yourself if programming is for you.

Oh! No! I'm sorry if that's what is sounds like. I came for help with this, not the answer. I'm just so lost. I'm a 4th year Computer Science major at my university and this is the first time I've ever used LINUX or C. My university has admitted that it was a flaw for them to keep them from us until now, but that hasn't kept them from assigning this (any many more to come) assignment.

What is the name of your university?