C++ shell with forks and pipes

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/
#include <iostream>
#include <fstream>
#include <sys/stat.h>
#include <cerrno>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
using namespace std;

// Will be used to create an array to hold individual arguments passed by
// the user on the command line.
const int MAX_ARGS = 256;

enum PipeRedirect {PIPE, REDIRECT, NEITHER};

// Splits a user's command into two commands, or a command and a file name.
PipeRedirect parse_command(int, char**, char**, char**);

// Pipes the first command's output into the second.
void pipe_cmd(char**, char**);

// Reads input from the user into the given array and returns the number of
// arguments taken in.
int read_args(char**);

// Redirects the output from the given command into the given file.
void redirect_cmd(char**, char**);

// Given the number of arguments and an array of arguments, this will execute
// those arguments.  The first argument in the array should be a command.
void run_cmd(int, char**);

// Given a string of user input, this will determine if the user wants to
// quit the shell.
bool want_to_quit(string);

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include "general.h"

// Takes user input until they quit the shell, and passes that input as
// arguments to be run.
int main() {
  char *argv[MAX_ARGS], *cmd1[MAX_ARGS], *cmd2[MAX_ARGS];
  PipeRedirect pipe_redirect;
  int argc;

  // Keep returning the user to the prompt ad infinitum unless they enter
  // 'quit' or 'exit' (without quotes).
  while (true) {
    // Display a prompt.
    cout << "SarahShell> ";

    // Read in a command from the user.
    argc = read_args(argv);

    // Decipher the command we just read in and split it, if necessary, into
    // cmd1 and cmd2 arrays.  Set pipe_redirect to a PipeRedirect enum value to
    // indicate whether the given command pipes, redirects, or does neither.
    pipe_redirect = parse_command(argc, argv, cmd1, cmd2);

    // Determine how to handle the user's command(s).
    if (pipe_redirect == PIPE)          // piping
      pipe_cmd(cmd1, cmd2);
    else if (pipe_redirect == REDIRECT) // redirecting
      redirect_cmd(cmd1, cmd2);
    else
      run_cmd(argc, argv);              // neither

    // Reset the argv array for next time.
    for (int i=0; i<argc; i++)
      argv[i] = NULL;
  }

  // Let the OS know everything is a-okay.
  return 0;
}

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include "general.h"

// Given the number of arguments (argc) in an array of arguments (argv), this
// will go through those arguments and, if necessary, bifurcate the arguments
// into arrays cmd1 and cmd2.  It will return a PipeRedirect enum representing
// whether there was a pipe in the command, a redirect to a file, or neither.
// cmd1 and cmd2 will only be populated if there was a pipe or a redirect.
PipeRedirect parse_command(int argc, char** argv, char** cmd1, char** cmd2) {
  // Assume no pipe or redirect will be found.
  PipeRedirect result = NEITHER;

  // Will hold the index of argv where the pipe or redirect is found.
  int split = -1;

  // Go through the array of arguments...
  for (int i=0; i<argc; i++) {
    // Pipe found!
    if (strcmp(argv[i], "|") == 0) {
      result = PIPE;
      split = i;

    // Redirect found!
    } else if (strcmp(argv[i], ">>") == 0) {
      result = REDIRECT;
      split = i;
    }
  }

  // If either a pipe or a redirect was found...
  if (result != NEITHER) {
    // Go through the array of arguments up to the point where the
    // pipe/redirect was found and store each of those arguments in cmd1.
    for (int i=0; i<split; i++)
      cmd1[i] = argv[i];

    // Go through the array of arguments from the point where the pipe/redirect
    // was found through the end of the array of arguments and store each
    // argument in cmd2.
    int count = 0;
    for (int i=split+1; i<argc; i++) {
      cmd2[count] = argv[i];
      count++;
    }

    // Terminate cmd1 and cmd2 with NULL, so that execvp likes them.
    cmd1[split] = NULL;
    cmd2[count] = NULL;
  }

  // Return an enum showing whether a pipe, redirect, or neither was found.
  return result;
}

// This pipes the output of cmd1 into cmd2.
void pipe_cmd(char** cmd1, char** cmd2) {
  int fds[2]; // file descriptors
  pipe(fds);
  pid_t pid;

  // child process #1
  if (fork() == 0) {
    // Reassign stdin to fds[0] end of pipe.
    dup2(fds[0], 0);

    // Not going to write in this child process, so we can close this end
    // of the pipe.
    close(fds[1]);

    // Execute the second command.
    execvp(cmd2[0], cmd2);
    perror("execvp failed");

  // child process #2
  } else if ((pid = fork()) == 0) {
    // Reassign stdout to fds[1] end of pipe.
    dup2(fds[1], 1);

    // Not going to read in this child process, so we can close this end
    // of the pipe.
    close(fds[0]);

    // Execute the first command.
    execvp(cmd1[0], cmd1);
    perror("execvp failed");

  // parent process
  } else
    waitpid(pid, NULL, 0);
}

// This will get input from the user, split the input into arguments, insert
// those arguments into the given array, and return the number of arguments as
// an integer.
int read_args(char **argv) {
  char *cstr;
  string arg;
  int argc = 0;

  // Read in arguments till the user hits enter
  while (cin >> arg) {
    // Let the user exit out if their input suggests they want to.
    if (want_to_quit(arg)) {
      cout << "Goodbye!\n";
      exit(0);
    }

    // Convert that std::string into a C string.
    cstr = new char[arg.size()+1];
    strcpy(cstr, arg.c_str());
    argv[argc] = cstr;

    // Increment our counter of where we're at in the array of arguments.
    argc++;

    // If the user hit enter, stop reading input.
    if (cin.get() == '\n')
      break;
  }

  // Have to have the last argument be NULL so that execvp works.
  argv[argc] = NULL;
 
  // Return the number of arguments we got.
  return argc;
}

void redirect_cmd(char** cmd, char** file) {
  int fds[2]; // file descriptors
  int count;  // used for reading from stdout
  int fd;     // single file descriptor
  char c;     // used for writing and reading a character at a time
  pid_t pid;  // will hold process ID; used with fork()

  pipe(fds);

  // child process #1
  if (fork() == 0) {
    // Thanks to http://linux.die.net/man/2/open for showing which headers
    // need to be included to use this function and its flags.
    fd = open(file[0], O_RDWR | O_CREAT, 0666);

    // open() returns a -1 if an error occurred
    if (fd < 0) {
      printf("Error: %s\n", strerror(errno));
      return;
    }

    dup2(fds[0], 0);

    // Don't need stdout end of pipe.
    close(fds[1]);

    // Read from stdout...
    while ((count = read(0, &c, 1)) > 0)
      write(fd, &c, 1); // Write to file.

    // Okay, so this is a bit contrived, but when I didn't have any kind of exec
    // function call here, I got my SarahShell prompt repeated over and over
    // again on the Multilab machines, I think because of this crazy child
    // process or something.  When I put this execlp here with the useless call
    // to echo, however, that looping stops and you can actually enter things
    // at the prompt again, hurray!
    execlp("echo", "echo", NULL);

  // child process #2
  } else if ((pid = fork()) == 0) {
    dup2(fds[1], 1);

    // Don't need stdin end of pipe.
    close(fds[0]);

    // Output contents of the given file to stdout.
    execvp(cmd[0], cmd);
    perror("execvp failed");

  // parent process
  } else {
    waitpid(pid, NULL, 0);
    close(fds[0]);
    close(fds[1]);
  }
}

// Given the number of arguments (argc) and an array of arguments (argv),
// this will fork a new process and run those arguments.
// Thanks to http://tldp.org/LDP/lpg/node11.html for their tutorial on pipes
// in C, which allowed me to handle user input with ampersands.
void run_cmd(int argc, char** argv) {
  pid_t pid;
  const char *amp;
  amp = "&";
  bool found_amp = false;
 
  // If we find an ampersand as the last argument, set a flag.
  if (strcmp(argv[argc-1], amp) == 0)
    found_amp = true;

  // Fork our process
  pid = fork();
 
  // error
  if (pid < 0)
    perror("Error (pid < 0)");
 
  // child process
  else if (pid == 0) {
    // If the last argument is an ampersand, that's a special flag that we
    // don't want to pass on as one of the arguments.  Catch it and remove
    // it here.
    if (found_amp) {
      argv[argc-1] = NULL;
      argc--;
    }

    execvp(argv[0], argv);
    perror("execvp error");

  // parent process
  } else if (!found_amp)
    waitpid(pid, NULL, 0); // only wait if no ampersand
}

// Given a string of user input, this determines whether or not the user
// wants to exit the shell.
bool want_to_quit(string choice) {
  // Lowercase the user input
  for (unsigned int i=0; i<choice.length(); i++)
    choice[i] = tolower(choice[i]);

  return (choice == "quit" || choice == "exit");
}