noob2.c

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

#define READ_END 0
#define WRITE_END 1

int tokenize(char *line, char *sep, char *argv[][2000],char **redirect_in, char **redirect_out) {
  int index = 0;
  int stage = 0;
  char *token = strtok(line, sep);
  while (token != NULL) {
    if (strcmp(token, ">") == 0) {
      token = strtok(NULL, sep);
      (*redirect_out) = token;
    }
    else if (strcmp(token, "<") == 0) {
      token = strtok(NULL, sep);
      (*redirect_in) = token;
    }
    else if (strcmp(token, "|") == 0) {
      argv[stage][index] = 0;
      index = 0;
      stage++;
    }
    else {
      argv[stage][index] = token;
      index++;
    }

    token = strtok(NULL, sep);
  }
  argv[stage][index] = 0;
  return stage + 1;
}

int main() {
  int pid1 = -1;
  int pid2 = -1;
  int pipefd[2];
  char line[2000];
  size_t bufsize = 2000;
  const char sep[50] = " ";

  char *buffer = line;

  int chars;
  
  //read a line
  while((chars = getline(&buffer, &bufsize, stdin)) != -1) {
    char *argv[2][2000];
    line[chars-1] = '\0';
    char* redirect_in;
    char* redirect_out;
    int index = 0;
    int command = 0;
    int stages = tokenize(line, sep, argv[command], &redirect_in, &redirect_out);
    printf("\nyash$ ");

    //piping
    if (stages > 1) {
      //happens in parent
      if (pipe(pipefd) == -1) {
        fprintf(stderr, "failed to created pipe\n");
        return -1;
      }
    }
    
    //actual forking
    
    //second command optionally
    if (stages > 1) {
      pid2 = fork();

      if (pid2 == -1) {
        fprintf(stderr, "Could not fork process\n");
        return -1;
      } else if (pid2 == 0) {
        //child process 2
        //redirection output
        if (redirect_out != NULL) {
          int fd = open(redirect_out, O_WRONLY|O_CREAT|O_TRUNC, 0666);
          dup2(fd, 1);
          close(fd);
        }

        //2nd command always reads from pipe
        if (dup2(pipefd[READ_END], STDIN_FILENO) == -1) {
          fprintf(stderr, "dup2 failed: cannot read from pipe");
          return -1;
        }
        close(pipefd[READ_END]);
        close(pipefd[WRITE_END]);

        execvp(argv[1][0], &argv[1][0]);
        //child process 2 ends here

        fprintf(stderr, "Ooops\n");
        return -1;
      }
    }

    //first command
    pid1 = fork();

    if (pid1 == -1) {
      fprintf(stderr, "Could not fork process\n");
      return -1;
    } else if (pid1 == 0) {
      // child process
      //redirection input
      if (redirect_in != NULL) {
        int fd = open(redirect_in, O_RDONLY, 0666);
        dup2(fd, STDIN_FILENO);
        close(fd);
      }

      if (stages <= 1) {
        //redirection to output file
        if (redirect_out != NULL) {
          int fd = open(redirect_out, O_WRONLY|O_CREAT|O_TRUNC, 0666);
          dup2(fd, 1);
          close(fd);
        }
      } else {
        //write to pipe
        if (dup2(pipefd[WRITE_END], STDOUT_FILENO) == -1) {
          fprintf(stderr, "dup2 failed: cannot write to pipe");
          return -1;
        }
        close(pipefd[READ_END]);
        close(pipefd[WRITE_END]);
      }
      execvp(argv[0][0], &argv[0][0]);
      //child process 1 ends here

      fprintf(stderr, "Ooops\n");
      return -1;
    }

    //closing both end because parent doesn't need pipes
    close(pipefd[READ_END]);
    close(pipefd[WRITE_END]);
    wait(&pid1);
    wait(&pid2);
  }

  return 0;
}