Thread: Pipelines and I/O redirections

I have to extend and modify the program to:

Redirect(>)standard output from a command to a file. If the file already exist, it will beerased and overwritten without warning.
Examples:

Code:

COP$ ls > 1
COP$ sort myshell.c > 2

Append( >>)standard output from a command to a file if the file exists; if the file does not exist,create one. Examples:

Code:

COP$ sort myshell.c >> 1
COP$ echo "Add A Line" >> 1

Redirect the standard input to be from a file, rather than the keyboard. For example:

Code:

COP$ sort < myshell.c
COP$ sort < myshell.c > 1
COP$ sort > 1 < myshell.c

and lastly pass ( |)the standard output of one command to another for further processing. Examples:

Code:

COP$ ls | sort
COP$ sort < myshell.c | grep main | cat > output

Heres what I got so far:

Code:

#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>


#define MAX_ARGS 20
#define BUFSIZ 1024


int get_args(char* cmdline, char* args[])
{
  int i = 0;


  /* if no args */
  if((args[0] = strtok(cmdline, "\n\t ")) == NULL)
    return 0;


  while((args[++i] = strtok(NULL, "\n\t ")) != NULL) {
    if(i >= MAX_ARGS) {
      printf("Too many arguments!\n");
      exit(1);
    }
  }
  /* the last one is always NULL */
  return i;
}


void execute(char* cmdline)
{
  int pid, async;
  char* args[MAX_ARGS];


  int nargs = get_args(cmdline, args);
  if(nargs <= 0) return;


  if(!strcmp(args[0], "quit") || !strcmp(args[0], "exit")) {
    exit(0);
  }


  /* check if async call */
  if(!strcmp(args[nargs-1], "&")) { async = 1; args[--nargs] = 0; }
  else async = 0;


  pid = fork();
  if(pid == 0) { /* child process */
    execvp(args[0], args);
    /* return only when exec fails */
    perror("exec failed");
    exit(-1);
  } else if(pid > 0) { /* parent process */
    if(!async) waitpid(pid, NULL, 0);
    else printf("this is an async call\n");
  } else { /* error occurred */
    perror("fork failed");
    exit(1);
  }
}


int main (int argc, char* argv [])
{
  char cmdline[BUFSIZ];


  for(;;) {
    printf("COP4338$ ");
    if(fgets(cmdline, BUFSIZ, stdin) == NULL) {
      perror("fgets failed");
      exit(1);
    }
    execute(cmdline) ;
  }
  return 0;
}

Ill look into dup(), how would I check for > or >> or | to do the according process?

Thank you Salem, you're the bomb.