Unix shell

Im doing an assignment, programming in C, in Unix.
The task is, stimulating a shell.
It should accept any command entered.

I have done it, and so far it could run commands entered from the prompt.

However, the problems are:

1. No error returning back when an invalid command entered.

2. Another task is, the pipe operator. I dont know how to do this. Any1 know
how to make it accept pipes like in normal shell? For example:

command1 paratemeters | command 2 ....

Thanks for reading.

Alan.




-----------------------------------------
Here's the code at the moment:
-----------------------------------------

#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

int main();
void parent(int pid);
void child();

char command[100];

int main() {
 int ret;

 do {
      printf("alan_ass2$");
      fgets(command,100,stdin);

      if ( strncmp(command,"logout\n",8) == 0 )
       exit(1);
      if ( strncmp(command,"\n",1) == 0 )
       continue;

      if( (ret=fork()) > 0 )
           parent(ret);
      else if ( ret == 0 )
           child();
      else
           perror("fork() failed");
     } while (1);

}

void parent(int pid)
{
    int childs_exit_val;

 /*
 ** The parent only invokes the wait() if the command is a foreground
command,
 ** i.e. there is no '&' at the end of the line
 */
}

void child()
{
     char *argv[100]; /* 100 is arbitrarily long enough for this assignment
*/
     char *tmp;
     int cnt,i;

     cnt = -1;
     printf("\n");
            tmp=strtok(command, "\n");
            tmp=strtok(tmp, " ");
            while ( tmp != NULL )
            {
                    argv[++cnt]= tmp;
                    tmp=strtok(NULL," ");
            }

     if ((strncmp(argv[0],"cd",2)==0)&&(strlen(argv[0])==2))
          argv[0]="chdir";

     for (i=0; i<=cnt; i++) {
          printf("argv[%d] = '%s'\n", i, argv[i]);
     }

     argv[cnt+1]=NULL;

     execvp(argv[0], argv);

 /*
 ** If we arrive here, an error must have occurred otherwise a different
 ** program image would be active.
 */
    perror("execvp");
    exit(1); /* must be careful to exit a child process */
}