Implementation of shell like feature within C program using unix system calls

Question

Can someone please suggest what is wrong with this program.I am trying to implement shell like feature by creating a child process here. On giving a command having single word like ls or pwd it works but command with multiple words like ls -lrt or who am i is not working. There is something silly mistake that I am doing but unable to debug.

    #include <stdio.h>
    #include <string.h>
    #include <unistd.h>
    #include <iostream>
    #include <wait.h>
    #include <errno.h>
    #include <sys/types.h>
    #include <cstdlib>
    #define BUFSIZE  200
    #define ARGVSIZE 40
    #define DELIM    "\n\t\r"

    int main ()
    {
      int i,n;
      char buf[BUFSIZE + 1] ;
      char * str = "Shell > ";
      char * clargs[ARGVSIZE] ;
      int returnstatus;
      for(;;)
      {
          n = 1;
          write(STDOUT_FILENO,str,strlen(str));

          read(STDIN_FILENO,buf,BUFSIZE);
          if(!strcmp(buf,"exit\n"))
          {
              perror("exit");
              exit(20);
          }

          clargs[0] = strtok(buf,DELIM);

          while((clargs[n] = strtok(NULL,DELIM)) != NULL)
              n++;

          clargs[n] = NULL;

          switch(fork())
          {
          case 0:
          if((execvp(clargs[0],&clargs[0])) < 0)
              exit(200);

          default:
          wait(&returnstatus);
          printf("Exit status of command : %d\n",WEXITSTATUS(returnstatus));
          for(int i =0; i <= n;i++)
              clargs[i] = "\0";

          for(int i =0; i < BUFSIZE+1;i++)
              buf[i] = '\0';
      }
  }

  return 0;

}

@Thomas...its not working for commands having multiple words like "ls -lrt" or say "who am i" — Kundan Kumar
– Kundan Kumar, Commented May 25, 2012 at 18:59
@OliCharlesworth...I was expecting that this should give the output when i run the command like ls -lrt or any other command. But its giving exit statis 200 in those cases where i give the command like "ps -aux" or any other command which has more than one word. I am unable to understand why this is not working — Kundan Kumar
– Kundan Kumar, Commented May 25, 2012 at 19:12

ugoren · Accepted Answer · 2012-05-25 19:27:52Z

4

You don't have a space in DELIM.
When trying to run ls -lrt, you want to run the ls executable, with two arguments - ls and -lrt.
But your strtok wouldn't break ls -lrt in two. So you're actually trying to run a program called ls -lrt, and there's no such program.

Adding a space to DELIM should solve it.

Not that it won't be good enough for some cases. E.g. when running echo "a b", you want "a b" to be one parameter, because of the parenthesis. strtok would break it into two. A real shell does more complicated parsing.

answered May 25, 2012 at 19:27

ugoren

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