help with creating linux shell using C

Question

I'm supposed to create a linux shell using C. Below is my code:

#include <stddef.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#define SHELL "/bin/sh"
#include "extern.h"

int mysystem (char *command)
{
  int status;
  pid_t pid;

  pid = fork ();
  if (pid == 0)
    {

      execl (SHELL, SHELL, "-c", command, NULL);
      _exit (EXIT_FAILURE);
    }
  else if (pid < 0)

    status = -1;
  else

    if (waitpid (pid, &status, 0) != pid)
      status = -1;
  return status;
}

Everything is right when I test the code using different commands like "ls", "man", etc. but when I use notepad to create a testfile containing the following:

echo "hello"
exit 2

the return code come out to be 512 when it's supposed to be just 2 . Can anyone help me fix my code?

Answer 1

status is not the exit code; it contains other information as well. Normally the return value is in bits 8-15 of status , but you should be using the macros in wait.h to extract the return value from status in a portable way.

Note that 512 is 2<<8 .

Answer 2

Make sure you're using the macros like WIFEXITED and WEXITSTATUS on your status value. See your operating system's man page for waitpid . Here is a description of the POSIX requirements on waitpid .

Answer 3

By notepad do you mean you're using a Windows program to create a Unix shell script? That doesn't work because you end up with CRLF at the end of each line instead of LF. Try the "dos2unix" command on the script to convert it to Unix format and then run it.

I assume you're aware that code is already available in the system() library call? Judging by your function name, I'd guess you're just trying to learn how to do it with system calls.

Answer 4

Try enclosing your command string you supply to /bin/sh with quotes, because otherwise the space character makes /bin/sh think you are supplying another option to the shell itself, not to the command you are calling. For example, try this in a terminal:

/bin/sh -c exit 2
echo $?

and

/bin/sh -c "exit 2"
echo $?

The first one gives 0 , and the second one gives the desired 2 .