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?

can anyone help me edit my question. there are things missing. thx — Brian
– Brian, Commented Nov 24, 2010 at 21:52

R.. GitHub STOP HELPING ICE · Accepted Answer · 2010-11-24 22:03:24Z

4

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.

answered Nov 24, 2010 at 22:03

R.. GitHub STOP HELPING ICE

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aschepler · Accepted Answer · 2010-11-24 22:04:08Z

4

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.

answered Nov 24, 2010 at 22:04

aschepler

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Comments

JOTN · Accepted Answer · 2010-11-24 22:03:01Z

0

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.

answered Nov 24, 2010 at 22:03

JOTN

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1 Comment

Brian Over a year ago

what do u mean that the code is already available in the system()library call? can you show me?

Blagovest Buyukliev · Accepted Answer · 2010-11-24 22:53:26Z

0

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.

edited Nov 24, 2010 at 22:53

answered Nov 24, 2010 at 22:19

Blagovest Buyukliev

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2 Comments

Brian Over a year ago

but when I do ./mysystem in the terminal, it still prints out 512.

Blagovest Buyukliev Over a year ago

It prints 512 because you don't use the WEXITSTATUS macro, as others have noted. Try changing the last line of your function to be return WEXITSTATUS(status).

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