redirect-demo.c

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#include <stdio.h>


int main (int argc, char **argv, char **envp) {
    if (argc != 2) {
        fprintf (stderr, "Usage: %s <filename>\n", argv[0]);
        return 1;
    }

    close (0);
    int fd = open (argv[1], O_RDONLY);

    if (fd < 0) {
        perror ("Could not open file");
        return 2;

    } else if (fd != 0) {
        fprintf (stderr,
                 "Tried to open %s as fd 0, got %d instead\n",
                 argv[1],
                 fd);
        return 3;
    }

    char * const arglist[] = {
        "/bin/cat",
        NULL
    };

    execve ("/bin/cat", arglist, envp);
    perror ("Could not exec");

    return 0;
}