learn how to use exec and what is file descriptor

upper.c

#include <ctype.h>
#include <stdio.h>

int main(void)
{
    int ch;
    while((ch = getchar()) != EOF) {
        putchar(toupper(ch));
    }
    return 0;
}

wrapper.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
    int fd;
    if(argc != 2){
        fputs("usage: wrapper file\n", stderr);
        exit(1);
    }
    // O_RDONLY = open file (read only)
    fd = open(argv[1], O_RDONLY);
    if(fd < 0){
        // failed to open file
        perror("open");
        exit(1);
    }
    // duplicate a file descriptor
    // These system calls create a copy of the file descriptor oldfd.
    // dup2() makes newfd be the copy of oldfd, closing newfd first if necessary
    // STDIN_FILENO means standard input
    dup2(fd, STDIN_FILENO);
    // so it will copy fd to standard input and close fd
    close(fd);
    execl("./upper", "upper", NULL);
    perror("exec upper");
    exit(1);
}