tee-clone.c

/**
 * This is a simple implementation of the Unix's tee command
 * It's a solution for the exercise 4-1 in "The Linux Programming Interface" book
 *
 * auther: Zain Saqer (github.com/zain-saqer)
 */

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

#define BUFFER_SIZE 1024

int main(int argc, char *argv[]) {
    int opt;
    char *file = NULL;
    char buf[BUFFER_SIZE];
    int fd, flags;

    opt = getopt(argc, argv, "a");

    int append = opt == 'a';
    if (append && optind >= argc) {
        fprintf(stderr, "Usage: %s [ [ -a ] [ File ] ]\n",
                argv[0]);
        exit(EXIT_FAILURE);
    }

    if (optind < argc) {
        file = argv[optind];
        flags = O_CREAT | O_WRONLY | O_TRUNC;
        if (append)
            flags = O_CREAT | O_WRONLY | O_APPEND;
        if ((fd = open(file, flags, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP)) == -1) {
            perror("open");
            exit(EXIT_FAILURE);
        }
    }


    ssize_t n;
    while ((n = read(STDIN_FILENO, buf, BUFFER_SIZE)) > 0) {
        // write to stdout
        ssize_t n2 = 0;
        while(n2 != -1 && n2 < n)
            n2 = write(STDOUT_FILENO, buf + n2, n - n2);

        if (file) {
            n2 = 0;
            while(n2 != -1 && n2 < n)
                n2 = write(fd, buf + n2, n - n2);
            if (n2 == -1) {
                perror("write to file");

                exit(EXIT_FAILURE);
            }

        }
    }

    if (file)
        close(fd);

    exit(EXIT_SUCCESS);
}