#include <sys/stat.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main()
{
    struct stat st;
    fstat(STDOUT_FILENO, &st);
    if(S_ISCHR(st.st_mode))
        fprintf(stderr, "terminal\n");
    else if(S_ISREG(st.st_mode))
        fprintf(stderr, "regular file\n");
    else
    {
        //what ever
    }

    return 0;
}