Shell program to display age of person given their date of birth.

age.c

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

void printhelp(){
    printf("Get age of person.\n\n");
    printf("Usage: age -d <day> -m <month> -y <year>\n");
    printf("Arguments\n");
    printf("  -h\tprint this.\n");
    printf("  -d\tday of birth. \n");
    printf("  -m\tmonth of birth.\n");
    printf("  -y\tyear of birth.\n");
}

int main(int argc, char** argv){

    int c;
    int d = -1;
    int m = -1;
    int y = -1;

    //Get age params
    while ((c = getopt(argc, argv, ":d:m:y:")) != -1) {
        switch(c) {
            case 'd':
                d = atoi(optarg);
                break;
            case 'm':
                m = atoi(optarg);
                break;
            case 'y':
                y = atoi(optarg);
                break;
            default:
                printhelp();
                return 1;
        }
    }
    //Make sure all parameters were given.
    if(d == -1 || m == -1 || y == -1){
        printhelp();
        return -1;
    }
    
    //Initialize time variables.
    time_t now = time(0);
    struct tm* now_tm = localtime(&now);

    struct tm dob;
    dob.tm_sec = 0;
    dob.tm_min = 0;
    dob.tm_hour = 0;
    dob.tm_year = y-1900;
    dob.tm_mon = m-1;
    dob.tm_mday = d-1;

    //Get age and display it.
    double age = difftime(now, mktime(&dob));
    printf("%f\n", age/60/60/24/365.242199);
    
    return 0;
}