Shell Programming and Scripting

D="20100421043335"
date -d "${D:0:8} ${D:8:2}:${D:10:2}:${D:12:2} 2 minutes ago"

date -d "now 2 minutes ago"

printf "%(%Y%m%d%H%M%S)T" "${D:0:4}-${D:4:2}-${D:6:2} ${D:8:2}:${D:10:2}:${D:12:2} 2 minutes ago"

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


int
main(int argc, char* argv[])
{
   struct tm tm1, *tm2;

   char buf[80];
   char format[50];
   char tmp[80];
   char date[80];
   char seconds[10];
   char *s, *d;

   time_t t1, t2;

   int c;
   int debug = 0;
   int errflg = 0;

   extern char *optarg;
   extern int optind, optopt;

   while ((c = getopt(argc, argv, "dh")) != -1) {
        switch(c) {
        case 'd':
            debug = 1;
            break;
        case 'h':
            errflg++;
            break;
        case '?':
            fprintf(stderr, "Unknown option: -%c\n", optopt);
            errflg++;
        }
   }
   if (errflg || (argc < 2)) {
       fprintf(stderr, "Usage: subtime [-d] datestr seconds\n");
       exit(1);
   }

   strcpy(tmp, argv[optind]);
   optind++;
   strcpy(seconds, argv[optind]);

   strcpy(format, "%Y %m %d %H %M %S");

   /* reformat date string to place a space betwen each token */
   /* strptime specification requires a space */
   s = tmp;
   d = date;
   c = 0;
   while(*s) {
      if (c == 4 || c == 6 || c == 8 || c == 10 || c == 12 || c == 14) {
         *d++ = ' ';
      }
      c++;
      *d++ = *s++;
   }
   *d = '\0';

   if (debug) {
       fprintf(stderr, "Format: %s\n", format);
       fprintf(stderr, "Date: %s\n", date);
       fprintf(stderr, "Seconds: %s\n", seconds);
   }

   if (!strptime(date, format, &tm1)) {
       fprintf(stderr, "strptime() error\n");
       exit(1);
   }
   strftime(buf, 50, "%a %Y %H:%M:%S", &tm1);

   if ((t1 = mktime(&tm1)) == -1) {
      fprintf(stderr, "mktime() error\n");
      exit(1);
   }

   t1 = t1 - atoi(seconds);
   tm2 = localtime(&t1);

   if (strftime(buf, sizeof(buf), "%Y%m%d%H%M%S", tm2) == 0) {
      fprintf(stderr, "strftime() error\n");
      exit(1);
   }

   printf("%s", buf);

   exit(0);
}