--- /dev/null
+.TH CRONNEXT 1 "Nov 30, 2015"
+.SH NAME
+cronnext \- time of next job cron will execute
+.SH SYNOPSIS
+.TP 9
+.B cronnext
+[-i users] [-e users] [-s] [-t time] [-v] [-h]
+.SH DESCRIPTION
+Determine the time cron will execute the next job.
+Without arguments, it prints that time considering all crontabs,
+in number of seconds since the Epoch.
+This number can be converted into other formats using date(1),
+like 'date --date @43243254'
+.SH OPTIONS
+.TP
+-i user,user,user,...
+consider only the crontabs of these users;
+.br
+use "*system*" for the system crontab
+.TP
+-e user,user,user,...
+do not consider the crontabs of these users
+.TP
+-s
+do not consider the system crontab, usually /etc/crontab;
+.br
+the system crontab usually contains the hourly, daily, weekly and montly
+crontabs, which might be better dealt with anacron(8)
+.TP
+-t time
+determine the next job from this time, instead of now;
+.br
+the time is expressed in number of seconds since the Epoch, as obtained for
+example by 'date +%s --date "now + 2 hours"'
+.TP
+-v
+verbose mode: scanning the crontab is done in verbose mode, and for each user
+and job the next execution time is printed both as number of seconds since the
+Epoch and in the standard format
+.TP
+-h
+summary of options
+.SH AUTHOR
+Marco Migliori <sgerwk@aol.com>
+.SH SEE ALSO
+cron(8), cron(1), crontab(5), crontab(1), anacron(8), anacrontab(5), atq(1),
+date(1)
+
--- /dev/null
+/*
+ cronnext - calculate the time cron will execute the next job
+ Copyright (C) 2016 Marco Migliori <sgerwk@aol.com>
+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 2 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License along
+ with this program; if not, write to the Free Software Foundation, Inc.,
+ 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+ The GNU General Public License can also be found in the file
+ `COPYING' that comes with the Anacron source distribution.
+*/
+
+#include "config.h"
+
+#define MAIN_PROGRAM
+
+#include <stdlib.h>
+#include <string.h>
+#include <pwd.h>
+
+#include "globals.h"
+#include "funcs.h"
+#include "cron-paths.h"
+
+/*
+ * print entry flags
+ */
+char *flagname[]= {
+ [MIN_STAR] = "MIN_STAR",
+ [HR_STAR] = "HR_STAR",
+ [DOM_STAR] = "DOM_STAR",
+ [DOW_STAR] = "DOW_STAR",
+ [WHEN_REBOOT] = "WHEN_REBOOT",
+ [DONT_LOG] = "DONT_LOG"
+};
+
+void printflags(int flags) {
+ int f;
+ printf("flags: 0x%d = ", flags);
+ for (f = 1; f < sizeof(flagname); f = f << 1)
+ if (flags & f)
+ printf("%s ", flagname[f]);
+ printf("\n");
+}
+
+/*
+ * print a crontab entry
+ */
+void printentry(entry *e, int system, time_t next) {
+ if (system)
+ printf("entry user: %s\n", e->pwd->pw_name);
+ printf("cmd: %s\n", e->cmd);
+ printflags(e->flags);
+ printf("delay: %d\n", e->delay);
+ printf("next: %d = ", next);
+ printf("%s", asctime(localtime(&next)));
+}
+
+/*
+ * print a crontab data
+ */
+void printcrontab(user *u) {
+ printf("==========================\n");
+ printf("user: %s\n", u->name);
+ printf("crontab: %s\n", u->tabname);
+ printf("system: %d\n", u->system);
+}
+
+/*
+ * basic algorithm: iterate over time from now to 8 year ahead in default steps
+ * of 1 minute, checking whether time matches a crontab entry at each step (8
+ * years is the largest interval between two crontab matches)
+ *
+ * to save iterations, use larger steps if month or day don't match the entry:
+ * - if the month doesn't match, skip to 00:00 of the first day of next month
+ * - for the day, avoid the complication of the different length of months: if
+ * neither the day nor the next day match, increase time of one day
+ */
+
+/*
+ * check whether time matches day of month and/or day of week; this requires
+ * checking dom if dow=*, dow if dom=*, either one otherwise; see comment "the
+ * dom/dow situation is odd..." in cron.c
+ */
+int matchday(entry *e, time_t time) {
+ struct tm current;
+ int d;
+
+ localtime_r(&time, ¤t);
+
+ if (e->flags & DOW_STAR)
+ return bit_test(e->dom, current.tm_mday - 1);
+ if (e->flags & DOM_STAR)
+ return bit_test(e->dow, current.tm_wday);
+ return bit_test(e->dom, current.tm_mday - 1) ||
+ bit_test(e->dow, current.tm_wday);
+}
+
+/*
+ * next time matching a crontab entry
+ */
+time_t nextmatch(entry *e, time_t start) {
+ time_t time;
+ time_t nexttime;
+ struct tm current;
+ int res;
+
+ /* maximum match interval is 8 years (<102 months of 28 days):
+ * crontab has '* * 29 2 *' and we are on 1 March 2096:
+ * next matching time will be 29 February 2104 */
+
+ for (time = start; time < start + 102 * 28 * 24 * 60 * 60; ) {
+ localtime_r(&time, ¤t);
+
+ /* month doesn't match: move to 1st of next month */
+ if (!bit_test(e->month, current.tm_mon)) {
+ current.tm_mon++;
+ if (current.tm_mon >= 12) {
+ current.tm_year++;
+ current.tm_mon = 0;
+ }
+ current.tm_mday = 1;
+ current.tm_hour = 0;
+ current.tm_min = 0;
+ time = mktime(¤t);
+ continue;
+ }
+
+ /* neither time nor time+1day match day: increase 1 day */
+ if (!matchday(e, time) && !matchday(e, time + 24 * 60 * 60)) {
+ time += 24 * 60 * 60;
+ continue;
+ }
+
+ /* if time matches, return time;
+ * check for month is redudant, but check for day is
+ * necessary because we only know that either time
+ * or time+1day match */
+ if (bit_test(e->month, current.tm_mon) &&
+ matchday(e, time) &&
+ bit_test(e->hour, current.tm_hour) &&
+ bit_test(e->minute, current.tm_min)
+ )
+ return time;
+
+ /* skip to next minute */
+ time += 60;
+ }
+
+ return -1;
+}
+
+/*
+ * match a user against a list
+ */
+int matchuser(char *user, char *list) {
+ char *pos;
+ int l = strlen(user);
+
+ for (pos = list; pos = strstr(pos, user); pos += l) {
+ if ((pos != list) && (*(pos - 1) != ','))
+ continue;
+ if ((pos[l] != '\0') && (pos[l] != ','))
+ continue;
+ return 1;
+ }
+ return 0;
+}
+
+/*
+ * find next sheduled job
+ */
+time_t cronnext(time_t start,
+ char *include, char *exclude, int system,
+ int verbose) {
+ time_t closest, next;
+ static cron_db database = {NULL, NULL, (time_t) 0};
+ user *u;
+ entry *e;
+
+ /* load crontabs */
+ load_database(&database);
+
+ /* find next sheduled time */
+ closest = -1;
+ for (u = database.head; u; u = u->next) {
+ if (include && !matchuser(u->name, include))
+ continue;
+ if (exclude && matchuser(u->name, exclude))
+ continue;
+ if (!system && u->system)
+ continue;
+
+ if (verbose)
+ printcrontab(u);
+
+ for (e = u->crontab; e; e = e->next) {
+ next = nextmatch(e, start);
+ if (next < 0)
+ continue;
+ if ((closest < 0) || (next < closest))
+ closest = next;
+ if (verbose)
+ printentry(e, u->system, next);
+ }
+ }
+
+ return closest;
+}
+
+void usage() {
+ printf("find the next time of a scheduled cron job\n");
+ printf("usage:\n");
+ printf("\tcronnext [-i users] [-e users] [-s] [-t time] [-v] [-h]\n");
+ printf("\t\t-i users\tinclude only the crontab of these users\n");
+ printf("\t\t-e users\texclude the crontab of these users\n");
+ printf("\t\t-s\t\tdo not include the system crontab\n");
+ printf("\t\t-t time\t\tstart from this time (seconds since epoch)\n");
+ printf("\t\t-v\t\tverbose mode\n");
+ printf("\t\t-h\t\tthis help\n");
+}
+
+/*
+ * main
+ */
+int main(int argn, char *argv[]) {
+ int opt;
+ char *include, *exclude;
+ int system, verbose;
+ time_t start, next;
+
+ include = NULL;
+ exclude = NULL;
+ system = 1;
+ start = time(NULL);
+ verbose = 0;
+
+ while (-1 != (opt = getopt(argn, argv, "i:e:st:vh"))) {
+ switch (opt) {
+ case 'i':
+ include = optarg;
+ break;
+ case 'e':
+ exclude = optarg;
+ break;
+ case 's':
+ system = 0;
+ break;
+ case 't':
+ start = atoi(optarg);
+ break;
+ case 'v':
+ verbose = 1;
+ break;
+ case 'h':
+ usage();
+ return EXIT_SUCCESS;
+ default:
+ printf("unrecognized option: %s\n",
+ argv[optind - 1]);
+ usage();
+ exit(EXIT_FAILURE);
+ }
+ }
+
+ /* debug cron */
+ if (verbose) {
+ printf("SPOOL_DIR=%s\n", SPOOL_DIR);
+ set_debug_flags("load");
+ }
+ /* "load,pars" for debugging loading and parsing, "" for nothing
+ see globals.h for symbolic names and macros.h for meaning */
+
+ /* print time of next scheduled command */
+ next = cronnext(start, include, exclude, system, verbose);
+ if (next == -1) {
+ if (verbose)
+ printf("no sheduled job\n");
+ return EXIT_FAILURE;
+ }
+ else
+ if (verbose)
+ printf("next of all jobs: %d=%s",
+ next, asctime(localtime(&next)));
+ else
+ printf("%d\n", next);
+
+ return EXIT_SUCCESS;
+}
+
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