Cron alternative that will run in your shitter.

sched.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/wait.h>

/* configurable constants */
#define TIME_MAX  2
#define LOG_FILE  "/root/logs/sched.log"

enum check_fields {
    CHK_MIN,
    CHK_HOUR,
    CHK_BOTH_HM
};

/* sleep for 1 minute -- don't change this */
struct timespec sleep_amt = {60, 0};

/* our scheduled tasks to run */
struct {
    const char *cmd;
    time_t last;
    int n_times;
    struct {
        int chk_fields;
        int tm_hour;
        int tm_min;
    } times[TIME_MAX]; /* increasing order of time */
} cron_tasks[] = {
    /* update void-packages git repo */
    {
        "cd $HOME/void-packages; git pull",
        0, 1, {{CHK_BOTH_HM, 0, 0}}
    },
    /* update system packages */
    {
        "xbps-install -Suy",
        0, 2, {{CHK_BOTH_HM, 0, 0}, {CHK_BOTH_HM, 12, 0}}
    },
};

inline int chk_min(struct tm *tm, int i, int ti)
{
    return tm->tm_min == cron_tasks[i].times[ti].tm_min;
}

inline int chk_hr(struct tm *tm, int i, int ti)
{
    return tm->tm_hour == cron_tasks[i].times[ti].tm_hour;
}

inline int zero_min(struct tm *tm)
{
    return tm->tm_min == 0;
}

int will_exec_task(time_t *t, int i)
{
    *t = time(NULL);

    if (*t - cron_tasks[i].last < 20)
        return 0;

    int ti;
    struct tm *tm = localtime(t);

    for (ti = 0; ti < cron_tasks[i].n_times; ti++) {
        switch (cron_tasks[i].times[ti].chk_fields) {
        case CHK_HOUR:
            if (zero_min(tm) && chk_hr(tm, i, ti))
                goto yes;
        case CHK_MIN:
            if (chk_min(tm, i, ti))
                goto yes;
        case CHK_BOTH_HM:
            if (chk_hr(tm, i, ti) && chk_min(tm, i, ti))
                goto yes;
        }
    }

    return 0;

yes:
    cron_tasks[i].last = *t;
    return 1;
}

inline const char *task(int i)
{
    return cron_tasks[i].cmd;
}

void run(int fd, time_t t, const char *cmd)
{
    if (fd != -1)
        dprintf(fd, "%lu\t%s\n", t, cmd);

    if (fork() == 0)
        execlp("sh", "sh", "-c", cmd, NULL);
}

void handle_sigchld(int signo)
{
    (void)signo;
    wait(NULL);
}

void handle_sigterm(int signo)
{
    (void)signo;
    exit(0);
}

int main(void)
{
    if (getuid() != 0)
        return 1;

    if (signal(SIGCHLD, handle_sigchld) == SIG_ERR)
        return 2;

    if (signal(SIGTERM, handle_sigterm) == SIG_ERR)
        return 3;

    int i;
    time_t t;

    const int n_tasks = sizeof(cron_tasks)/sizeof(cron_tasks[0]);

    for (;;) {
        int fd = open(LOG_FILE, O_WRONLY|O_CREAT|O_APPEND);
        for (i = 0; i < n_tasks; i++) {
            if (will_exec_task(&t, i))
                run(fd, t, task(i));
        }
        close(fd);
        nanosleep(&sleep_amt, NULL);
    }
}