A daemon to show notification if battery is low.

battery_daemon.c

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <unistd.h>
#include <sys/types.h>
#include <libnotify/notify.h>

#define LIMIT 20

/*
 * Make a notification popup with the current battery status.
 * From:
 * https://wiki.archlinux.org/index.php/Desktop_notifications#Usage_in_programming
 */
void notify(int cap)
{
	char c[100];

	notify_init("Battery");
	sprintf(c, "Battery is at %d", cap);
	NotifyNotification *batt =
	    notify_notification_new("Battery", c, "battery-low");
	notify_notification_show(batt, NULL);
	g_object_unref(G_OBJECT(batt));
	notify_uninit();
}

int main(void)
{
	/*
	 * Fork process, detach from terminal and background - basically turn it
	 * into a daemon.
	 * Alternative to using daemon is doing everything from:
	 *   http://www.thegeekstuff.com/2012/02/c-daemon-process/
	 */
	int ret = daemon(0, 0);

	if (ret < -1) {
		printf("Daemon failed\n");
		exit(1);
	}
	/*
	 * Create PID file - only works as root. Libnotify however, does not
	 * work as root.
	 */
	FILE *fp;
	pid_t pid = getpid();

	fp = fopen("/var/run/battery.pid", "w");
	if (fp != NULL) {
		fprintf(fp, "%d\n", pid);
		fclose(fp);
	}
	/* Check battery capacity, notify if low and sleep 1 minute. */
	while (true) {
		int cap = 0;

		fp = fopen("/sys/class/power_supply/BAT1/capacity", "r");
		if (fp == NULL)
			goto rest;
		fscanf(fp, "%d", &cap);
		if (cap < LIMIT)
			notify(cap);
		fclose(fp);
	rest:
		sleep(60);
	}

	return 0;
}

Makefile

CC = gcc
NOTIFY += $(shell pkg-config --cflags --libs libnotify)

.PHONY: clean

battery_daemon: battery_daemon.c
	$(CC) -o $@ $(NOTIFY) $(CFLAGS) $<

clean:
	rm -rf battery_daemon

To kill the daemon check what PID it has with:

ps -ax | grep battery_daemon

and then kill that PID.