A simple C program for feeding data into a lemonbar-like panel

panel.c

/* Copyright 2015 David Koňařík
 * 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 3 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, see <http://www.gnu.org/licenses/>.
 */

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
#include <math.h>
#include <sys/time.h>
#include <unistd.h>
#include <errno.h>

#define OUTLEN 2048
#define CMDOUTLEN 512

struct command
{
	struct timeval interval;
	char *command;
	//Caches the output of the comand, set to "" on init
	char curr_out[CMDOUTLEN];
	//When the command will be run, set to 0 on init 
	struct timeval next_run;
};

#define CMD(sec, msec, cmd) {{sec, msec * 1000}, cmd, "", {0}}

struct command commands[] = {
	CMD(10, 0, "amixer get 'Master' | sed -nEe "
	"'/^  Front Left/ {s/.*\\[([0-9]+%)\\].*/\\1/g; p}'"),
	CMD(10, 0, "mpc volume | cut -d ' ' -f 2 | sed -e 's/volume://g'"),
	CMD(1, 0, "~/.config/lemonbar/mpd_playing_now.sh"),
	CMD(10, 0, "~/.config/lemonbar/desktops.sh"),
	CMD(10, 0, "~/.config/lemonbar/battery.sh"),
	CMD(0, 500, "date +'%d.%m.%Y %H:%M'")
};

static const char format[] = "%{l}"
	"%{B#000549} \ue152 %{B-} $0  " //Volume
	"%{B#0F0F85} \ue0ed %{B-} $1  " //mpd volume
	"$2" //mpd "Playing now"
	"%{c}"
	"$3" //Desktops
	"%{r}"
	"$4  " //Battery
	"%{B#780808} \ue015 %{B-} $5"; //Clock

void process_command(struct command *cmd)
{
	char cmdout[CMDOUTLEN];
	cmd->curr_out[0] = '\0';

	FILE *cmdfp = popen(cmd->command, "r");
	if(cmdfp == NULL)
	{
		printf("Command %s failed to run: %s\n", cmd->command,
			strerror(errno));
		exit(1);
	}
	
	while(fgets(cmdout, CMDOUTLEN, cmdfp) != NULL)
	{
		if(strlen(cmdout) + strlen(cmd->curr_out) > CMDOUTLEN)
		{
			printf("Command buffer is full\n");
			exit(2);
		}
		if(cmdout[strlen(cmdout) - 1] == '\n')
		{
			cmdout[strlen(cmdout) - 1] = '\0'; //Strip newlines
		}
		strcat(cmd->curr_out, cmdout);
	}
	
	fclose(cmdfp);

	struct timeval time;
	gettimeofday(&time, NULL);
	timeradd(&time, &cmd->interval, &cmd->next_run);
}

void create_output(char *out)
{
	size_t i;
	size_t cmdnum = 0;
	unsigned int command_pos = 0;
	size_t tmp_out_strlen = 0;
	for(i = 0; i < strlen(format); i++)
	{
		if(command_pos)
		{
			if(isdigit(format[i]))
			{
				cmdnum += (format[i] - '0') * pow(10, command_pos - 1);
			}
			else
			{
				strcat(out, commands[cmdnum].curr_out);
				command_pos = 0;
				cmdnum = 0;
			}
		}
		if(format[i] == '$')
		{
			command_pos = 1;
		}
		if(!command_pos)
		{
			tmp_out_strlen = strlen(out);
			out[tmp_out_strlen] = format[i];
			out[tmp_out_strlen + 1] = '\0';
		}
	}
	if(command_pos != 0)
	{
		strcat(out, commands[cmdnum].curr_out);
	}
}

int main(int argc, char *argv[])
{
	size_t i = 0;
	for(i = 0; i < sizeof(commands) / sizeof(struct command); i++)
	{
		process_command(&commands[i]);
	}
	
	char out[OUTLEN] = "";
	create_output(out);
	printf("%s\n", out);
	fflush(stdout);

	struct timeval time;
	struct command *next_command;
	struct timeval delta;
	while(1)
	{
		gettimeofday(&time, NULL);
		next_command = &commands[0];
		for(i = 0; i < sizeof(commands) / sizeof(struct command); i++)
		{
			if(timercmp(&commands[i].next_run, &next_command->next_run, <))
			{
				next_command = &commands[i];
			}
		}
		timersub(&next_command->next_run, &time, &delta);
		long time_to_wait = delta.tv_sec * 1000 * 1000 + delta.tv_usec;
		usleep(time_to_wait > 0 ? time_to_wait : 0);

		process_command(next_command);
		out[0] = '\0';
		create_output(out);
		printf("%s\n", out);
		fflush(stdout);
	}

}