contrib: Add simple battery monitoring status-command

This status displays a clock and monitors the laptop battery, nagging
via swaynag when the battery capacity drops below the configured
threshold. It nags once each subsequent percentage point drop.
Also displays the local time which is updated roughly each second.

contrib/laptop_status.c

@@ -0,0 +1,78 @@
+#include <signal.h>
+#include <stdbool.h>
+#include <unistd.h>
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+#if !defined(__linux__)
+#error This program uses Linux-specific sysfs features
+#endif
+
+/* Simple sway status_command for laptops. Prints the date and the current
+ * battery status. Warns using swaynag when the battery drops to an unreasonable
+ * level. For every percentage point demise below that level, renags.
+ *
+ * Should compile without arguments with any reasonable Linux compiler.
+ * It should be easy to modify the battery status handling code for other
+ * systems
+ *
+ * example installation and configuration:
+ *      cc laptop_status.c -o ~/.local/bin/laptop-status
+ *      sed -i 's/status_command.+/status_command ~/.local/bin/laptop-status' \
+ *          ~/.config/sway/config'
+ */
+
+static const char *cap_file = "/sys/class/power_supply/BAT0/capacity";
+static const char *stat_file = "/sys/class/power_supply/BAT0/status";
+static const int bat_warn_below = 10;
+
+int main(void)
+{
+    setvbuf(stdout, NULL, _IOLBF, 0);
+    int status = open(stat_file, O_RDONLY);
+    int cap = open(cap_file, O_RDONLY);
+    if (cap < 0 || status < 0) {
+        fprintf(stderr, "no battery found\n");
+        exit(EXIT_FAILURE);
+    }
+
+    char timestamp[128] = {0}, bat_status[32] = {0}, bat_capacity[32] = {0};
+
+    int nagged = 0;
+    pid_t nag = 0;
+    for (;;) {
+        pread(cap, bat_capacity, sizeof bat_capacity - 1, 0);
+        pread(status, bat_status, sizeof bat_status - 1, 0);
+        bool discharging = !!strstr(bat_status, "Discharging");
+        int percent = (int)strtol(bat_capacity, NULL, 10);
+        if (discharging && percent < bat_warn_below) {
+            if (!nagged || percent < nagged) {
+                char msg[64];
+                snprintf(msg, sizeof msg, "battery is low (%d%%)", percent);
+                char *cmd[] = {"swaynag", "-m", msg, NULL};
+                if (!(nag = fork())) {
+                    execvp(cmd[0], cmd);
+                }
+            }
+            nagged = percent;
+        } else if (nag) {
+            kill(nag, SIGINT);
+            nag = 0;
+        }
+        time_t now = time(NULL);
+        strftime(timestamp, sizeof timestamp - 1, "%c", localtime(&now));
+        printf(
+            "%s [% 3d%%]%s\n",
+            timestamp,
+            percent,
+            strstr(bat_status, "Discharging") ? " " : "+"
+        );
+        sleep(1);
+    }
+}