* ram_total total memory size in GB NULL
* ram_used used memory in GB NULL
* run_command custom shell command command (echo foo)
+ * separator string to echo NULL
* swap_free free swap in GB NULL
* swap_perc swap usage in percent NULL
* swap_total total swap size in GB NULL
* uptime system uptime NULL
* username username of current user NULL
* vol_perc OSS/ALSA volume in percent mixer file (/dev/mixer)
+ * NULL on OpenBSD
* wifi_perc WiFi signal in percent interface name (wlan0)
* wifi_essid WiFi ESSID interface name (wlan0)
*/