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+$name battd
+$source https://the-tk.com/cgit/battd/
+
+battd is a simple battery daemon written for use with a daemontools-style
+supervisor.
+
+$desc
+
+The daemon `run` script polls the status of a user defined `check` function
+every `long_interval` or `short_interval` seconds. The interval is determined by
+the status of the `interval` function. On the transition from a successful
+`check` exit status to an unsuccessful `check` exit status the `failed` function
+is called. These functions and optionally the variables `long_interval` and
+`short_interval` need to be provided in a `conf` file located in the same
+directory as the script.
+
+The following `conf` file contains an example configuration which increases the
+polling interval from 5 minutes to 30 seconds when the battery level is below 16
+percent or fewer than 20 minutes of battery life remain and puts the machine in
+a hybrid sleep once the battery level is below 8 percent or fewer than 10
+minutes of battery life remain.
+
+```sh
+battery=/sys/class/power_supply/BAT0/uevent
+interval() { ./battcheck min_time=1200 min_pct=16 "$battery"; }
+check() { ./battcheck min_time=600 min_pct=8 "$battery"; }
+failed() { zzz -H; }
+```
+
+Finally, the `battcheck` awk script takes two optional command line variable
+assignments for `min_pct` and `min_time` specifying the remaining battery
+percentage and remaining battery time in seconds respectively after which the
+script will produce an unsuccessful exit code.
+
+These files should be placed inside a service directory making sure that `run`
+and `battcheck` are both executable. The service handles SIGHUP to reload its
+configuration at runtime.
+
+The `battcheck` script is quite versatile and may be useful in other contexts, in
+which case it is recommended to install it to `/usr/local/sbin` or another
+appropriate location.
+
+If the script crashes before a success to failure transition of the `check`
+function and is re-started after the transition then the script will incorrectly
+ignore this transition and not call the `failed` function. This could be fixed
+by storing the previous status in on a temporary file system but this was not
+done due to the unlikelihood of the event and in order to keep the code simple.