Add doc on detemining whether the machine is a laptop or desktop

docs/TEMPLATING.md

@@ -19,6 +19,8 @@
 * [Using `.chezmoitemplates`](#using-chezmoitemplates)
 * [Using `.chezmoitemplates` for creating similar files](#using-chezmoitemplates-for-creating-similar-files)
   * [Passing multiple arguments](#passing-multiple-arguments)
+* [Useful templates](#useful-templates)
+  * [Determine whether the current machine is a laptop or desktop](#determine-whether-the-current-machine-is-a-laptop-or-desktop)
 
 ---
 
@@ -472,3 +474,29 @@ it with a dictionary, for example `.local/share/chezmoi/small-font.yml.tmpl`:
 ```
 
 ---
+
+## Useful templates
+
+---
+
+### Determine whether the current machine is a laptop or desktop
+
+The following template sets the `$chassisType` variable to `"desktop"` or
+`"laptop"` on macOS, Linux, and Windows.
+
+```
+{{- $chassisType := "desktop" }}
+{{- if (eq .chezmoi.os "darwin") }}
+{{-   if contains "MacBook" (output "sysctl" "-n" "hw.model") }}
+{{-     $chassisType = "laptop" }}
+{{-   else }}
+{{-     $chassisType = "desktop" }}
+{{-   end }}
+{{- else if (eq .chezmoi.os "linux") }}
+{{-   $chassisType = (output "hostnamectl" "--json=short" | mustFromJson).Chassis }}
+{{- else if (eq .chezmoi.os "windows") }}
+{{-   $chassisType = (output "powershell.exe" "-noprofile" "-command" "if (Get-WmiObject -Class win32_battery -ComputerName localhost) { echo laptop } else { echo desktop }") }}
+{{- end }}
+```
+
+---