add: environment variable for input device

src/main.rs

@@ -18,6 +18,13 @@ Grabs input device through a kernel syscall. This prevents other devices
 (including kernel-internal ones such as rfkill) from receiving events 
 from the device.
 
+In case of overlap in options provided in command line and evenironment variables,
+command line options take precedence.
+
+# Environment
+
+DPK_INPUT_DEVICE can be used to provide the path to the input device
+
 # Usage
 
 dpk [/PATH/TO/INPUT/DEVICE]
@@ -83,13 +90,16 @@ fn run(
 }
 
 /// Open the device on the path provided in the first cli argument
-/// or, if none present, allow user to choose one
+/// or in DPK_INPUT_DEVICE environment variable
+/// or, if neither present, allow user to choose one
 fn pick_device() -> evdev::Device {
     let mut args = std::env::args_os();
     args.next();
 
     if let Some(dev_file) = args.next() {
         evdev::Device::open(dev_file).unwrap()
+    } else if let Ok(dev_file) = std::env::var("DPK_INPUT_DEVICE") {
+        evdev::Device::open(dev_file).unwrap()
     } else {
         let mut devices = evdev::enumerate().collect::<Vec<_>>();
         devices.reverse();