configure I2C bus and address via environment

README.md

@@ -102,26 +102,26 @@ For other Linux distributions, it might not be this simple -- for instance, in A
    git clone https://github.com/MaffooClock/gpsd-i2c.git
    ```
 
-2. Edit [gpsd-i2c.py](gpsd-i2c.py) and set the I²C bus and address of your GPS module.
-
-3. Test the service:
+2. Test the service (use the I²C bus and address of your GPS module):
    ```bash
-   python3 /opt/gpsd-i2c/gpsd-i2c.py
+   I2C_BUS=0 I2C_ADDRESS=0x42 python3 /opt/gpsd-i2c/gpsd-i2c.py
    ```
    If everything is correct, you should immediately see a bunch of NMEA sentences in the console.  Just <kbd>Ctrl</kbd>+<kbd>C</kbd> to stop, and then continue with the remaining steps below.
 
    If not, you'll need to resolve whatever errors you see before proceeding.
 
-4. Setup the systemd service:
+3. Setup the systemd service (use the I²C bus and address of your GPS module):
    ```bash
    cd /opt/gpsd-i2c
    cp gpsd-i2c.service /etc/systemd/system/
+   mkdir /etc/systemd/system/gpsd-i2c.service.d/
+   printf "[Service]\nEnvironment=I2C_BUS=0 I2C_ADDRESS=0x42\n" > \
+     /etc/systemd/system/gpsd-i2c.service.d/override.conf
    ```
 
-5. Start the service automatically on boot:
+4. Start the service automatically on boot:
    ```bash
-   systemctl enable gpsd-i2c.service
-   systemctl start gpsd-i2c.service
+   systemctl enable --now gpsd-i2c.service
    ```
 
 Of course, you don't have to install this into `/opt` -- anywhere you prefer is fine, just be sure to update the `WorkingDirectory=` line in the [gpsd-i2c.service](gpsd-i2c.service) file.

gpsd-i2c.py

@@ -1,9 +1,21 @@
 #!/usr/bin/env python3
 
-# Configure these as necessary:
-I2C_BUS = 0
-I2C_ADDRESS = 0x42
-
+import os
+def parse_int(val, default):
+    try:
+        result = int(val)
+    except (TypeError, ValueError):
+        try:
+            result = int(val,16)
+        except (TypeError, ValueError):
+            result = default
+    except Exception:
+        result = default
+    return result
+
+# Default to device 0-0042; can be overriden via environment
+I2C_BUS = parse_int(os.environ.get("I2C_BUS", None), 0)
+I2C_ADDRESS = parse_int(os.environ.get("I2C_ADDRESS", None), 0x42)
 
 ##### Don't edit anything below this line #####