Allow overriding flag defaults with environment variables.

Thanks to kwd42 for this feature suggestion.

README.md

@@ -55,6 +55,24 @@ rtltcp specific:
   -tunerxtalfreq=0: set tuner xtal frequency
 ```
 
+Flag default values may be overridden via environment variables which are a flag's name in all-caps prefixed by `RTLAMR_`. Flags passed at time of execution will override any values set by environment variable.
+
+```bash
+rtlamr -h
+
+...
+  -msgtype=scm: message type to receive: scm, scm+, idm, r900 and r900bcd
+...
+```
+
+```bash
+RTLAMR_MSGTYPE=idm rtlamr -h
+
+...
+  -msgtype=idm: message type to receive: scm, scm+, idm, r900 and r900bcd
+...
+```
+
 Running the receiver is as simple as starting an `rtl_tcp` instance and then starting the receiver:
 
 ```bash

flags.go

@@ -92,7 +92,7 @@ func RegisterFlags() {
 			}
 
 			format := "  -%s=%s: %s\n"
-			fmt.Fprintf(os.Stderr, format, f.Name, f.DefValue, f.Usage)
+			fmt.Fprintf(os.Stderr, format, f.Name, f.Value, f.Usage)
 		})
 	}
 
@@ -106,6 +106,22 @@ func RegisterFlags() {
 	}
 }
 
+func EnvOverride() {
+	flag.VisitAll(func(f *flag.Flag) {
+		envName := "RTLAMR_" + strings.ToUpper(f.Name)
+		flagValue := os.Getenv(envName)
+		if flagValue != "" {
+			log.Printf("Environment variable %q overrides flag %q with %q\n", envName, f.Name, flagValue)
+			if err := flag.Set(f.Name, flagValue); err != nil {
+				log.Fatalf(
+					"Environment variable %q failed to override flag %q with value %q: %q\n",
+					envName, f.Name, flagValue, err,
+				)
+			}
+		}
+	})
+}
+
 func HandleFlags() {
 	var err error
 

main.go

@@ -213,6 +213,7 @@ var (
 func main() {
 	rcvr.RegisterFlags()
 	RegisterFlags()
+	EnvOverride()
 
 	flag.Parse()
 	if *version {