Add environment variables as config options

Fixes #24

README.md

@@ -23,10 +23,20 @@ Simple usage:
       target_groups:
         - targets: ['localhost:9301']
 
-To get all the parameteres
+To get all the parameteres, command line arguments always override default and environment variables configs:
 
     squid-exporter -help
 
+The following environmnet variables can be used to override default parameters:
+
+```
+SQUID_EXPORTER_LISTEN
+SQUID_EXPORTER_METRICS_PATH
+SQUID_HOSTNAME
+SQUID_PORT
+SQUID_LOGIN
+SQUID_PASSWORD
+```
 
 Usage with docker:
 ------
@@ -38,6 +48,10 @@ Setup with Squid running on a different host
 
     docker run -p 9301:9301 -d boynux/squid-exporter -squid-hostname "192.168.0.2" -squid-port 3128 -listen ":9301"
 
+With environment variables
+
+    docker run -p 9301:9301 -d -e SQUID_PORT="3128" -e SQUID_HOSTNAME="192.168.0.2" -e SQUID_EXPORTER_LISTEN=":9301" boynux/squid-exporter
+
 
 Build:
 --------

config.go

@@ -2,6 +2,9 @@ package main
 
 import (
 	"flag"
+	"log"
+	"strconv"
+	"os"
 )
 
 const (
@@ -32,17 +35,45 @@ type Config struct {
 func NewConfig() *Config {
 	c := &Config{}
 
-	flag.StringVar(&c.ListenAddress, "listen", defaultListenAddress, "Address and Port to bind exporter, in host:port format")
-	flag.StringVar(&c.MetricPath, "metrics-path", defaultMetricsPath, "Metrics path to expose prometheus metrics")
+	flag.StringVar(&c.ListenAddress, "listen",
+		loadEnvStringVar("SQUID_EXPORTER_LISTEN", defaultListenAddress), "Address and Port to bind exporter, in host:port format")
+	flag.StringVar(&c.MetricPath, "metrics-path",
+		loadEnvStringVar("SQUID_EXPORTER_METRICS_PATH", defaultMetricsPath), "Metrics path to expose prometheus metrics")
 
-	flag.StringVar(&c.SquidHostname, "squid-hostname", defaultSquidHostname, "Squid hostname")
-	flag.IntVar(&c.SquidPort, "squid-port", defaultSquidPort, "Squid port to read metrics")
+	flag.StringVar(&c.SquidHostname, "squid-hostname",
+		loadEnvStringVar("SQUID_HOSTNAME", defaultSquidHostname), "Squid hostname")
+	flag.IntVar(&c.SquidPort, "squid-port",
+		loadEnvIntVar("SQUID_PORT", defaultSquidPort), "Squid port to read metrics")
+
+	flag.StringVar(&c.Login, "squid-login", loadEnvStringVar("SQUID_LOGIN", ""), "Login to squid service")
+	flag.StringVar(&c.Password, "squid-password", loadEnvStringVar("SQUID_PASSWORD", ""), "Password to squid service")
 
-	flag.StringVar(&c.Login, "squid-login", "", "Login to squid service")
-	flag.StringVar(&c.Password, "squid-password", "", "Password to squid service")
 	versionFlag = flag.Bool("version", false, "Print the version and exit")
 
 	flag.Parse()
 
 	return c
 }
+
+func loadEnvStringVar(key, def string) string {
+	val := os.Getenv(key)
+	if val == "" {
+		return def
+	}
+
+	return val
+}
+
+func loadEnvIntVar(key string, def int) int {
+	valStr := os.Getenv(key)
+	if valStr != "" {
+		val, err := strconv.ParseInt(valStr, 0, 32)
+		if err == nil {
+			return int(val)
+		}
+
+		log.Printf("Error parsing  %s='%s'. Integer value expected", key, valStr)
+	}
+
+	return def
+}