Add support for environment variables

README.md

@@ -48,14 +48,14 @@ To start the exporter we use the [docker run](https://docs.docker.com/engine/ref
 ```
 Usage of ./nginx-prometheus-exporter:
   -nginx.plus
-        Start the exporter for NGINX Plus. By default, the exporter is started for NGINX.
+        Start the exporter for NGINX Plus. By default, the exporter is started for NGINX. The default value can be overwritten by NGINX_PLUS environment variable.
   -nginx.scrape-uri string
         A URI for scraping NGINX or NGINX Plus metrics.
-        For NGINX, the stub_status page must be available through the URI. For NGINX Plus -- the API. (default "http://127.0.0.1:8080/stub_status")
+        For NGINX, the stub_status page must be available through the URI. For NGINX Plus -- the API. The default value can be overwritten by SCRAPE_URI environment variable. (default "http://127.0.0.1:8080/stub_status")
   -web.listen-address string
-        An address to listen on for web interface and telemetry. (default ":9113")
+        An address to listen on for web interface and telemetry. The default value can be overwritten by LISTEN_ADDRESS environment variable. (default ":9113")
   -web.telemetry-path string
-        A path under which to expose metrics. (default "/metrics")
+        A path under which to expose metrics. The default value can be overwritten by TELEMETRY_PATH environment variable. (default "/metrics")
 ```
 
 ### Exported Metrics 

exporter.go

@@ -4,25 +4,56 @@ import (
 	"flag"
 	"log"
 	"net/http"
+	"os"
+	"strconv"
 
 	"github.com/nginxinc/nginx-prometheus-exporter/client"
 	"github.com/nginxinc/nginx-prometheus-exporter/collector"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
+func getEnv(key, defaultValue string) string {
+	value, ok := os.LookupEnv(key)
+	if !ok {
+		return defaultValue
+	}
+	return value
+}
+
+func getEnvBool(key string, defaultValue bool) bool {
+	value, ok := os.LookupEnv(key)
+	if !ok {
+		return defaultValue
+	}
+	b, err := strconv.ParseBool(value)
+	if err != nil {
+		log.Fatalf("Environment Variable value for %s must be a boolean", key)
+	}
+	return b
+}
+
 var (
 	// Set during go build
 	version   string
 	gitCommit string
 
+	// Defaults values
+	defaultListenAddress = getEnv("LISTEN_ADDRESS", ":9113")
+	defaultMetricsPath   = getEnv("TELEMETRY_PATH", "/metrics")
+	defaultNginxPlus     = getEnvBool("NGINX_PLUS", false)
+	defaultScrapeURI     = getEnv("SCRAPE_URI", "http://127.0.0.1:8080/stub_status")
+
 	// Command-line flags
-	listenAddr  = flag.String("web.listen-address", ":9113", "An address to listen on for web interface and telemetry.")
-	metricsPath = flag.String("web.telemetry-path", "/metrics", "A path under which to expose metrics.")
-	nginxPlus   = flag.Bool("nginx.plus", false, "Start the exporter for NGINX Plus. By default, the exporter is started for NGINX.")
-	scrapeURI   = flag.String("nginx.scrape-uri", "http://127.0.0.1:8080/stub_status",
+	listenAddr = flag.String("web.listen-address", defaultListenAddress,
+		"An address to listen on for web interface and telemetry. The default value can be overwritten by LISTEN_ADDRESS environment variable.")
+	metricsPath = flag.String("web.telemetry-path", defaultMetricsPath,
+		"A path under which to expose metrics. The default value can be overwritten by TELEMETRY_PATH environment variable.")
+	nginxPlus = flag.Bool("nginx.plus", defaultNginxPlus,
+		"Start the exporter for NGINX Plus. By default, the exporter is started for NGINX. The default value can be overwritten by NGINX_PLUS environment variable.")
+	scrapeURI = flag.String("nginx.scrape-uri", defaultScrapeURI,
 		`A URI for scraping NGINX or NGINX Plus metrics.
-	For NGINX, the stub_status page must be available through the URI. For NGINX Plus -- the API.`)
+	For NGINX, the stub_status page must be available through the URI. For NGINX Plus -- the API. The default value can be overwritten by SCRAPE_URI environment variable.`)
 )
 
 func main() {