Run updates on a timer loop

The config yaml provides an interval period which is used to determine
how often to update the IP addresses on the domains. The local IP
address is checked each time before updating the domains.

README.md

@@ -32,6 +32,7 @@ Create a file `.digitalocean-dynamic-ip.json` (dot prefix to hide the file) and
 
 ```json
 {
+  "interval": "30m",
   "apikey": "samplekeydasjkdhaskjdhrwofihsamplekey",
   "doPageSize": 20,
   "useIPv4": true,
@@ -90,13 +91,3 @@ By default the tool will not output anything on success. If you wish to see debu
 #run:
 ./digitalocean-dynamic-ip -debug /path/to/my/config.json
 ```
-
-You can either set this to run periodically with a cronjob or use your own method.
-
-```bash
-# run `crontab -e` to edit your crontab
-# sample cron job task
-
-# m h  dom mon dow   command
-*/5 * * * * /home/user/digitalocean-dynamic-dns-ip/digitalocean-dynamic-ip
-```

digitalocean-dynamic-ip.go

@@ -12,7 +12,10 @@ import (
 	"net/http"
 	"net/url"
 	"os"
+	"os/signal"
 	"strconv"
+	"syscall"
+	"time"
 )
 
 func checkError(err error) {
@@ -48,6 +51,7 @@ var config ClientConfig
 
 // ClientConfig : configuration json
 type ClientConfig struct {
+	Interval        string   `json:"interval"`
 	APIKey          string   `json:"apiKey"`
 	DOPageSize      int      `json:"doPageSize"`
 	UseIPv4         *bool    `json:"useIPv4"`
@@ -366,8 +370,9 @@ func toIPv6String(ip net.IP) (currentIP string) {
 // 	return true
 // }
 
-func main() {
-	config = GetConfig()
+// updateAllDomains retrieves the current IP addresses and loops through all
+// domains in the config to update them with the current IPs.
+func updateAllDomains(config ClientConfig) {
 	currentIPv4, currentIPv6 := CheckLocalIPs()
 	if currentIPv4 == nil && currentIPv6 == nil {
 		logError("Current IP addresses are not valid, or both are disabled in the config. Check your configuration and internet connection.")
@@ -379,3 +384,30 @@ func main() {
 		log.Printf("%s: END", domain.Domain)
 	}
 }
+
+func main() {
+	config = GetConfig()
+
+	interval, err := time.ParseDuration(config.Interval)
+	checkError(err)
+
+	ticker := time.NewTicker(interval)
+	cancelChan := make(chan os.Signal)
+	signal.Notify(cancelChan, syscall.SIGTERM, syscall.SIGINT)
+
+	// Run once immediately before waiting for the ticker
+	updateAllDomains(config)
+
+	func() {
+		for {
+			select {
+			case <-cancelChan:
+				return
+			case <-ticker.C:
+				updateAllDomains(config)
+			}
+		}
+	}()
+
+	ticker.Stop()
+}

digitalocean-dynamic-ip.sample.json

@@ -1,4 +1,5 @@
 {
+  "interval": "30m",
   "apikey": "samplekeydasjkdhaskjdhrwofihsamplekey",
   "doPageSize": 20,
   "useIPv4": true,