Add support for value format string

Fixes #1

README.md

@@ -16,10 +16,10 @@ rkt run \
    --insecure-options=image \
    --net=flannel \
    docker://redis \
-   monder.cc/rkt-sidekick:v0.0.1 -- --cidr 10.0.0.0/16 /services/redis-a6f43b/ip
+   monder.cc/rkt-sidekick:v0.0.2 -- --cidr 10.0.0.0/16 -f '{"host":"$ip", "port":3000}' /services/redis-a6f43b/ip
 ```
 
-The script above will launch redis and a sidekick in the same pod. The sidekick will enumerate all network interfaces and write the first one matching `10.0.0.0/16` to the key `/services/redis-a6f43b/ip`
+The script above will launch redis and a sidekick in the same pod. The sidekick will enumerate all network interfaces and write the first one matching `10.0.0.0/16` to the formatted string to `/services/redis-a6f43b/ip` 
 
 Please note how to pass arguments to multiple images: https://coreos.com/rkt/docs/latest/subcommands/run.html#passing-arguments
 
@@ -31,6 +31,7 @@ Please note how to pass arguments to multiple images: https://coreos.com/rkt/doc
  Flags:
        --cidr string            cidr to match the ip (default "0.0.0.0/0")
    -e, --etcd-endpoint string   an etcd address in the cluster (default "http://172.16.28.1:2379")
+   -f, --format string          format of the etcd key value. '$ip' will be replace by container's ip address (default "$ip")
    -i, --interval duration      refresh interval (default 1m0s)
 ```
 

main.go

@@ -9,19 +9,22 @@ import (
 	"net"
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 	"time"
 )
 
 var flags struct {
 	etcdAddress string
 	cidr        string
+	format      string
 	interval    time.Duration
 }
 
 func init() {
 	pflag.StringVarP(&flags.etcdAddress, "etcd-endpoint", "e", "http://172.16.28.1:2379", "an etcd address in the cluster")
 	pflag.StringVar(&flags.cidr, "cidr", "0.0.0.0/0", "cidr to match the ip")
+	pflag.StringVarP(&flags.format, "format", "f", "$ip", "format of the etcd key value. '$ip' will be replace by container's ip address")
 	pflag.DurationVarP(&flags.interval, "interval", "i", time.Minute, "refresh interval")
 	pflag.Usage = func() {
 		fmt.Fprintf(os.Stderr, "Usage:\n  %s /key/in/etcd\n\nFlags:\n", os.Args[0])
@@ -48,7 +51,9 @@ func main() {
 		log.Fatal(err)
 	}
 
-	_, err = etcd.Set(context.Background(), pflag.Arg(0), ip, &client.SetOptions{TTL: 2 * flags.interval})
+	value := strings.Replace(flags.format, "$ip", ip, -1)
+
+	_, err = etcd.Set(context.Background(), pflag.Arg(0), value, &client.SetOptions{TTL: 2 * flags.interval})
 	if err != nil {
 		log.Fatal(err)
 	}