check_cert_expire.go

package main

import (
	"fmt"
	"log"
	"math"
	"net/http"
	"os"
	"os/exec"
	"time"
)

const (
	hoursInDay       = 24
	shouldUpdateDays = 1
)

func main() {
	if len(os.Args) != 2 {
		log.Fatal("must supply an HTTPS URL...")
	}

	resp, err := http.Get(os.Args[1])
	if err != nil {
		log.Fatalf("error performing HTTPS request: %s\n", err)
	}

	if resp.TLS == nil {
		log.Fatal("did not supply an HTTPS URL...")
	}

	for _, v := range resp.TLS.PeerCertificates {
		// A client certificate is not a certificate authority, so check only those.
		if !v.IsCA {
			duration := time.Since(v.NotAfter)
			expiresInDays := math.Abs(duration.Hours() / hoursInDay)

			if int(expiresInDays) == shouldUpdateDays {
				out, err := exec.Command("certbot", "renew").Output()
				if err != nil {
					log.Fatalf("error running certificate renew: %s\n", err)
				}

				fmt.Println(string(out))
			} else {
				fmt.Println("no update needed...")
			}
		}
	}
}