feat: replace interval with a proper cron schedule

README.md

@@ -46,12 +46,13 @@ Where all backup files will be stored.
 
 Default: `/backups`
 
-### `BACKUP_INTERVAL`
+### `BACKUP_SCHEDULE`
 
-The interval at which backups will happen. Must be a parsable string
+The cron schedule at which the backup job runs, using the common unix cron syntax.
-as [time.Duration](https://pkg.go.dev/time#ParseDuration). Must be positive.
 
-Default: `3h`
+Check out [crontab.dev](https://crontab.dev/) for a nice explanation of the schedule.
+
+Default: `* */6 * * * *` (every 6 hours)
 
 ### `BACKUP_MAX`
 

config.go

@@ -15,7 +15,7 @@ import (
 type conf struct {
 	Folder                string // Backup folder _with_ trailing slash
 	fullCurrentBackupPath string
-	Interval              time.Duration
+	Schedule              string
 	MaxBackups            int
 }
 
@@ -26,14 +26,14 @@ var (
 
 const (
 	envBackupFolder = `BACKUP_FOLDER`
-	envInterval     = `BACKUP_INTERVAL`
+	envSchedule     = `BACKUP_SCHEDULE`
 	envMax          = `BACKUP_MAX`
 )
 
 func init() {
 	config = &conf{
 		Folder:     "/backups/",
-		Interval:   time.Hour * 6,
+		Schedule:   "* */6 * * * *",
 		MaxBackups: 12,
 	}
 
@@ -46,14 +46,9 @@ func init() {
 		config.Folder = folder
 	}
 
-	var err error
+	schedule, has := os.LookupEnv(envSchedule)
-
-	interval, has := os.LookupEnv(envInterval)
 	if has {
-		config.Interval, err = time.ParseDuration(interval)
+		config.Schedule = schedule
-		if err != nil {
-			log.Fatalf("Invalid interval: %s\n", err)
-		}
 	}
 
 	max, has := os.LookupEnv(envMax)

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/gorilla/mux v1.8.0 // indirect
 	github.com/moby/term v0.0.0-20210619224110-3f7ff695adc6 // indirect
 	github.com/morikuni/aec v1.0.0 // indirect
+	github.com/robfig/cron/v3 v3.0.1
 	golang.org/x/time v0.0.0-20210723032227-1f47c861a9ac // indirect
 	google.golang.org/grpc v1.40.0 // indirect
 )

go.sum

@@ -515,6 +515,8 @@ github.com/prometheus/procfs v0.1.3/go.mod h1:lV6e/gmhEcM9IjHGsFOCxxuZ+z1YqCvr4O
 github.com/prometheus/procfs v0.2.0/go.mod h1:lV6e/gmhEcM9IjHGsFOCxxuZ+z1YqCvr4OA4YeYWdaU=
 github.com/prometheus/procfs v0.6.0/go.mod h1:cz+aTbrPOrUb4q7XlbU9ygM+/jj0fzG6c1xBZuNvfVA=
 github.com/prometheus/tsdb v0.7.1/go.mod h1:qhTCs0VvXwvX/y3TZrWD7rabWM+ijKTux40TwIPHuXU=
+github.com/robfig/cron/v3 v3.0.1 h1:WdRxkvbJztn8LMz/QEvLN5sBU+xKpSqwwUO1Pjr4qDs=
+github.com/robfig/cron/v3 v3.0.1/go.mod h1:eQICP3HwyT7UooqI/z+Ov+PtYAWygg1TEWWzGIFLtro=
 github.com/rogpeppe/fastuuid v0.0.0-20150106093220-6724a57986af/go.mod h1:XWv6SoW27p1b0cqNHllgS5HIMJraePCO15w5zCzIWYg=
 github.com/rogpeppe/fastuuid v1.2.0/go.mod h1:jVj6XXZzXRy/MSR5jhDC/2q6DgLz+nrA6LYCDYWNEvQ=
 github.com/rogpeppe/go-internal v1.3.0/go.mod h1:M8bDsm7K2OlrFYOpmOWEs/qY81heoFRclV5y23lUDJ4=

main.go

@@ -1,8 +1,8 @@
 package main
 
 import (
+	"github.com/robfig/cron/v3"
 	"log"
-	"time"
 )
 
 func main() {
@@ -11,7 +11,8 @@ func main() {
 		log.Fatalf("Could not create client: %s", err)
 	}
 
-	for {
+	cr := cron.New()
+	_, err = cr.AddFunc(config.Schedule, func() {
 		updateFullBackupPath()
 
 		containers, err := getContainers(c)
@@ -32,8 +33,9 @@ func main() {
 		}
 
 		log.Println("Done.")
-		log.Printf("Sleeping for %s\n", config.Interval)
+	})
-
+	if err != nil {
-		time.Sleep(config.Interval)
+		log.Fatalf("Could not create cron job: %s\n", err)
 	}
+	cr.Start()
 }