Snapshot schedules are real cron jobs, written and owned by Ham Lab

A schedule is now a line in /etc/cron.d/hamlab-zfs, which Ham Lab owns and rewrites whole
whenever the schedules change; the line calls `hamlab zfs-snapshot <id>` as the account Ham
Lab runs as, reaching zfs through sudo exactly as the daemon does. The daemon's own ticker
is gone. Snapshots are backup-shaped and should not stop because a web console is down,
being upgraded, or uninstalled — and a cron line is greppable by anyone looking at the
machine the normal way, which a schedule hidden inside a daemon is not.

A cron expression is now the single description of when a schedule fires: the four common
shapes render to one, a custom schedule states it outright, and the next run is read back
with the same evaluator the Jobs tab uses on everyone else's lines. So the page, the file
and the prediction cannot disagree.

Schedules gain a custom cron expression and an optional snapshot name prefix. What a run
did moves out of the policy into zfs-runs/<id>.json beside the config, because the cron job
is a separate process and must never write state.json behind the daemon's back.

The detected external job can be adopted: its schedule, retention and dataset are read out
of the cron line into a new Ham Lab schedule, ready to check rather than retype.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

PLAN.md

@@ -276,6 +276,28 @@ something long in one, switch away and back, refresh — it's all still there.
   a dashboard could silently reset the sampling rate for the whole machine, overriding the
   fleet refresh rate set in Settings. The sampler now has one source: `refreshMs` in the UI
   prefs, which the hub fans out, remembered per machine as `sampler_ms`.
+- **ZFS tab, with scheduled snapshots** — **shipped 2026-09-24** (`internal/storage/snapshots.go`,
+  `internal/api/zfs.go`, `ZFS.tsx`). Pools, datasets and snapshots moved out of Disks —
+  Disks is the hardware, ZFS is the filesystem on top of it — and the tab gained what it was
+  split out for: **snapshot schedules**. Hourly, daily, weekly or monthly, per dataset,
+  optionally recursive, with a **retention period** and a floor that keeps the newest few
+  whatever their age. A missed window (the machine was off) runs once on return rather than
+  being skipped.
+  **The naming is zfSnap's, deliberately.** `grid` has run zfSnap v1.11.1 from root cron at
+  02:30 with a 6-week TTL since before this repo existed, and it works. So rather than
+  invent a second convention for two tools to disagree over, this writes and reads exactly
+  what zfSnap does — `tank@2026-09-24_02.30.01--6w`, **retention encoded in the name**. Three
+  consequences: Ham Lab shows and expires the snapshots zfSnap already took; the two can run
+  side by side without fighting, since both only remove what has passed the expiry written
+  into its own name; and changing a schedule never re-dates old snapshots, because each
+  carries the rule it was taken under. The page **detects the existing cron job**
+  (zfSnap, zfs-auto-snapshot, sanoid, or a bare `zfs snapshot`) and says so, warning if a Ham
+  Lab schedule is running as well.
+  **Nothing is destroyed unseen.** Every schedule answers *what would it delete?* with the
+  list, sizes and dates, before Run now touches anything; only names that parse as a stamp
+  and a TTL are candidates, so a snapshot taken by hand is invisible to it; and a snapshot
+  deleted by hand needs its full name typed back. The scheduler is a 30 s tick in the daemon,
+  and `Run now` calls the same code, so the button and the timer cannot drift apart.
 - **Skins** — 13 since 2026-09-19 (`themes.ts`: a palette is one `mk(…)` line): Dark,
   Light, Cyberpunk, Retro, **HUD** (hairlines of cyan on near-black, corner ticks, thin
   type), Nord, Gruvbox, Catppuccin + Latte, Dracula, Tokyo Night, Solarized Dark + Light;
@@ -511,32 +533,6 @@ something long in one, switch away and back, refresh — it's all still there.
   - *Not app work, but the trigger:* nothing here blocks starting the migration except the
     inventory. Do that first.
 
-- **Rename the Space tab** — asked for 2026-09-23, not started. "Space" is vague; the tab
-  is WizTree for a disk or pool. Candidates: **Usage**, **Disk Usage**, **Storage**. Note
-  `Disks` already exists and `Storage` would muddle with it, so Usage is probably the one.
-  The rename touches the rail label, the `View` id, the hash route (`#/<machine>/space`,
-  which people may have bookmarked — decide whether to keep `space` as the id and change
-  only the label), the footer's page name and `Space.tsx`'s own strings.
-
-- **ZFS gets its own tab, under Disks** — asked for 2026-09-23, not started. Today ZFS is a
-  section inside `Disks.tsx`: pools with health, capacity, fragmentation, scrub controls, a
-  dataset tree and per-dataset snapshot lists with "Snapshot now". Moving it out makes room
-  for what is actually wanted: **scheduled snapshots with a retention policy**, configured
-  from the console rather than by hand. What exists to build on — `grid` runs `zfSnap`
-  nightly at 02:30 from root cron with 6-week expiry (`zfsnap-cron-6w-daily.sh`, predates
-  this repo), and Ham Lab's Cron tab can already read, write and run root crontabs, so the
-  job can be managed rather than reinvented. Decide: does Ham Lab drive `zfSnap`/`zfs-auto-
-  snapshot` if present, or write and own its own timer? Per-dataset policies or one for the
-  pool? And it must show what a retention rule *will* delete before it runs.
-
-- **Rename Cron to something generic** — asked for 2026-09-23, not started. Mitchell's
-  reasoning: the Services tab is systemd but "Services" is the right generic name for it,
-  so the scheduling tab should be named for the job it does, not the tool. **Scheduled** or
-  **Jobs**. The tab already covers more than cron — `/etc/crontab`, `/etc/cron.d`, user
-  crontabs, `run-parts` directories *and* systemd timers — so the generic name is the
-  honest one. Same rename surface as the Space tab: rail label, `View` id, hash route,
-  footer name.
-
 - **A Virtualization tab** — asked for 2026-09-23, not started, and **large**. Run and
   manage **LXC containers and virtual machines** from the console, the way Proxmox does —
   which is the comparison Ham Lab has invited since its first line. Nothing exists yet.

cmd/hamlab/main.go

@@ -4,6 +4,7 @@ package main
 import (
 	"bufio"
 	"context"
+	"encoding/json"
 	"errors"
 	"flag"
 	"fmt"
@@ -32,6 +33,7 @@ import (
 	"hamlab/internal/metrics"
 	"hamlab/internal/nodes"
 	"hamlab/internal/services"
+	"hamlab/internal/priv"
 	"hamlab/internal/setup"
 	"hamlab/internal/space"
 	"hamlab/internal/storage"
@@ -59,6 +61,7 @@ func main() {
   serve                run the console in the foreground (what the service runs)
   passwd [user]        set a user's password (makes the account if it is new; reads stdin when piped)
   users                list the accounts
+  zfs-snapshot <id>    run one ZFS snapshot schedule (what its cron line calls)
   user-rm <user>       remove an account and sign it out everywhere
   check                verify the config and state load, then exit
   version
@@ -129,6 +132,10 @@ func main() {
 		if err := passwd(cfg, flag.Arg(1)); err != nil {
 			log.Fatal(err)
 		}
+	case "zfs-snapshot":
+		if err := zfsSnapshot(cfg, flag.Arg(1)); err != nil {
+			log.Fatal(err)
+		}
 	case "users":
 		migrateUsers(cfg)
 		if len(cfg.Users) == 0 {
@@ -178,6 +185,58 @@ func main() {
 
 // migrateUsers gives a pre-users config its first account, named after this OS user, so
 // the CLI sees the same accounts the daemon will.
+// zfsSnapshot runs one snapshot schedule: take it, then expire what has aged out. This is
+// what the cron line Ham Lab writes actually calls, so a schedule keeps running whether or
+// not the console is up — and keeps running if Ham Lab is never opened again.
+//
+// It deliberately does not write state.json. The daemon owns that file and would clobber
+// anything written from out here; the outcome goes beside it instead, in zfs-runs/.
+func zfsSnapshot(cfg *config.Config, id string) error {
+	if strings.TrimSpace(id) == "" {
+		return errors.New("usage: hamlab zfs-snapshot <schedule-id>")
+	}
+	st, err := store.Open(filepath.Join(cfg.DataDir(), "state.json"))
+	if err != nil {
+		return err
+	}
+	var raw json.RawMessage
+	st.View(func(s *store.State) { raw = s.ZFS })
+	var z struct {
+		Policies []storage.Policy `json:"policies"`
+	}
+	if len(raw) > 0 {
+		if err := json.Unmarshal(raw, &z); err != nil {
+			return fmt.Errorf("reading the schedules: %w", err)
+		}
+	}
+	var p *storage.Policy
+	for i := range z.Policies {
+		if z.Policies[i].ID == id {
+			p = &z.Policies[i]
+		}
+	}
+	if p == nil {
+		return fmt.Errorf("no snapshot schedule %q — it may have been removed; this cron line is stale", id)
+	}
+	res, runErr := storage.New(priv.Available()).Run(context.Background(), p)
+	rec := storage.RunRecord{At: time.Now(), Taken: res.Taken, Pruned: res.Pruned}
+	if runErr != nil {
+		rec.Error = runErr.Error()
+	}
+	if err := storage.WriteRun(cfg.DataDir(), id, rec); err != nil {
+		fmt.Fprintln(os.Stderr, "recording the run:", err)
+	}
+	if runErr != nil {
+		return runErr
+	}
+	fmt.Printf("%s: took %s", p.Name, res.Taken)
+	if len(res.Pruned) > 0 {
+		fmt.Printf(", expired %d", len(res.Pruned))
+	}
+	fmt.Println()
+	return nil
+}
+
 func migrateUsers(cfg *config.Config) {
 	name := os.Getenv("USER")
 	if u, err := user.Current(); err == nil && u.Username != "" {

internal/api/server.go

@@ -137,7 +137,6 @@ func New(cfg *config.Config, sessions *auth.Sessions, mgr *term.Manager, col *me
 		go s.keepFleetInStep()
 	}
 	go s.watchForUpdates(context.Background())
-	go s.runSchedules(context.Background())
 
 	m := http.NewServeMux()
 	m.HandleFunc("GET /healthz", func(w http.ResponseWriter, _ *http.Request) { w.WriteHeader(204) })

internal/api/zfs.go

@@ -16,8 +16,9 @@ import (
 	"io"
 	"log"
 	"net/http"
+	"os"
+	"path/filepath"
 	"strings"
-	"sync"
 	"time"
 
 	"hamlab/internal/storage"
@@ -28,11 +29,6 @@ type zfsState struct {
 	Policies []storage.Policy `json:"policies"`
 }
 
-type zfsEngine struct {
-	s  *Server
-	mu sync.Mutex
-}
-
 func (s *Server) zfsLoad() zfsState {
 	var raw json.RawMessage
 	s.store.View(func(st *store.State) { raw = st.ZFS })
@@ -99,45 +95,22 @@ func randomID() (string, error) {
 	return hex.EncodeToString(b), nil
 }
 
-// ---- the schedule runs itself ----
+// ---- the cron file ----
+//
+// A schedule is a cron line, written to /etc/cron.d/hamlab-zfs, which Ham Lab owns and
+// rewrites whole whenever the schedules change. Nothing in the daemon runs them: snapshots
+// are backup-shaped and should not stop because a web console is down, being upgraded, or
+// uninstalled. The line calls `hamlab zfs-snapshot <id>` as the account Ham Lab runs as,
+// which reaches zfs through sudo exactly as the daemon does.
 
-// runSchedules takes the snapshots that are due. Every 30 s, which is fine for a schedule
+// syncCron puts the current schedules on the machine.
-// whose finest grain is hourly, and cheap when nothing is due.
+func (s *Server) syncCron(ctx context.Context, z zfsState) error {
-func (s *Server) runSchedules(ctx context.Context) {
+	exe, err := os.Executable()
-	t := time.NewTicker(30 * time.Second)
-	defer t.Stop()
-	for {
-		select {
-		case <-ctx.Done():
-			return
-		case <-t.C:
-		}
-		z := s.zfsLoad()
-		now := time.Now()
-		changed := false
-		for i := range z.Policies {
-			p := &z.Policies[i]
-			if !p.Due(now) {
-				continue
-			}
-			res, err := s.storage.Run(ctx, p)
-			p.LastRun = now
-			p.LastTaken, p.Pruned = res.Taken, len(res.Pruned)
 	if err != nil {
-				p.LastError = err.Error()
+		return err
-				log.Printf("zfs schedule %q: %v", p.Name, err)
-			} else {
-				p.LastError = ""
-				log.Printf("zfs schedule %q: took %s, expired %d", p.Name, res.Taken, len(res.Pruned))
-			}
-			changed = true
-		}
-		if changed {
-			if err := s.zfsSave(z); err != nil {
-				log.Printf("zfs schedule: saving: %v", err)
-			}
-		}
 	}
+	exe, _ = filepath.EvalSymlinks(exe)
+	return s.storage.WriteCronFile(ctx, z.Policies, s.user, exe, s.cfg.Path())
 }
 
 // ---- endpoints ----
@@ -145,12 +118,16 @@ func (s *Server) runSchedules(ctx context.Context) {
 func (s *Server) zfsOverview(w http.ResponseWriter, r *http.Request) {
 	z := s.zfsLoad()
 	now := time.Now()
+	dir := s.cfg.DataDir()
 	out := make([]map[string]any, 0, len(z.Policies))
 	for i := range z.Policies {
 		p := z.Policies[i]
-		m := map[string]any{"policy": p}
+		expr, _ := p.CronExpr()
+		m := map[string]any{"policy": p, "cron": expr, "last": storage.ReadRun(dir, p.ID)}
 		if p.Enabled {
-			m["next"] = p.NextRun(now)
+			if n := p.NextRun(now); !n.IsZero() {
+				m["next"] = n
+			}
 		}
 		out = append(out, m)
 	}
@@ -173,7 +150,6 @@ func (s *Server) zfsPolicyPut(w http.ResponseWriter, r *http.Request) {
 	found := false
 	for i := range z.Policies {
 		if z.Policies[i].ID == p.ID && p.ID != "" {
-			p.LastRun, p.LastError, p.LastTaken, p.Pruned = z.Policies[i].LastRun, z.Policies[i].LastError, z.Policies[i].LastTaken, z.Policies[i].Pruned
 			z.Policies[i] = p
 			found = true
 		}
@@ -193,7 +169,12 @@ func (s *Server) zfsPolicyPut(w http.ResponseWriter, r *http.Request) {
 		jsonErr(w, 500, err.Error())
 		return
 	}
-	log.Printf("zfs schedule %q saved by %s (%s, %s, keep %s)", p.Name, clientIP(r), p.Dataset, p.Every, p.KeepFor)
+	if err := s.syncCron(r.Context(), z); err != nil {
+		jsonErr(w, 500, "saved, but writing "+storage.CronFile+" failed: "+err.Error())
+		return
+	}
+	expr, _ := p.CronExpr()
+	log.Printf("zfs schedule %q saved by %s (%s, %q, keep %s)", p.Name, clientIP(r), p.Dataset, expr, p.KeepFor)
 	jsonOK(w, 200, p)
 }
 
@@ -218,6 +199,11 @@ func (s *Server) zfsPolicyDelete(w http.ResponseWriter, r *http.Request) {
 		jsonErr(w, 500, err.Error())
 		return
 	}
+	if err := s.syncCron(r.Context(), z); err != nil {
+		jsonErr(w, 500, "removed, but writing "+storage.CronFile+" failed: "+err.Error())
+		return
+	}
+	storage.ForgetRun(s.cfg.DataDir(), id)
 	log.Printf("zfs schedule %s removed by %s (its snapshots are left alone)", id, clientIP(r))
 	w.WriteHeader(204)
 }
@@ -257,18 +243,11 @@ func (s *Server) zfsRunNow(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	res, err := s.storage.Run(r.Context(), &p)
-	now := time.Now()
+	rec := storage.RunRecord{At: time.Now(), Taken: res.Taken, Pruned: res.Pruned}
-	z := s.zfsLoad()
-	for i := range z.Policies {
-		if z.Policies[i].ID == id {
-			z.Policies[i].LastRun, z.Policies[i].LastTaken, z.Policies[i].Pruned = now, res.Taken, len(res.Pruned)
-			z.Policies[i].LastError = ""
 	if err != nil {
-				z.Policies[i].LastError = err.Error()
+		rec.Error = err.Error()
 	}
-		}
+	_ = storage.WriteRun(s.cfg.DataDir(), id, rec)
-	}
-	_ = s.zfsSave(z)
 	if err != nil {
 		jsonErr(w, 500, err.Error())
 		return

internal/cron/schedule.go

@@ -132,6 +132,17 @@ func (s *schedule) dayMatches(t time.Time) bool {
 	}
 }
 
+// NextRun is the first time strictly after `from` that a cron expression fires. Exported so
+// anything that writes a cron line — the ZFS snapshot schedules — can say when it runs next
+// using the same evaluator that reads everyone else's.
+func NextRun(expr string, from time.Time) (time.Time, bool) { return nextRun(expr, from) }
+
+// ValidSchedule reports whether a cron expression parses.
+func ValidSchedule(expr string) error {
+	_, err := parseSchedule(expr)
+	return err
+}
+
 // nextRun finds the first time strictly after `from` that the schedule fires.
 func nextRun(expr string, from time.Time) (time.Time, bool) {
 	s, err := parseSchedule(expr)

internal/storage/snapshots.go

@@ -18,13 +18,21 @@ package storage
 // to it. And every policy can be asked what it would delete, before it deletes anything.
 
 import (
+	"bytes"
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
 	"sort"
 	"strconv"
 	"strings"
 	"time"
+
+	"hamlab/internal/cron"
+	"hamlab/internal/priv"
 )
 
 // stampLayout is zfSnap's timestamp: 2026-09-24_02.30.01.
@@ -41,12 +49,15 @@ type Policy struct {
 	Dataset string `json:"dataset"`
 	// Recursive takes one atomic snapshot of the dataset and everything under it (zfs -r).
 	Recursive bool `json:"recursive"`
-	// Every is hourly | daily | weekly | monthly. At is "HH:MM" (the minute alone is used
+	// Every is hourly | daily | weekly | monthly | custom. At is "HH:MM" (the minute alone
-	// for hourly). Weekday is 0=Sunday, Day is 1–28.
+	// is used for hourly). Weekday is 0=Sunday, Day is 1–28. With "custom", Cron is a plain
+	// cron expression and the rest is ignored — the escape hatch for anything the four
+	// common shapes cannot say.
 	Every   string `json:"every"`
 	At      string `json:"at"`
 	Weekday int    `json:"weekday"`
 	Day     int    `json:"day"`
+	Cron    string `json:"cron,omitempty"`
 	// KeepFor is how long a snapshot lives: "6w", "30d", "12h". It is written into the
 	// snapshot's own name, zfSnap-style, so each one carries the rule it was taken under.
 	// KeepMin is a floor — that many newest snapshots survive however old they are, so a
@@ -56,10 +67,6 @@ type Policy struct {
 	Prefix  string `json:"prefix"`
 	Enabled bool   `json:"enabled"`
 
-	LastRun   time.Time `json:"last_run,omitempty"`
-	LastError string    `json:"last_error,omitempty"`
-	LastTaken string    `json:"last_taken,omitempty"` // the snapshot the last run made
-	Pruned    int       `json:"pruned,omitempty"`     // how many the last run destroyed
 }
 
 func (p *Policy) prefix() string { return strings.TrimSpace(p.Prefix) }
@@ -74,8 +81,15 @@ func (p *Policy) Validate() error {
 	}
 	switch p.Every {
 	case "hourly", "daily", "weekly", "monthly":
+	case "custom":
+		if strings.TrimSpace(p.Cron) == "" {
+			return errors.New("a custom schedule needs a cron expression")
+		}
 	default:
-		return errors.New("every: hourly, daily, weekly or monthly")
+		return errors.New("every: hourly, daily, weekly, monthly or custom")
+	}
+	if _, err := p.CronExpr(); err != nil {
+		return err
 	}
 	if _, _, err := p.clock(); err != nil {
 		return err
@@ -144,67 +158,50 @@ func ParseAge(s string) (time.Duration, error) {
 	return 0, fmt.Errorf("%q: the unit is m, h, d or w", s)
 }
 
-// lastDue is the most recent moment this policy was supposed to run, at or before now.
+// CronExpr renders the schedule as a cron expression — which is what actually runs it. The
-// A policy whose LastRun is older than that is due — so a machine that was off through
+// four common shapes are sugar over this; a custom policy states it outright. One expression
-// its window takes one snapshot when it comes back, rather than skipping silently.
+// is the only description of when a schedule fires, so the page, the cron file and the next
-func (p *Policy) lastDue(now time.Time) time.Time {
+// run can never disagree.
-	hour, min, _ := p.clock()
+func (p *Policy) CronExpr() (string, error) {
-	y, mo, d := now.Date()
+	if p.Every == "custom" {
-	loc := now.Location()
+		expr := strings.Join(strings.Fields(p.Cron), " ")
+		if err := cron.ValidSchedule(expr); err != nil {
+			return "", fmt.Errorf("cron expression: %w", err)
+		}
+		if strings.HasPrefix(expr, "@reboot") {
+			return "", errors.New("@reboot is not a schedule for snapshots")
+		}
+		return expr, nil
+	}
+	hour, min, err := p.clock()
+	if err != nil {
+		return "", err
+	}
 	switch p.Every {
 	case "hourly":
-		t := time.Date(y, mo, d, now.Hour(), min, 0, 0, loc)
+		return fmt.Sprintf("%d * * * *", min), nil
-		if t.After(now) {
-			t = t.Add(-time.Hour)
-		}
-		return t
 	case "daily":
-		t := time.Date(y, mo, d, hour, min, 0, 0, loc)
+		return fmt.Sprintf("%d %d * * *", min, hour), nil
-		if t.After(now) {
-			t = t.AddDate(0, 0, -1)
-		}
-		return t
 	case "weekly":
-		t := time.Date(y, mo, d, hour, min, 0, 0, loc)
+		return fmt.Sprintf("%d %d * * %d", min, hour, p.Weekday), nil
-		back := (int(t.Weekday()) - p.Weekday + 7) % 7
-		t = t.AddDate(0, 0, -back)
-		if t.After(now) {
-			t = t.AddDate(0, 0, -7)
-		}
-		return t
 	case "monthly":
-		t := time.Date(y, mo, p.Day, hour, min, 0, 0, loc)
+		return fmt.Sprintf("%d %d %d * *", min, hour, p.Day), nil
-		if t.After(now) {
-			t = t.AddDate(0, -1, 0)
 	}
-		return t
+	return "", errors.New("unknown schedule")
-	}
-	return time.Time{}
 }
 
-// NextRun is when this policy runs next, for the UI to show.
+// NextRun is when cron will next fire this schedule, read with the same evaluator the Jobs
+// tab uses on everyone else's lines.
 func (p *Policy) NextRun(now time.Time) time.Time {
-	last := p.lastDue(now)
+	expr, err := p.CronExpr()
-	switch p.Every {
+	if err != nil {
-	case "hourly":
-		return last.Add(time.Hour)
-	case "daily":
-		return last.AddDate(0, 0, 1)
-	case "weekly":
-		return last.AddDate(0, 0, 7)
-	case "monthly":
-		return last.AddDate(0, 1, 0)
-	}
 		return time.Time{}
 	}
-
+	t, ok := cron.NextRun(expr, now)
-// Due reports whether the policy should run now.
+	if !ok {
-func (p *Policy) Due(now time.Time) bool {
+		return time.Time{}
-	if !p.Enabled {
-		return false
 	}
-	d := p.lastDue(now)
+	return t
-	return !d.IsZero() && p.LastRun.Before(d)
 }
 
 // SnapTime reads a snapshot's name in zfSnap's convention and returns when it was taken,
@@ -349,3 +346,131 @@ func (m *Manager) Run(ctx context.Context, p *Policy) (RunResult, error) {
 	}
 	return res, nil
 }
+
+// ---- what happened when it ran ----
+//
+// The policies are a definition the daemon owns; what a run did is written by whatever ran
+// it, which is a cron job in its own process. Keeping the two apart means the CLI never has
+// to write state.json behind the daemon's back — the last writer would win and one of them
+// would lose its work.
+
+// RunRecord is the outcome of one run of one schedule.
+type RunRecord struct {
+	At     time.Time `json:"at"`
+	Taken  string    `json:"taken,omitempty"`
+	Pruned []string  `json:"pruned,omitempty"`
+	Error  string    `json:"error,omitempty"`
+}
+
+func runPath(dir, id string) string { return filepath.Join(dir, "zfs-runs", id+".json") }
+
+// WriteRun records what a run did, beside the config.
+func WriteRun(dir, id string, r RunRecord) error {
+	if err := os.MkdirAll(filepath.Join(dir, "zfs-runs"), 0o700); err != nil {
+		return err
+	}
+	b, err := json.Marshal(r)
+	if err != nil {
+		return err
+	}
+	p := runPath(dir, id)
+	tmp := p + ".tmp"
+	if err := os.WriteFile(tmp, b, 0o600); err != nil {
+		return err
+	}
+	return os.Rename(tmp, p)
+}
+
+// ReadRun is the last thing a schedule did, or the zero value if it has never run.
+func ReadRun(dir, id string) RunRecord {
+	var r RunRecord
+	b, err := os.ReadFile(runPath(dir, id))
+	if err == nil {
+		_ = json.Unmarshal(b, &r)
+	}
+	return r
+}
+
+// ForgetRun drops a schedule's record, for when the schedule itself goes.
+func ForgetRun(dir, id string) { _ = os.Remove(runPath(dir, id)) }
+
+// ---- the cron file ----
+
+// CronFile is where Ham Lab writes the snapshot schedules. One file it owns entirely and
+// rewrites whole, so there is never a half-updated set of lines, and so a schedule removed
+// in the UI is removed from the machine.
+const CronFile = "/etc/cron.d/hamlab-zfs"
+
+// CronFileText renders every schedule as cron lines. user is who they run as — the account
+// Ham Lab itself runs as, which reaches zfs through sudo exactly as the daemon does, so the
+// files it writes stay owned by that account rather than by root.
+func CronFileText(policies []Policy, user, exe, config string) (string, error) {
+	var b strings.Builder
+	b.WriteString("# Ham Lab snapshot schedules — written by the ZFS tab, rewritten whole on every change.\n")
+	b.WriteString("# Edit them there, not here: anything added by hand is lost the next time a schedule changes.\n")
+	b.WriteString("SHELL=/bin/sh\n")
+	b.WriteString("PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin\n\n")
+	for i := range policies {
+		p := &policies[i]
+		if !p.Enabled {
+			b.WriteString("# " + p.Name + " — paused\n")
+			continue
+		}
+		expr, err := p.CronExpr()
+		if err != nil {
+			return "", fmt.Errorf("%s: %w", p.Name, err)
+		}
+		name := strings.ReplaceAll(p.Name, "\n", " ")
+		fmt.Fprintf(&b, "# %s — %s, keep %s\n", name, p.Dataset, p.KeepFor)
+		fmt.Fprintf(&b, "%s %s %s -config %s zfs-snapshot %s\n\n", expr, user, exe, config, p.ID)
+	}
+	return b.String(), nil
+}
+
+// WriteCronFile puts the schedules on the machine, or removes the file when there are none.
+func (m *Manager) WriteCronFile(ctx context.Context, policies []Policy, user, exe, config string) error {
+	live := false
+	for i := range policies {
+		if policies[i].Enabled {
+			live = true
+		}
+	}
+	if !live {
+		out, code, err := m.run(ctx, "rm", "-f", CronFile)
+		if err != nil {
+			return err
+		}
+		if code != 0 {
+			return errors.New(strings.TrimSpace(string(out)))
+		}
+		return nil
+	}
+	text, err := CronFileText(policies, user, exe, config)
+	if err != nil {
+		return err
+	}
+	return m.writeRoot(ctx, CronFile, text)
+}
+
+// writeRoot puts text at a root-owned path, through the same privilege escalation every
+// other write here uses.
+func (m *Manager) writeRoot(ctx context.Context, path, text string) error {
+	args := []string{"tee", path}
+	if m.sudo {
+		args = priv.Wrap(args...)
+	}
+	ctx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+	cmd := exec.CommandContext(ctx, args[0], args[1:]...)
+	cmd.Stdin = strings.NewReader(text)
+	var errb bytes.Buffer
+	cmd.Stdout, cmd.Stderr = nil, &errb
+	if err := cmd.Run(); err != nil {
+		msg := strings.TrimSpace(errb.String())
+		if msg == "" {
+			msg = err.Error()
+		}
+		return errors.New(msg)
+	}
+	return nil
+}

internal/storage/snapshots_test.go

@@ -1,8 +1,11 @@
 package storage
 
 import (
+	"strings"
 	"testing"
 	"time"
+
+	"hamlab/internal/cron"
 )
 
 func TestSnapTimeReadsZfSnapNames(t *testing.T) {
@@ -33,39 +36,62 @@ func TestSnapTimeReadsZfSnapNames(t *testing.T) {
 	}
 }
 
-func TestScheduleIsDueAndNext(t *testing.T) {
+func TestCronExprAndNextRun(t *testing.T) {
 	now := time.Date(2026, 9, 24, 9, 15, 0, 0, time.Local)
-	daily := &Policy{Every: "daily", At: "02:30", Enabled: true}
+	for _, c := range []struct {
-	if d := daily.lastDue(now); d.Hour() != 2 || d.Day() != 24 {
+		p    Policy
-		t.Fatalf("daily lastDue = %v", d)
+		want string
+	}{
+		{Policy{Every: "daily", At: "02:30"}, "30 2 * * *"},
+		{Policy{Every: "hourly", At: "00:30"}, "30 * * * *"},
+		{Policy{Every: "weekly", At: "04:00", Weekday: 0}, "0 4 * * 0"},
+		{Policy{Every: "monthly", At: "05:15", Day: 12}, "15 5 12 * *"},
+		{Policy{Every: "custom", Cron: "*/15  9-17 * * 1-5"}, "*/15 9-17 * * 1-5"},
+	} {
+		got, err := c.p.CronExpr()
+		if err != nil || got != c.want {
+			t.Errorf("CronExpr(%+v) = %q, %v; want %q", c.p.Every, got, err, c.want)
 		}
-	if !daily.Due(now) {
-		t.Fatal("never run: must be due")
 	}
-	daily.LastRun = now.Add(-time.Hour) // ran at 08:15 today, after 02:30
+	// the next run is read with the same evaluator the Jobs tab uses
-	if daily.Due(now) {
+	daily := Policy{Every: "daily", At: "02:30"}
-		t.Fatal("already ran since the window: not due")
+	if n := daily.NextRun(now); n.Hour() != 2 || n.Minute() != 30 || n.Day() != 25 {
+		t.Fatalf("NextRun = %v, want 25th 02:30", n)
 	}
-	if n := daily.NextRun(now); n.Day() != 25 || n.Hour() != 2 {
+	// a custom expression that does not parse is refused, not silently ignored
-		t.Fatalf("daily NextRun = %v", n)
+	if _, err := (&Policy{Every: "custom", Cron: "not a schedule"}).CronExpr(); err == nil {
+		t.Fatal("a bad cron expression must be refused")
 	}
-	// a machine that was off through its window runs once when it comes back, not never
+	if _, err := (&Policy{Every: "custom", Cron: "@reboot"}).CronExpr(); err == nil {
-	daily.LastRun = now.AddDate(0, 0, -2)
+		t.Fatal("@reboot is not a snapshot schedule")
-	if !daily.Due(now) {
-		t.Fatal("a missed window must still be due")
 	}
-	// disabled never runs
-	daily.Enabled = false
-	if daily.Due(now) {
-		t.Fatal("disabled must never be due")
 }
-	weekly := &Policy{Every: "weekly", At: "04:00", Weekday: int(time.Sunday), Enabled: true}
+
-	if d := weekly.lastDue(now); d.Weekday() != time.Sunday || d.After(now) {
+func TestCronFileText(t *testing.T) {
-		t.Fatalf("weekly lastDue = %v (%v)", d, d.Weekday())
+	ps := []Policy{
+		{ID: "abc", Name: "tank nightly", Dataset: "tank", Every: "daily", At: "02:30", KeepFor: "6w", Enabled: true},
+		{ID: "def", Name: "paused one", Dataset: "tank/apps", Every: "hourly", At: "00:05", KeepFor: "2d"},
+	}
+	text, err := CronFileText(ps, "core", "/usr/local/bin/hamlab", "/home/core/apps/hamlab/config.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(text, "30 2 * * * core /usr/local/bin/hamlab -config /home/core/apps/hamlab/config.json zfs-snapshot abc") {
+		t.Fatalf("the enabled schedule is missing or malformed:\n%s", text)
+	}
+	if strings.Contains(text, "zfs-snapshot def") {
+		t.Fatalf("a paused schedule must not be scheduled:\n%s", text)
+	}
+	// every line cron will read must parse as a schedule
+	for _, line := range strings.Split(text, "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" || strings.HasPrefix(line, "#") || strings.Contains(line, "=") {
+			continue
+		}
+		f := strings.Fields(line)
+		if err := cron.ValidSchedule(strings.Join(f[:5], " ")); err != nil {
+			t.Errorf("line %q: %v", line, err)
 		}
-	hourly := &Policy{Every: "hourly", At: "00:30", Enabled: true}
-	if d := hourly.lastDue(now); d.Minute() != 30 || d.Hour() != 8 {
-		t.Fatalf("hourly lastDue at 09:15 with minute 30 = %v, want 08:30", d)
 	}
 }
 
@@ -77,6 +103,8 @@ func TestValidateRefusesNonsense(t *testing.T) {
 	bad := []Policy{
 		{Dataset: "", Every: "daily", At: "02:30", KeepFor: "6w"},
 		{Dataset: "tank", Every: "yearly", At: "02:30", KeepFor: "6w"},
+		{Dataset: "tank", Every: "custom", KeepFor: "6w"},
+		{Dataset: "tank", Every: "custom", Cron: "not a schedule", KeepFor: "6w"},
 		{Dataset: "tank", Every: "daily", At: "25:00", KeepFor: "6w"},
 		{Dataset: "tank", Every: "daily", At: "02:30"},                          // no retention
 		{Dataset: "tank", Every: "daily", At: "02:30", KeepFor: "6 weeks"},      // unparseable

web/src/ZFS.tsx

@@ -9,7 +9,7 @@
 // disagree over. An existing cron job is detected and shown, so the page tells the truth
 // about the machine whether Ham Lab or something else is doing the work.
 import { FormEvent, useCallback, useEffect, useState } from 'react'
-import { api, ZFSExternal, ZFSInfo, ZFSOverview, ZFSPolicy, ZFSSnapshot } from './api'
+import { api, ZFSExternal, ZFSInfo, ZFSOverview, ZFSPolicy, ZFSRun, ZFSSnapshot } from './api'
 import { fmtBytes } from './fmt'
 import { useRefreshMs } from './prefs'
 import { askText, askYes, Btn, Loading, Meter, Spinner, toast, useEscape } from './ui'
@@ -26,7 +26,8 @@ const soon = (t?: string) => {
 }
 const WEEKDAYS = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
 const describe = (p: ZFSPolicy) => {
-  const when = p.every === 'hourly' ? `every hour at :${(p.at.split(':')[1] ?? '00').padStart(2, '0')}`
+  const when = p.every === 'custom' ? `on ${p.cron}`
+    : p.every === 'hourly' ? `every hour at :${(p.at.split(':')[1] ?? '00').padStart(2, '0')}`
     : p.every === 'daily' ? `every day at ${p.at}`
       : p.every === 'weekly' ? `every ${WEEKDAYS[p.weekday] ?? 'Sunday'} at ${p.at}`
         : `on day ${p.day} of the month at ${p.at}`
@@ -107,7 +108,7 @@ function DatasetRow({ d, onChanged }: { d: ZFSInfo['datasets'][number]; onChange
 
 // ---- schedules ----
 
-function ExternalNotice({ x, mine }: { x: ExternalJob; mine: number }) {
+function ExternalNotice({ x, mine, onAdopt }: { x: ExternalJob; mine: number; onAdopt: () => void }) {
   return (
     <div className={'notice' + (mine > 0 ? ' attn' : '')} style={{ marginBottom: 8 }}>
       <b>{x.tool}</b> is already taking snapshots on this machine, from cron as <b>{x.user}</b>:
@@ -115,13 +116,16 @@ function ExternalNotice({ x, mine }: { x: ExternalJob; mine: number }) {
       <span className="muted">in {x.source}{x.last ? ` · last ran ${ago(x.last)}` : ''}</span>
       {mine > 0
         ? <div className="warn" style={{ marginTop: 6 }}>A Ham Lab schedule is running as well. Both use the same names and both only remove snapshots that have passed the expiry written into them, so they will not fight — but they will take twice as many. Keep one.</div>
-        : <div className="muted small" style={{ marginTop: 6 }}>Ham Lab reads the snapshots it makes and shows when each expires. Add a schedule here only if you mean to replace it; edit this one on the Jobs tab.</div>}
+        : <div className="svc-actions" style={{ marginTop: 6 }}>
+            <Btn className="btn" onClick={onAdopt}>Adopt as a schedule</Btn>
+            <span className="muted small">Copies it into a Ham Lab schedule you can edit here. Turn the old cron job off on the Jobs tab once you are happy with it.</span>
+          </div>}
     </div>
   )
 }
 type ExternalJob = ZFSExternal
 
-function ScheduleCard({ p, next, onChanged, onEdit }: { p: ZFSPolicy; next?: string; onChanged: () => void; onEdit: () => void }) {
+function ScheduleCard({ p, cron, next, last, onChanged, onEdit }: { p: ZFSPolicy; cron: string; next?: string; last: ZFSRun; onChanged: () => void; onEdit: () => void }) {
   const [expired, setExpired] = useState<ZFSSnapshot[] | null>(null)
   const [busy, setBusy] = useState('')
   const preview = async () => {
@@ -146,7 +150,7 @@ function ScheduleCard({ p, next, onChanged, onEdit }: { p: ZFSPolicy; next?: str
   return (
     <section className="card">
       <div className="sub-head">
-        <span className={'dot ' + (p.last_error ? 'bad' : p.enabled ? 'ok' : '')} />
+        <span className={'dot ' + (last.error ? 'bad' : p.enabled ? 'ok' : '')} />
         <span className="dname">{p.name || p.dataset}</span>
         {!p.enabled && <span className="badge">paused</span>}
         <span className="spacer" />
@@ -155,11 +159,12 @@ function ScheduleCard({ p, next, onChanged, onEdit }: { p: ZFSPolicy; next?: str
       <div className="props" style={{ marginTop: 6 }}>
         <div><span className="k">Dataset</span><span className="mono">{p.dataset}</span></div>
         <div><span className="k">When</span><span>{describe(p)}</span></div>
-        <div><span className="k">Last run</span><span>{ago(p.last_run)}{p.pruned ? ` · ${p.pruned} expired removed` : ''}</span></div>
+        <div><span className="k">Cron</span><span className="mono small" title={`in ${'/etc/cron.d/hamlab-zfs'}`}>{cron}</span></div>
-        {p.last_taken && <div><span className="k">Last snapshot</span><span className="mono small">{p.last_taken.split('@')[1]}</span></div>}
+        <div><span className="k">Last run</span><span>{ago(last.at)}{last.pruned?.length ? ` · ${last.pruned.length} expired removed` : ''}</span></div>
+        {last.taken && <div><span className="k">Last snapshot</span><span className="mono small">{last.taken.split('@')[1]}</span></div>}
         {p.keep_min > 0 && <div><span className="k">Floor</span><span>always keep the newest {p.keep_min}</span></div>}
       </div>
-      {p.last_error && <div className="error" style={{ marginTop: 8 }}>{p.last_error}</div>}
+      {last.error && <div className="error" style={{ marginTop: 8 }}>{last.error}</div>}
       {expired && (
         <div className="notice" style={{ marginTop: 8 }}>
           {expired.length === 0 ? 'Nothing has expired — a run now would only take a snapshot.' : (
@@ -210,14 +215,20 @@ function ScheduleDialog({ p, datasets, onClose, onSaved }: { p: Partial<ZFSPolic
         </label>
         <label className="muted">How often
           <span className="chips">
-            {(['hourly', 'daily', 'weekly', 'monthly'] as const).map((k) => (
+            {(['hourly', 'daily', 'weekly', 'monthly', 'custom'] as const).map((k) => (
               <button type="button" key={k} className={'chip' + (f.every === k ? ' on' : '')} onClick={() => set('every', k)}>{k}</button>
             ))}
           </span>
         </label>
+        {f.every === 'custom' ? (
+          <label className="muted">Cron expression — minute hour day month weekday
+            <input className="mono" value={f.cron ?? ''} onChange={(e) => set('cron', e.target.value)} placeholder="0 */6 * * *" spellCheck={false} required />
+          </label>
+        ) : (
           <label className="muted">At
             <input type="time" value={f.at} onChange={(e) => set('at', e.target.value)} />
           </label>
+        )}
         {f.every === 'weekly' && (
           <label className="muted">On
             <select value={f.weekday} onChange={(e) => set('weekday', Number(e.target.value))}>
@@ -230,6 +241,9 @@ function ScheduleDialog({ p, datasets, onClose, onSaved }: { p: Partial<ZFSPolic
             <input type="number" min={1} max={28} value={f.day} onChange={(e) => set('day', Number(e.target.value))} />
           </label>
         )}
+        <label className="muted">Snapshot name prefix — optional; the timestamp and retention follow it
+          <input value={f.prefix ?? ''} onChange={(e) => set('prefix', e.target.value)} placeholder="none" spellCheck={false} />
+        </label>
         <label className="muted">Keep for — written into each snapshot's name, as zfSnap does it: 6w, 30d, 12h
           <input value={f.keep_for} onChange={(e) => set('keep_for', e.target.value)} placeholder="6w" spellCheck={false} required />
         </label>
@@ -255,6 +269,26 @@ function ScheduleDialog({ p, datasets, onClose, onSaved }: { p: Partial<ZFSPolic
   )
 }
 
+// adopt reads an existing cron job into the fields of a new schedule, so replacing it is a
+// matter of checking the numbers rather than retyping them. zfSnap's own flags are read
+// where they can be: -a <age> is the retention, -r is recursive.
+function adopt(x: ExternalJob, fallbackDataset: string): Partial<ZFSPolicy> {
+  const cmd = x.command
+  const keep = /-a\s+(\d+[mhdwMy])/.exec(cmd)?.[1] ?? /--\s*(\d+[mhdw])\b/.exec(cmd)?.[1] ?? '6w'
+  const f = x.schedule.trim().split(/\s+/)
+  const min = Number(f[0]), hour = Number(f[1])
+  const simple = f.length === 5 && Number.isInteger(min) && Number.isInteger(hour) && f[2] === '*' && f[3] === '*' && f[4] === '*'
+  const ds = /\b(tank[\w/-]*)\b/.exec(cmd)?.[1] ?? fallbackDataset
+  return {
+    ...BLANK, name: `${x.tool} (adopted)`, dataset: ds,
+    recursive: /\s-\w*r/.test(cmd) || true,
+    every: simple ? 'daily' : 'custom',
+    at: simple ? `${String(hour).padStart(2, '0')}:${String(min).padStart(2, '0')}` : '02:30',
+    cron: simple ? '' : x.schedule.trim(),
+    keep_for: keep, keep_min: 1, enabled: true,
+  }
+}
+
 // ---- page ----
 
 export default function ZFS() {
@@ -286,10 +320,10 @@ export default function ZFS() {
       <div className="dash-bar" style={{ marginTop: 12 }}><h2 className="section">Snapshot schedules</h2><span className="spacer" />
         <Btn className="btn on" onClick={() => setEditing({ ...BLANK, dataset: zfs.pools[0]?.name ?? '' })}>+ New schedule</Btn>
       </div>
-      {ov.external && <ExternalNotice x={ov.external} mine={mine} />}
+      {ov.external && <ExternalNotice x={ov.external} mine={mine} onAdopt={() => setEditing(adopt(ov.external!, zfs.pools[0]?.name ?? ''))} />}
       {ov.policies.length === 0 && !ov.external && <div className="notice">Nothing is taking snapshots on a schedule. Snapshots are written the way zfSnap writes them, with the retention in the name, so anything already here is understood.</div>}
-      <div className="cards">{ov.policies.map(({ policy, next }) => (
+      <div className="cards">{ov.policies.map(({ policy, cron, next, last }) => (
-        <ScheduleCard key={policy.id} p={policy} next={next} onChanged={load} onEdit={() => setEditing(policy)} />
+        <ScheduleCard key={policy.id} p={policy} cron={cron} next={next} last={last ?? {}} onChanged={load} onEdit={() => setEditing(policy)} />
       ))}</div>
 
       <div className="dash-bar" style={{ marginTop: 12 }}><h2 className="section">Datasets</h2><span className="spacer" /><span className="muted small">used · referenced · in snapshots · count · compression · mount</span></div>

web/src/api.ts

@@ -133,9 +133,10 @@ export interface SpaceNode { name: string; dir: boolean; size: number; apparent:
 export interface SpaceTree extends SpaceNode { path: string; children?: SpaceTree[]; more_n?: number; more_size?: number; total: number }
 export interface SpaceFile { path: string; size: number; apparent: number; mtime: number }
 
-export interface ZFSPolicy { id: string; name: string; dataset: string; recursive: boolean; every: 'hourly' | 'daily' | 'weekly' | 'monthly'; at: string; weekday: number; day: number; keep_for: string; keep_min: number; prefix?: string; enabled: boolean; last_run?: string; last_error?: string; last_taken?: string; pruned?: number }
+export interface ZFSPolicy { id: string; name: string; dataset: string; recursive: boolean; every: 'hourly' | 'daily' | 'weekly' | 'monthly' | 'custom'; at: string; weekday: number; day: number; cron?: string; keep_for: string; keep_min: number; prefix?: string; enabled: boolean }
+export interface ZFSRun { at?: string; taken?: string; pruned?: string[]; error?: string }
 export interface ZFSExternal { tool: string; schedule: string; command: string; user: string; source: string; id: string; next?: number; last?: number }
-export interface ZFSOverview { zfs: ZFSInfo; policies: { policy: ZFSPolicy; next?: string }[]; external?: ZFSExternal | null }
+export interface ZFSOverview { zfs: ZFSInfo; policies: { policy: ZFSPolicy; cron: string; next?: string; last: ZFSRun }[]; external?: ZFSExternal | null }
 
 export interface FileEntry { name: string; type: 'dir' | 'file' | 'link' | 'other'; size: number; mode: string; perm: string; mtime: number; user: string; group: string; target?: string; broken?: boolean }
 export interface FileInfo extends FileEntry { path: string; text: boolean }