KindleCron (kron)

Cron-like task scheduling for jailbroken Kindles that survives deep sleep.

KindleCron runs scheduled jobs on a jailbroken Kindle reliably and to the second, while letting the device sleep between them. It is a single static binary with nothing to install on the device.

Ordinary cron is unreliable here. To save power, the Kindle suspends (CPU off) after a few minutes of inactivity, and a suspended device cannot run crond. The common workaround, holding the device in its active state, drains the battery.

KindleCron takes the opposite approach. During the brief readyToSuspend window just before the device sleeps, it programs an RTC wake-up for the next job's due time; set this late in the cycle, the value is not overwritten by powerd. The Kindle then suspends normally and conserves power. When the job is due, the RTC fires, the device wakes into screenSaver state, becomes responsive and KindleCron runs the job.

That underlying trick is not new. It has been known in the Kindle community for years (see Credits) and various scripts implement it ad hoc for one purpose each. What KindleCron adds is the packaging: a single, self-contained binary that turns the trick into a general-purpose scheduler with a cron grammar, persistent job state, keep-awake handling, per-job timeouts, logging and a defined exit-code contract. The power-state handling lives in one place and is maintained there, so any project that needs work to happen on a sleeping Kindle can schedule it in one command instead of reimplementing the powerd procedure itself.

1. Download the latest release to your computer
2. Unzip it
3. Connect the Kindle over USB
4. Copy the extensions and kron folders to the root of the Kindle drive (/mnt/us)
5. Run Setup once (KUAL: kron -> Setup, or a terminal: /mnt/us/kron/kron setup)

kron [global flags] <command> [args]

Commands:
  daemon                              run the scheduler (default; start this at boot)
  stop                                tell a running daemon to shut down
  kill-jobs [NAME]                    kill orphaned job process group(s); all, or just NAME
  add [-timeout DUR] NAME SCHEDULE COMMAND...
                                      register or replace a job; NAME is a plain name
                                      (no '/' or leading '.')
  remove NAME                         delete a job and its state
  enable NAME                         re-enable a disabled job (resumes scheduling)
  disable NAME                        stop scheduling a job without deleting it
  purge                               delete ALL jobs and their state (-y to skip prompt)
  clear-logs                          empty kron.log and all per-job logs (-y to skip prompt)
  list                                show all jobs, when each next runs, and any timeout
  setup                               symlink kron into /usr/bin so a bare `kron` works everywhere
  unlink                              remove the /usr/bin/kron symlink that setup created
  version | help

Global flags:
  -dir PATH        data directory for jobs/state/log
                   (default: the binary's own directory; overrides $KRON_DIR)
  -logmax KB       cap kron.log at KB kilobytes, dropping oldest lines
                   (0 = unlimited; default 256)
  -keepawake DUR   keep the device awake (no suspend) when the next job is due within
                   DUR (default 3m); 'off' to always suspend regardless of interval
  -wakelead DUR    wake this long BEFORE a job is due as a safety margin (default 15s;
                   0 disables it)
  -jobtimeout DUR  global default run-time limit per job (default 10m); a job exceeding
                   its effective limit is killed. Override per job with add -timeout
  -version         print version
  -eips            also show the result on the Kindle screen (centred, lower area) via
                   eips. Meant for KUAL, where there is no terminal. Supported by
                   daemon, stop, version, purge, kill-jobs, clear-logs, setup and unlink.
  -help            show help

Global flags must come before the subcommand (kron -keepawake 5m daemon), or, for commands that take no positional arguments (daemon, list, stop, version, setup, unlink), in either position.

kron's exit code is the success signal, so another program (a launcher, an installer, Scopae) can drive it without parsing human-readable stdout:

stdout stays human-readable and may change wording (e.g. the keep-awake note), so callers should rely on the exit code.

A cron expression is the primary form, and the cron keyword is optional, so a bare expression is the preferred way to write a schedule. once is the only form with no cron equivalent; every and at are convenience shorthands for cadences that cron already covers.

<cron expr>                 5- or 6-field cron, e.g. 0 9 * * 1 (Mondays 09:00)
once YYYY-MM-DD HH:MM[:SS]  one-shot; runs once (catches up if missed) then self-removes
every <N>[s|m|h|d]          convenience: fixed interval since last run, e.g. every 30m
at HH:MM[,HH:MM,...]        convenience: daily at local times (same as cron M H * * *)

kron add weekly   "0 9 * * 1"             /mnt/us/app/weekly.sh
kron add poll     "*/30 * * * * *"        /mnt/us/app/poll.sh
kron add reminder "once 2026-06-15 14:30" /mnt/us/app/notify.sh

every (a fixed interval since the last run, and the only form that also does arbitrary sub-minute intervals like 90s) and at (daily wall-clock times, the same as cron M H * * *) are sugars you can use when they read more clearly.

A cron schedule takes a standard cron expression. The cron keyword is optional: a bare expression is treated as cron, so kron add j "0 9 * * 1" ... is the same as kron add j "cron 0 9 * * 1" .... The field count is detected automatically: 5 fields is the classic form, 6 fields adds a leading seconds field.

5-field:      min  hour  dom  month  dow
6-field:  sec min  hour  dom  month  dow

  sec    second        0-59   (6-field form only)
  min    minute        0-59
  hour   hour          0-23
  dom    day of month  1-31
  month  month         1-12 or jan-dec
  dow    day of week   0-7 or sun-sat (0 and 7 are both Sunday)

Each field accepts * (all values), a single value, a-b ranges, */s or a-b/s steps, v/s (from v to the field maximum, step s), and comma lists of any of these. Month and day-of-week also accept three-letter names.

kron add j "30 7 * * 1-5"      ...   # weekdays at 07:30
kron add j "0 0 1 * *"         ...   # midnight on the 1st of every month
kron add j "0 */2 * * *"       ...   # every 2 hours on the hour
kron add j "*/30 * * * * *"    ...   # every 30 seconds (6-field)
kron add j "0 9 * * mon,thu"   ...   # Mondays and Thursdays at 09:00

When both the day-of-month and day-of-week fields are restricted (neither is *), a day matches if either field matches, following the Vixie-cron convention. So 0 0 13 * 5 fires on every Friday and on the 13th of every month. Wall-clock times that do not exist on a DST spring-forward day are skipped for that day.

A job may declare its own run-time limit, which overrides the global -jobtimeout default (in either direction). A job that exceeds its effective limit is killed (its whole process group), so a hung job can never pin the device awake.

kron add sync -timeout 2m "every 1h" /mnt/us/app/sync.sh

The limit is shown by kron list, and stored in the job file as timeout=2m.

KindleCron drives the device through Kindle's powerd service. The states and properties below were reverse-engineered (see Credits) and are the behavior KindleCron relies on. Timings are approximate and vary by model.

All are write-only integers, set with lipc-set-prop -i com.lab126.powerd <property> <value>.

• yparitcher - the RTC wakeup idea
• KindleModding - the powerd documentation

Copyright (C) 2026 Lennard Ollesch

KindleCron is free software, licensed under the GNU Affero General Public License v3.0 or later (AGPL-3.0-or-later). It comes with no warranty. You may use, study, share, and modify it under the terms of the license; if you distribute it or a modified version, you must pass on the same freedoms and make the source available. See the LICENSE file for the full text.