# cron(3tcl) 2.1 cron "cron"

## Name

cron - Tool for automating the period callback of commands

## Synopsis

- package require Tcl 8.6
- package require cron ?2.1?

- ::cron::at ?processname? timecode command
- ::cron::cancel processname
- ::cron::every processname frequency command
- ::cron::in ?processname? timecode command
- ::cron::object_coroutine object coroutine ?info?
- ::cron::sleep milliseconds
- ::cron::task delete process
- ::cron::task exists process
- ::cron::task info process
- ::cron::task set process field value ?field...? ?value...?
- ::cron::wake ?who?
- ::cron::clock_step milleseconds
- ::cron::clock_delay milleseconds
- ::cron::clock_sleep seconds ?offset?
- ::cron::clock_set newtime

## Description

The cron package provides a Pure-tcl set of tools to allow programs to schedule tasks to occur at regular intervals. Rather than force each task to issue it's own call to the event loop, the cron system mimics the cron utility in Unix: on task periodically checks to see if something is to be done, and issues all commands for a given time step at once.

Changes in version 2.0

While cron was originally designed to handle time scales > 1 second, the latest version's internal understand time granularity down to the millisecond, making it easier to integrate with other timed events. Version 2.0 also understands how to properly integrate coroutines and objects. It also adds a facility for an external (or script driven) clock. Note that vwait style events won't work very well with an external clock.

## Commands

::cron::at ?processname? timecode command

  This command registers a command to be called at the time specified by timecode. If timecode is expressed as an integer, the timecode is assumed to be in unixtime. All other inputs will be interpreted by clock scan and converted to unix time. This task can be modified by subsequent calls to this package's commands by referencing processname. If processname exists, it will be replaced. If processname is not given, one is generated and returned by the command.

      ::cron::at start_coffee {Tomorrow at 9:00am}  {remote::exec::coffeepot power on}
      ::cron::at shutdown_coffee {Tomorrow at 12:00pm}  {remote::exec::coffeepot power off}

::cron::cancel processname

  This command unregisters the process processname and cancels any pending commands. Note: processname can be a process created by either ::cron::at or ::cron::every.

      ::cron::cancel check_mail

::cron::every processname frequency command

  This command registers a command to be called at the interval of frequency. frequency is given in seconds. This task can be modified by subsequent calls to this package's commands by referencing processname. If processname exists, it will be replaced.

      ::cron::every check_mail 900  ::imap_client::check_mail
      ::cron::every backup_db  3600 {::backup_procedure ::mydb}

::cron::in ?processname? timecode command

  This command registers a command to be called after a delay of time specified by timecode. timecode is expressed as an seconds. This task can be modified by subsequent calls to this package's commands by referencing processname. If processname exists, it will be replaced. If processname is not given, one is generated and returned by the command.

::cron::object_coroutine object coroutine ?info?

  This command registers a coroutine, associated with object to be called given the parameters of info. If now parameters are given, the coroutine is assumed to be an idle task which will self-terminate. info can be given in any form compadible with ::cron::task set

::cron::sleep milliseconds

  When run within a coroutine, this command will register the coroutine for a callback at the appointed time, and immediately yield.

  If the ::cron::time variable is > 0 this command will advance the internal time, 100ms at a time.

  In all other cases this command will generate a fictious variable, generate an after call, and vwait the variable:

      set eventid [incr ::cron::eventcount]
      set var ::cron::event_#$eventid
      set $var 0
      ::after $ms "set $var 1"
      ::vwait $var
      ::unset $var

  Usage:

      ::cron::sleep 250

::cron::task delete process

  Delete the process specified the process

::cron::task exists process

  Returns true if process is registered with cron.

::cron::task info process

  Returns a dict describing process. See ::cron::task set for a description of the options.

::cron::task set process field value ?field...? ?value...?

  If process does not exist, it is created. Options Include:

  command If coroutine is black, a global command which implements this process. If coroutine is not black, the command to invoke to create or recreate the coroutine. coroutine The name of the coroutine (if any) which implements this process. frequency If -1, this process is terminated after the next event. If 0 this process should be called during every idle event. If positive, this process should generate events periodically. The frequency is an interger number of milleseconds between events. object The object associated with this process or coroutine. scheduled If non-zero, the absolute time from the epoch (in milleseconds) that this process will trigger an event. If zero, and the frequency is also zero, this process is called every idle loop. running A boolean flag. If true it indicates the process never returned or yielded during the event loop, and will not be called again until it does so.
::cron::wake ?who?

  Wake up cron, and arrange for its event loop to be run during the next Idle cycle.

      ::cron::wake {I just did something important}

Several utility commands are provided that are used internally within cron and for testing cron, but may or may not be useful in the general cases.

::cron::clock_step milleseconds

  Return a clock time absolute to the epoch which falls on the next border between one second and the next for the value of milleseconds

::cron::clock_delay milleseconds

  Return a clock time absolute to the epoch which falls on the next border between one second and the next milleseconds in the future.

::cron::clock_sleep seconds ?offset?

  Return a clock time absolute to the epoch which falls exactly seconds in the future. If offset is given it may be positive or negative, and will shift the final time to before or after the second would flip.

::cron::clock_set newtime

  Sets the internal clock for cron. This command will advance the time in 100ms increment, triggering events, until the internal time catches up with newtime.

  newtime is expressed in absolute milleseconds since the beginning of the epoch.

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category odie of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

When proposing code changes, please provide unified diffs, i.e the output of diff -u.

Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar.

## Keywords

cron, odie

## Category

System

## Copyright

Copyright © 2016-2018 Sean Woods <yoda@etoyoc.com>