mcron/doc/mcron.texi

\input texinfo
@c %**start of header
@setfilename mcron.info
@include config.texi
@include version.texi
@settitle mcron @value{VERSION}
@c %**end of header

@syncodeindex fn cp

@copying This manual is for GNU mcron (version @value{VERSION}), which is a
program for running jobs at scheduled times.

Copyright @copyright{}  2003, 2005, 2006, 2012, 2014  Dale Mellor
Copyright @copyright{}  2018  Mathieu Lirzin
Copyright @copyright{}  2021  Maxim Cournoyer

@quotation
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with no Invariant Sections, no Front-Cover Texts and
no Back-Cover Texts.  A copy of the license is included in the
section entitled ``GNU Free Documentation License''.
@end quotation
@end copying


@ifinfo

@dircategory Individual utilities

@direntry
* mcron: (mcron).       Run jobs at scheduled times.
@end direntry

@end ifinfo


@titlepage
@title mcron - Mellor's cron daemon
@author Dale Mellor

@page
@vskip 0pt plus 1fill
@c @insertcopying

@end titlepage

@contents

@ifnottex
@node Top, Introduction, (dir), (dir)
@top mcron

This file documents the @code{mcron} command (Mellor's cron) for
running jobs at scheduled times.

@c @insertcopying
@end ifnottex

@menu
* Introduction::                Introducing mcron.
* Simple examples::             How to use mcron 99.9% of the time.
* Syntax::                      All the possibilities for configuring cron jobs.
* Invoking::                    What happens when you run the mcron command.
* Guile modules::               Incorporating mcron into another Guile program.
* GNU Free Documentation License::  The license of this manual.
* Index::                       The complete index.

@detailmenu
 --- The Detailed Node Listing ---

Simple examples

* Guile Simple Examples::
* Vixie Simple Examples::

Full available syntax

* Guile Syntax::
* Extended Guile examples::
* Vixie Syntax::

Extended Guile examples

* AT commands::
* Every second Sunday::
* Two hours every day::
* Missing the first appointment::
* Penultimate day of every month::

Vixie

* Paul Vixie's copyright::
* Crontab file::
* Incompatibilities with old Unices::

Detailed invoking

* Invoking mcron::
* Invoking cron or crond::
* Invoking crontab::
* Behaviour on laptops::
* Exit codes::

Guile modules

* The base module::             The job list and execution loop.
* The redirect module::         Sending output of jobs to a mail box.
* The vixie-time module::       Parsing vixie-style time specifications.
* The job-specifier module::    All commands for scheme configuration files.
* The vixie-specification module::  Commands for reading vixie-style crontabs.

@end detailmenu
@end menu

@node Introduction, Simple examples, Top, Top
@chapter Introducing mcron
@cindex introduction
@cindex mcron
The mcron program represents a complete re-think of the cron concept
originally found in the Berkeley and AT&T unices, and subsequently
rationalized by Paul Vixie.  The original idea was to have a daemon
that wakes up every minute, scans a set of files under a special
directory, and determines from those files if any shell commands
should be executed in this minute.

The new idea is to read the required command instructions, work out
which command needs to be executed next, and then sleep until the
inferred time has arrived.  On waking the commands are run, and the
time of the next command is computed.  Furthermore, the specifications
are written in scheme, allowing at the same time simple command
execution instructions and very much more flexible ones to be composed
than the original Vixie format.  This has several useful advantages
over the original idea.  (Changes to user crontabs are signalled
directly to mcron by the crontab program; cron must still scan the
/etc/crontab file once every minute, although use of this file is
highly discouraged and this behaviour can be turned off).

@cindex advantages of mcron
@itemize @bullet
@item
Does not consume CPU resources when not needed.  Many cron daemons only
run jobs once an hour, or even just once a day.
@item
Can easily allow for finer time-points to be specified,
i.e. seconds.  In principle this could be extended to microseconds, but
this is not implemented.
@item
Times can be more or less regular.  For example, a job that runs
every 17 hours can be specified, or a job that runs on the first
Sunday of every month.
@item
Times can be dynamic.  Arbitrary Guile (scheme) code can be provided to
compute the next time that a command needs to be run.  This could, for
example, take the system load into consideration.
@item
Turns out to be easy to provide complete backwards compatibility with
Vixie cron.
@item
Each user looks after their own files in their own directory.  They can use
more than one to break up complicated cron specifications.
@item
Each user can run their own daemon.  This removes the need for suid
programs to manipulate the crontabs, and eliminates many security
concerns that surround all existing cron programs.
@item
The user can obtain an advance schedule of all the jobs that are due
to run.
@item
Vixie cron is implemented in 4500 lines of C code; mcron is 2000 lines
of scheme, despite the fact that it offers many more features and much
more flexibility, and complete compatibility with Vixie cron.
@end itemize

A full discussion of the design and philosophy of mcron can be found
in the white paper at
@url{http://www.gnu.org/software/mcron/design.html}.


@node Simple examples, Syntax, Introduction, Top
@chapter Simple examples
The vast majority of uses of cron are sublimely simple: run a program
every hour, or every day.  With this in mind the design of mcron has
been to allow such simple specifications to be made easily.  The
examples show how to create the command descriptions, and subsequently
how to run mcron to make them happen.
@menu
* Guile Simple Examples::
* Vixie Simple Examples::
@end menu

@node Guile Simple Examples, Vixie Simple Examples, Simple examples, Simple examples
@section Guile
@cindex guile examples
@cindex examples, guile
@cindex example, run a program every hour
You have an executable @code{my-program} in your home directory, which
you want to run every hour.  Create a file @code{job.guile} in
directory @code{~/.config/cron} (this path may be altered by the
@code{$XDG_CONFIG_HOME} environment variable) with the following
contents

@example
(job '(next-hour) "my-program")
@end example

then run the command @code{mcron}.

Want the program to run fifteen minutes past the hour, every two
hours? Edit the file to read

@example
(job
   '(next-minute-from
      (next-hour (range 0 24 2))
      15)
   "my-program")
@end example

and run the command @code{mcron}.

Or, if you are not comfortable with Scheme, you could use (and see
also the next section)

@example
(job "15 */2 * * *" "my-program")
@end example

and run the @code{mcron} command.

If you want to run other jobs, you can either add more lines to this
file, or you can create other files in your @code{.config/cron} directory
with the @code{.guile} extension.  Alternatively, you can use any file
you want and pass it as an argument to @code{mcron}, or even pipe the
commands into the standard input.


@node Vixie Simple Examples,  , Guile Simple Examples, Simple examples
@section Vixie
@cindex examples
@cindex examples, vixie
@cindex vixie examples
You have an executable @code{my-program} in your home directory, which
you want to run every hour.  Create a file @code{job.vixie} in directory
@code{~/.cron} with the following contents

@example
0 * * * * my-program
@end example

then run the command @code{mcron}.

@cindex vixie compatibility
@cindex compatibility
Alternatively (full compatibility with Vixie cron), set your
environment variable @code{EDITOR} to your favorite editor, run
@code{crontab -e}, put the above line into the edit buffer, save and
exit.  For this to work the @code{cron} daemon must be already running
on your system, as root.

@node Syntax, Invoking, Simple examples, Top
@chapter Full available syntax
@menu
* Guile Syntax::
* Extended Guile examples::
* Vixie Syntax::
@end menu
@node Guile Syntax, Extended Guile examples, Syntax, Syntax
@section Guile Syntax
@subsection Job specification
@cindex guile syntax
@cindex syntax, guile
@findex job
In Guile-formatted configuration files each command that needs executing is
introduced with the @code{job} function.  This function always takes two
arguments, the first a time specification, and the second a command
specification.  An optional third argument may contain a string to display
when this job is listed in a schedule.  Additionally a @var{user} keyword
argument can be supplied to use a different user than the one defined in
@code{configuration-user} global variable.

@cindex time specification, procedure
@cindex procedure time specification
The first argument can be a procedure, a list, or a string.  If a
function is supplied, it must take exactly one argument, which will be
the ``current'' time in UNIX format, and the return value of the
function must be the time in UNIX format when this action should next
be run.  The following functions are available to facilitate the
computation:

@findex next-second-from
@code{(next-second-from time . args)} without arguments this
returns the second after the current one.  With the extra arguments,
these form a list of seconds in the minute when the action should run,
and the function will return the time of the next allowed second
(which may be in the next minute of the hour).  @footnote{Note that
while commands can be scheduled to run at any second, it is unlikely
that they will be executed then but some time shortly thereafter,
depending on the load on the system and the number of jobs that mcron
has to start at the same time.}

@findex next-minute-from
@findex next-hour-from
@findex next-day-from
@findex next-week-from
@findex next-month-from
@findex next-year-from
Similarly to @code{next-second-from}, there are also
@code{next-minute-from}, @code{next-hour-from}, @code{next-day-from},
@code{next-week-from}, @code{next-month-from}, @code{next-year-from}.

@findex range
Furthermore, the optional argument can be fulfilled by the function
@code{(range start end . step)}, which will provide a list of values
from start to (but not including) end, with the step if given.  For
example @code{(range 0 10 2)} will yield the list @code{'(0 2 4 6 8)}.

@findex next-second
@findex next-minute
@findex next-hour
@findex next-day
@findex next-week
@findex next-month
@findex next-year
@cindex time specification, list
@cindex list time specification
If the first argument to the @code{job} function is a list, it is
taken to be program code made up of the functions @code{(next-second
. args)}, @code{(next-minute...)}, etc, where the optional arguments
can be supplied with the @code{(range)} function above (these
functions are analogous to the ones above except that they implicitly
assume the current time; it is supplied by the mcron base when the
list is eval'd).

@cindex time specification
@cindex time specification, string
@cindex string time specification
@cindex time specification, vixie-style
@cindex vixie-style time specification
If the first argument to the @code{job} function is a string, it is
expected to be a Vixie cron-style time specification.  See the section
on Vixie syntax for this.

@cindex job execution
@cindex command execution
@cindex execution
The second argument to the @code{(job)} function can be either a string, a
list, or a function.  The command is executed in the home directory and with
the UID of @var{user}.  If a string is passed, it is assumed to be shell
script and is executed with the user's default shell.  If a list is passed it
is assumed to be scheme code and is eval'd as such.  A supplied function
should take exactly zero arguments, and will be called at the pertinent times.

@subsection Sending output as e-mail
@cindex email output
@cindex email from guile script
@cindex standard input to commands
@findex with-mail-out
When jobs are specified in a vixie-style configuration, the command is
broken at a percentage sign, and the stuff that comes after this is
sent into the command's standard input.  Furthermore, any output from
the command is mailed to the user.  This functionality is provided for
compatibility with Vixie cron, but it is also available to scheme
configuration files.  The command (with-mail-out action . user) can be
used to direct output from the action (which may be a procedure, list,
or string) into an e-mail to the user.

In the case that the action is a string, then percentage signs are
processed as per the vixie specifications, and information is piped to
the shell command's standard input.

@subsection Setting environment variables
@cindex environment variables in scheme
@cindex setting environment variables
@findex append-environment-mods
Also for compatibility with Vixie cron, mcron has the ability to set
environment variables in configuration files.  To access this
functionality from a scheme configuration file, use the command
(append-environment-mods name value), where name is the name of an
environment variable, and value is the value put to it.  A value of #f
will remove the variable from the environment.

Note that environment modifications are accumulated as the
configuration file is processed, so when a job actually runs, its
environment will be modified according to the modifications specified
before the job specification in the configuration file.


@node Extended Guile examples, Vixie Syntax, Guile Syntax, Syntax
@section Extended Guile examples
@cindex examples, extended guile
@cindex extended guile examples
While Guile gives you flexibility to do anything, and the power to
represent complex requirements succinctly, things are not always as
they seem.  The following examples illustrate some pitfalls, and
demonstrate how to code around them.

@menu
* AT commands::
* Every second Sunday::
* Two hours every day::
* Missing the first appointment::
* Penultimate day of every month::
@end menu

@node AT commands, Every second Sunday, Extended Guile examples, Extended Guile examples
@subsection Synthesizing ``at'' commands
@cindex at command
The current implementation of mcron does not provide for an at command
(a command-line program that allows the user to specify that a job
runs exactly once at a certain time).  This can, however, be achieved.

Suppose the program @code{my-program} needs to be run at midnight
tonight.  A Guile script like the following would work (but a printed
schedule, obtained with the @code{--schedule} option, will show
superfluous entries).

@example
(job '(next-day)
     (lambda () (system "my-program")
                (kill (getppid) SIGINT)))
@end example

@node Every second Sunday, Two hours every day, AT commands, Extended Guile examples
@subsection Every second Sunday
@cindex examples, every second sunday
To run @code{my-program} on the second Sunday of every month, a Guile
script like the following should suffice (it is left as an exercise to
the student to understand how this works!).

@example
(job (lambda (current-time)
       (let* ((next-month (next-month-from current-time))
              (first-day (tm:wday (localtime next-month)))
              (second-sunday (if (eqv? first-day 0)
                                 7
                                 (- 14 first-day))))
         (+ next-month (* 24 60 60 second-sunday))))
     "my-program")
@end example


@node Two hours every day, Missing the first appointment, Every second Sunday, Extended Guile examples
@subsection Two hours every day
@cindex examples, two hours every day
@cindex pitfalls, two hours every day
Surprisingly perhaps, the following will @strong{not} have the desired
effect.

@example
(job '(next-hour-from (next-day) '(1 2))
     "my-program")
@end example

Rather than running the my-program program at one o'clock and two
o'clock every day, it will only run it at one o'clock.  This is because
each time mcron has to compute the next time to run the command, it
first obtains the next day, and then finds the earliest hour in that
day to run at.  Thus, after running the command at one o'clock, the
program first skips forwards to the next midnight (missing the two
o'clock appointment), and then finds the next one o'clock schedule.

The following simple command is the correct way to specify this
behaviour.

@example
(job '(next-hour '(1 2)) "my-program")
@end example


@node Missing the first appointment, Penultimate day of every month, Two hours every day, Extended Guile examples
@subsection Missing the first appointment
@cindex examples, missing the first appointment
@cindex pitfalls, missing the first appointment
The command

@example
(job '(next-hour-from (next-day) '(16))
     "my-program")
@end example

will run @code{my-program} every day at four o'clock in the
afternoon.  However, if mcron is started with this script at midday,
the first time the command will run will be four o'clock tomorrow;
today's appointment will be missed (one time only).

The correct way to specify this requirement is simply

@example
(job '(next-hour '(16))
     "my-program")
@end example


@node Penultimate day of every month,  , Missing the first appointment, Extended Guile examples
@subsection Penultimate day of every month
@cindex examples, penultimate day of every month
The following will run the @code{my-program} program on the
second-to-last day of every month.

@example
(job '(- (next-month-from (next-month)) (* 48 3600))
     "my-program")
@end example



@node Vixie Syntax,  , Extended Guile examples, Syntax
@section Vixie
@cindex syntax, vixie
@cindex vixie syntax
@cindex vixie definition
@cindex vixie compatibility
@cindex compatibility, vixie
@emph{NOTE} that this section is definitive.  If there is a difference in
behaviour between the mcron program and this part of the manual, then
there is a bug in the program.  This section is also copied verbatim
from Paul Vixie's documentation for their cron program, and their
copyright notice is duly reproduced below.

There are three problems with this specification.

@cindex zero'th day of month
@cindex 0'th day of month
1.  It is allowed to specify days of the month in the range 0-31.  What
does it mean to specify day 0? Looking at the Vixie source code, it
seems that if this date appears as part of a list, it has no
effect.  However, if it appears on its own, the effect is to say
``don't run on any particular day of the month, only take the week-day
specification into account.'' Mcron has been coded to mimic this
behaviour as a special case (unmodified mcron logic implies that this
date specification would cause jobs to run on the last day of the
previous month).

@cindex thirteenth month of year
@cindex 13th month of year
2.  Similarly to the above (but different), months of the year can be
specified in the range 0-12.  In the case of mcron (don't know what
Vixie cron did) month 12 will cause the program to wait until January
of the following year (but don't rely on this).

@cindex shell
@cindex environment variables, shell
@cindex /etc/passwd
3.  Somewhere it says that cron sets the SHELL environment variable to
/bin/sh, and elsewhere it implies that the default behaviour is for
the user's default shell to be used to execute commands.  Mcron sets
the variable and runs the command in the user's default shell, as
advertised by the /etc/passwd file.

@menu
* Paul Vixie's copyright::
* Crontab file::
* Incompatibilities with old Unices::
@end menu


@node Paul Vixie's copyright, Crontab file, Vixie Syntax, Vixie Syntax
@subsection Paul Vixie's copyright
@cindex copyright, Paul Vixie's
@cindex Paul Vixie's copyright
@quotation
Copyright 1988,1990,1993,1994 by Paul Vixie
All rights reserved

Distribute freely, except: don't remove my name from the source or
documentation (don't take credit for my work), mark your changes (don't
get me blamed for your possible bugs), don't alter or remove this
notice.  May be sold if buildable source is provided to buyer.  No
warrantee of any kind, express or implied, is included with this
software; use at your own risk, responsibility for damages (if any) to
anyone resulting from the use of this software rests entirely with the
user.
@end quotation




@node  Crontab file, Incompatibilities with old Unices, Paul Vixie's copyright, Vixie Syntax
@subsection Crontab files
@cindex crontab file
@cindex vixie crontab file
A @code{crontab} file contains instructions to the @code{cron} daemon
of the general form: ``run this command at this time on this date''.
Each user has their own crontab, and commands in any given crontab
will be executed as the user who owns the crontab.  Uucp and News will
usually have their own crontabs, eliminating the need for explicitly
running @code{su} as part of a cron command.

@cindex comments, vixie-style
Blank lines and leading spaces and tabs are ignored.  Lines whose first
non-space character is a pound-sign (#) are comments, and are ignored.
Note that comments are not allowed on the same line as cron commands, since
they will be taken to be part of the command.  Similarly, comments are not
allowed on the same line as environment variable settings.

An active line in a crontab will be either an environment setting or a cron
command.  An environment setting is of the form,

@cindex environment setting, vixie-style
@example
name = value
@end example

where the spaces around the equal-sign (=) are optional, and any
subsequent non-leading spaces in @code{value} will be part of the
value assigned to @code{name}.  The @code{value} string may be placed
in quotes (single or double, but matching) to preserve leading or
trailing blanks.

@cindex environment variables, SHELL
@cindex environment variables, LOGNAME
@cindex environment variables, HOME
@cindex SHELL environment variable
@cindex LOGNAME environment variable
@cindex HOME environment variable
@cindex /etc/passwd
Several environment variables are set up automatically by the
@code{cron} daemon.  SHELL is set to /bin/sh, and LOGNAME and HOME are
set from the /etc/passwd line of the crontab's owner.  HOME and SHELL
may be overridden by settings in the crontab; LOGNAME may not.

@cindex environment variables, USER
@cindex USER environment variable
@cindex BSD
(Another note: the LOGNAME variable is sometimes called USER on BSD systems...
on these systems, USER will be set also.) @footnote{mcron has not been
ported to BSD, so these notes are not relevant.}

@cindex environment variables, MAILTO
@cindex MAILTO environment variable
In addition to LOGNAME, HOME, and SHELL, @code{cron} will look at
MAILTO if it has any reason to send mail as a result of running
commands in ``this'' crontab.  If MAILTO is defined (and non-empty),
mail is sent to the user so named.  If MAILTO is defined but empty
(MAILTO=""), no mail will be sent.  Otherwise mail is sent to the
owner of the crontab.  This option is useful if you decide on
/bin/mail instead of /usr/lib/sendmail as your mailer when you install
cron -- /bin/mail doesn't do aliasing, and UUCP usually doesn't read
its mail.

The format of a cron command is very much the V7 standard, with a number of
upward-compatible extensions.  Each line has five time and date fields,
followed by a user name if this is the system crontab file,
followed by a command.  Commands are executed by @code{cron}
when the minute, hour, and month of year fields match the current
time, @strong{and} when at least one of the two day fields (day of month, or day of week)
match the current time (see ``Note'' below).  @code{cron} examines cron entries once every minute.
The time and date fields are:

@cindex vixie time specification fields
@cindex fields, vixie time specification
@multitable @columnfractions .2 .5
@item Field @tab Allowed values
@item ----- @tab --------------
@item minute @tab 0-59
@item hour @tab 0-23
@item day of month @tab 0-31
@item month @tab 0-12 (or names, see below)
@item day of week @tab 0-7 (0 or 7 is Sun, or use names)
@end multitable

A field may be an asterisk (*), which always stands for ``first-last''.

@cindex ranges in vixie time specifications
Ranges of numbers are allowed.  Ranges are two numbers separated
with a hyphen.  The specified range is inclusive.  For example,
8-11 for an ``hours'' entry specifies execution at hours 8, 9, 10
and 11.

@cindex lists in vixie time specifications
Lists are allowed.  A list is a set of numbers (or ranges)
separated by commas.  Examples: ``1,2,5,9'', ``0-4,8-12''.

@cindex steps in vixie time specifications
Step values can be used in conjunction with ranges.  Following
a range with ``/<number>'' specifies skips of the number's value
through the range.  For example, ``0-23/2'' can be used in the hours
field to specify command execution every other hour (the alternative
in the V7 standard is ``0,2,4,6,8,10,12,14,16,18,20,22'').  Steps are
also permitted after an asterisk, so if you want to say ``every two
hours'', just use ``*/2''.

@cindex names in vixie-style time specifications
Names can also be used for the ``month'' and ``day of week''
fields.  Use the first three letters of the particular
day or month (case doesn't matter).  Ranges or
lists of names are not allowed.  @footnote{Mcron allows any alphabetic
characters after a name, so full names of days or months are also valid.}

@cindex % character on vixie-style commands
@cindex standard input, vixie-style
The ``sixth'' field (the rest of the line) specifies the command to be
run.
The entire command portion of the line, up to a newline or %
character, will be executed by /bin/sh or by the shell
specified in the SHELL variable of the cronfile.
Percent-signs (%) in the command, unless escaped with backslash
(\\), will be changed into newline characters, and all data
after the first % will be sent to the command as standard
input.

@cindex day specification, vixie-style
@cindex vixie-style day specification
Note: The day of a command's execution can be specified by two
fields -- day of month, and day of week.  If both fields are
restricted (ie, aren't *), the command will be run when
@emph{either}
field matches the current time.  For example,

``30 4 1,15 * 5''

would cause a command to be run at 4:30 am on the 1st and 15th of each
month, plus every Friday.

EXAMPLE CRON FILE

@example
# use /bin/sh to run commands, no matter what /etc/passwd says
SHELL=/bin/sh
# mail any output to `paul', no matter whose crontab this is
MAILTO=paul
#
# run five minutes after midnight, every day
5 0 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
# run at 2:15pm on the first of every month -- output mailed to paul
15 14 1 * *     $HOME/bin/monthly
# run at 10 pm on weekdays, annoy Joe
0 22 * * 1-5	mail -s "It's 10pm" joe%Joe,%%Where are your kids?%
23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
5 4 * * sun     echo "run at 5 after 4 every sunday"
@end example

@node  Incompatibilities with old Unices,  , Crontab file, Vixie Syntax
@subsection Extensions and incompatibilities
@cindex incompatibilities with old Unices
@cindex extensions, vixie over old Unices
This section lists differences between Paul Vixie's cron and the
olde-worlde BSD and AT&T programs, for the benefit of system
administrators and users who are upgrading all the way.

@itemize @bullet
@item
@cindex day 7
When specifying day of week, both day 0 and day 7 will be considered Sunday.
BSD and AT&T seem to disagree about this.

@item
Lists and ranges are allowed to co-exist in the same field.  "1-3,7-9" would
be rejected by AT&T or BSD cron -- they want to see "1-3" or "7,8,9" ONLY.

@item
Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".

@item
Names of months or days of the week can be specified by name.

@item
Environment variables can be set in the crontab.  In BSD or AT&T, the
environment handed to child processes is basically the one from /etc/rc.

@item
Command output is mailed to the crontab owner (BSD can't do this), can be
mailed to a person other than the crontab owner (SysV can't do this), or the
feature can be turned off and no mail will be sent at all (SysV can't do this
either).

@end itemize


@node Invoking, Guile modules, Syntax, Top
@chapter Detailed invoking
@cindex invoking
@cindex personality
@cindex mcron program
@cindex cron program
@cindex crond program
@cindex crontab program
The program adopts one of three different personalities depending on
the name used to invoke it.  In a standard installation, the program is
installed in the system under the names mcron, cron and crontab
(installed SUID).

The recommended way to invoke the program is via the mcron personality
described in the next section.  The program can also be run as cron by
root, and by the SUID program crontab by individual users to gain
backwards compatibility with Vixie cron.  However, due to the fact that
this daemon process is shared by, and under control of, all the users
of the system it is possible (though very unlikely) that it may become
unusable, hence the recommendation to use the mcron personality.

@cindex deprecated, vixie personality
Furthermore, the Vixie personality is considered deprecated by this
author (it offers not a single advantage over the mcron personality,
and bloats the code by a factor of three).  It is unlikely that this
personality will ever actually go away, but the program may in future
be split into two distinct parts, and new developments will only take
place in the part which implements the mcron personality.



@menu
* Invoking mcron::
* Invoking cron or crond::
* Invoking crontab::
* Behaviour on laptops::
* Exit codes::
@end menu

@node Invoking mcron, Invoking cron or crond, Invoking, Invoking
@section Invoking mcron
@cindex invoking mcron
@cindex mcron options
@cindex mcron arguments
@cindex command line, mcron
@cindex mcron command line
Mcron should be run by the user who wants to schedule their jobs.  It
may be made a background job using the facilities of the shell.  The
basic command is @code{mcron [OPTION ...] [file ...]}  which has the
effect of reading all the configuration files specified (subject to
the options) and then waiting until it is time to execute some
command.  If no files are given on the command line, then mcron will
look in the user's cron configuration directories: these are ~/.cron
(deprecated), the directory indicated by the @code{XDG_CONFIG_HOME}
environment variable, or ~/.config/cron if this variable is not set.
In any case, files which end in the extension .vixie or .vix will be
assumed to contain Vixie-style crontabs, and files ending .guile or
.gle will be assumed to contain scheme code and will be executed as
such; ANY OTHER FILES WILL BE IGNORED - specify a file name of ``-''
and then pipe the files into the standard input if you really want to
read them, possibly using the @code{stdin} option to specify the type
of file.

The program accepts the following options.

@table @option
@item -s count
@itemx --schedule=count
@cindex printout of jobs schedule
@cindex schedule of jobs, listing
@cindex options, schedule
@cindex options, -s
@cindex -s option
@cindex --schedule option
With this option specified no commands are run.  Instead, the program
computes the times the commands would be run and prints the
information to the screen, and then immediately exits.

The count indicates the number of commands to display.

@cindex daemon option
@cindex options, daemon
@cindex options, -d
@cindex -d option
@cindex --daemon option
@item -d
@itemx --daemon
With this option the program will detach itself from the controlling
terminal and run as a daemon process.

@cindex stdin option
@cindex options, stdin
@cindex options, -i
@cindex -i option
@cindex --stdin option
@cindex standard input, configuring from
@cindex configuring from standard input
@item -i (vixie|guile)
@itemx --stdin=(vixie|guile)
This option is used to indicate whether the configuration information
being passed on the standard input is in Vixie format or Guile
format.  Guile is the default.

@cindex log-format option
@cindex options, log-format
@cindex --log-format option
@cindex logging output, configuration
@cindex configuring the logging output
@anchor{--log-format}
@item --log-format
This option accepts an @code{(ice-9 format)} format string that can be used to
customize the appearance of the output.  The format string is applied to
@code{format} with the following four arguments:
@enumerate
@item A date/time string.
@item The job process PID (as as number).
@item The action name.
@item The message to log.
@end enumerate

It defaults to @code{"~a ~2@*~a: ~a~%"}, which produces output messages like:

@example
2021-08-17T12:01:01 some-job: completed in 0.218s
@end example

If you'd rather see the job process PID instead of a timestamp, you could
instead specify the format string as @code{"~1@*~a ~a: ~a~%"}, which would
result in something like:

@example
39234 some-job: completed in 0.218s
@end example

To learn about all the possibilities offered by @code{(ice-9 format)}, refer
to @ref{Formatted Output,,, guile, GNU@tie{}Guile@tie{}Reference@tie{}Manual}.

@cindex date-format option
@cindex options, date-format
@cindex --date-format option
@cindex timestamp, modification
@cindex changing the default timestamp
@anchor{--date-format}
@item --date-format
This option accepts a @code{(srfi srfi-19)} date string format, to customize
the appearance of the timestamp in output messages.  It defaults to
@code{"~5"}, which corresponds to a local ISO-8601 date/time format
(@pxref{SRFI-19 Date to string,,,
guile,GNU@tie{}Guile@tie{}Reference@tie{}Manual}).

@cindex -v option
@cindex --version option
@cindex options, -v
@cindex options, version
@item -v
@itemx --version
This option causes a message to be printed on the standard output with
information about the version and copyright for the current program.

@cindex -h option
@cindex --help option
@cindex options, -h
@cindex options, --help
@item -h
@itemx --help
This causes a short but complete usage message to be displayed on
standard output.

@end table

@node Invoking cron or crond, Invoking crontab, Invoking mcron, Invoking
@section Invoking cron or crond
@cindex cron, invokation
@cindex invoking cron
@cindex crond, invokation
@cindex invoking crond
@cindex @value{CONFIG_SPOOL_DIR}
@cindex @value{CONFIG_SOCKET_FILE}
NOTE THAT THIS SECTION ONLY APPLIES IF THE @code{cron} or
@code{crond}, and @code{crontab} PROGRAMS HAVE BEEN INSTALLED BY THE
SYSTEM ADMINISTRATOR.

If the program runs by the name of @code{cron} or @code{crond}, then
it will read all the files in @code{@value{CONFIG_SPOOL_DIR}} (which
should only be readable by root) and the file @code{/etc/crontab}, and
then detaches itself from the terminal to live forever as a daemon
process.  Additionally, it creates a UNIX socket at
@code{@value{CONFIG_SOCKET_FILE}}, and listens for messages sent to
that socket consisting of a user name whose crontabs have been
changed.  In this case, the program will re-read that user's crontab.
This is for correct functioning with the crontab program.

Further, unless the @code{--noetc} option is used, a job is scheduled to run
every minute to check if @code{/etc/crontab} has been modified.  If so, this
file will also be re-read.

The options which may be used with this program are as follows.

@table @option

@cindex -v option
@cindex --version option
@cindex options, -v
@cindex options, version
@item -v
@itemx --version
This option causes a message to be printed on the standard output with
information about the version and copyright for the current program.

@cindex -h option
@cindex --help option
@cindex options, -h
@cindex options, --help
@item -h
@itemx --help
This causes a short but complete usage message to be displayed on
standard output.

@item -s [count]
@itemx --schedule[=count]
@cindex printout of jobs schedule
@cindex schedule of jobs, listing
@cindex options, schedule
@cindex options, -s
@cindex -s option
@cindex --schedule option
With this option specified no commands are run.  Instead, the program
computes the times the commands would be run and prints the
information to the screen, and then immediately exits.

The count, if supplied, indicates the number of commands to
display.  The default value is 8.

@cindex -n option
@cindex --noetc option
@cindex options, -n
@cindex options, --noetc
@item -n
@itemx --noetc
This tells cron not to add a job to the system which wakes up every
minute to check for modifications to @code{/etc/crontab}.  It is
recommended that this option be used (and further that the
@code{/etc/crontab} file be taken off the system altogether!)

@item --log-format
Analogous to mcron's @ref{--log-format}.

@item --date-format
Analogous to mcron's @ref{--date-format}.

@end table

@node Invoking crontab, Behaviour on laptops, Invoking cron or crond, Invoking
@section Invoking crontab
@cindex crontab, invoking
@cindex invoking crontab
This program is run by individual users to inspect or modify their
crontab files.  If a change is made to the file, then the root daemon
process will be given a kick, and will immediately read the new
configuration.  A warning will be issued to standard output if it
appears that a cron daemon is not running.

The command is used as

@code{crontab [-u user] file}

or

@code{crontab [-u user] ( -l | -e | -r )}

Only the root user can use the -u option, to specify the manipulation
of another user's crontab file.  In the first instance, the entire
crontab file of the user is replaced with the contents of the
specified file, or standard input if the file is ``-''.

In the latter case, the program behaves according to which of the
(mutually exclusive) options was given (note that the long options are
an mcron extension).

@table @option

@cindex -l option
@cindex list option, crontab
@cindex options, -l
@cindex options, --list
@cindex viewing a crontab
@cindex listing a crontab
@item -l
@itemx --list
Print the user's crontab file to the standard output, and exit.

@cindex -r option
@cindex remove option
@cindex options, -r
@cindex options, --remove
@cindex deleting a crontab
@cindex removing a crontab
@item -r
@item --remove
Delete the user's crontab file, and exit.

@cindex -e option
@cindex edit option
@cindex options, -e
@cindex options, --edit
@cindex editing a crontab
@cindex creating a crontab
@item -e
@item --edit
Using the editor specified in the user's VISUAL or EDITOR environment
variables, allow the user to edit their crontab.  Once the user exits the
editor, the crontab is checked for parseability, and if it is okay
then it is installed as the user's new crontab and the daemon is
notified that a change has taken place, so that the new file will
become immediately effective.

@end table


@node Behaviour on laptops, Exit codes, Invoking crontab, Invoking
@section Behaviour on laptops
@cindex laptops
@cindex power suspend
While mcron has not been designed to work anachronistically, the behaviour of
mcron when a laptop emerges from a suspended state is well defined, and the
following description explains what happens in this situation.

When a laptop awakes from a suspended state, all jobs which would have run while
the laptop was suspended will run exactly once immediately (and simultaneously)
when the laptop awakes, and then the next time that those jobs run will be
computed based on the time the laptop was awoken.  Any jobs which would not have
run during the suspense period will be unaffected, and will still run at their
proper times.


@node Exit codes,  , Behaviour on laptops, Invoking
@section Exit codes
@cindex exit codes
@cindex error conditions
@cindex errors
The following are the status codes returned to the operating system
when the program terminates.

@table @asis
@item 0
No problems.

@item 1
An attempt has been made to start cron but there is already a
@value{CONFIG_PID_FILE} file.  If there really is no other cron daemon
running (this does not include invokations of mcron) then you should
remove this file before attempting to run cron.

@item 2
In parsing a guile configuration file, a @code{job} command has been
seen but the second argument is neither a procedure, list or
string.  This argument is the job's action, and needs to be specified
in one of these forms.

@item 3
In parsing a guile configuration file, a @code{job} command has been
seen but the first argument is neither a procedure, list or
string.  This argument is the job's next-time specification, and needs
to be specified in one of these forms.

@item 4
An attempt to run cron has been made by a user who does not have
permission to access the crontabs in @value{CONFIG_SPOOL_DIR}.  These
files should be readable only by root, and the cron daemon must be run
as root.

@item 5
An attempt to run mcron has been made, but there are no jobs to
schedule!

@item 6
The system administrator has blocked this user from using crontab with
the files @value{CONFIG_ALLOW_FILE} and @value{CONFIG_DENY_FILE}.

@item 7
Crontab has been run with more than one of the arguments @code{-l},
@code{-r}, @code{-e}.  These are mutually exclusive options.

@item 8
Crontab has been run with the -u option by a user other than
root.  Only root is allowed to use this option.

@item 9
An invalid vixie-style time specification has been supplied.

@item 10
An invalid vixie-style job specification has been supplied.

@item 11
A bad line has been seen in /etc/crontab.

@item 12
The last component of the name of the program was not one of
@code{mcron}, @code{cron}, @code{crond} or @code{crontab}.

@item 13
Either none of the user's configuration directories exist, or there is a problem
reading the files there.  The configuration directories are ~/.cron
and the directory pointed to by the @code{XDG_CONFIG_HOME} environment
variable, or ~/.config/cron if this is not set.

@c @item 14
@c There is a problem writing to /var/cron/update.  This is probably
@c because the crontab program is not installed SUID root, as it should
@c be.

@item 15
Crontab has been run without any arguments at all.  There is no default
behaviour in this case.

@item 16
Cron has been run by a user other than root.

@end table



@node Guile modules, Index, Invoking, Top
@chapter Guile modules
Some of the key parts of mcron are implemented as modules so they can
be incorporated into other Guile programs, or even into C-sourced
programs if they are linked against libguile.

It may be, for example, that a program needs to perform house-keeping
functions at certain times of the day, in which case it can spawn
(either fork or thread) a sub-process which uses a built-in
mcron.  Another example may be a program which must sleep until some
non-absolute time specified on the Gregorian calendar (the first day
of next week, for example).  Finally, it may be the wish of the user to
provide a program with the functionality of mcron plus a bit extra.

The base module maintains mcron's internal job lists, and provides the
main wait-run-wait loop that is mcron's main function.  It also
introduces the facilities for accumulating a set of environment
modifiers, which take effect when jobs run.

@menu
* The base module::             The job list and execution loop.
* The redirect module::         Sending output of jobs to a mail box.
* The vixie-time module::       Parsing vixie-style time specifications.
* The job-specifier module::    All commands for scheme configuration files.
* The vixie-specification module::  Commands for reading vixie-style crontabs.
@end menu

@node The base module, The redirect module, Guile modules, Guile modules
@section The base module
@cindex guile module
@cindex base module
@cindex modules, base

This module may be used by including @code{(use-modules (mcron base))}
in a program.  The main functions are @code{add-job} and
@code{run-job-loop}, which allow a program to create a list of job
specifications to run, and then to initiate the wait-run-wait loop
firing the jobs off at the requisite times.  However, before they are
introduced two functions which manipulate the environment that takes
effect when a job runs are defined.

@cindex environment
The environment is a set of name-value pairs which is built up
incrementally.  Each time the @code{add-job} function is called, the
environment modifiers that have been accumulated up to that point are
stored with the new job specification, and when the job actually runs
these name-value pairs are used to modify the run-time environment in
effect.

@deffn{Scheme procedure} append-environment-mods name value
When a job is run make sure the environment variable @var{name} has
the value @var{value}.
@end deffn

@deffn{Scheme procedure} clear-environment-mods
This procedure causes all the environment modifiers that have been
specified so far to be forgotten.
@end deffn

@deffn{Scheme procedure} add-job time-proc action displayable @
  configuration-time configuration-user @
  [#:schedule @var{%global-schedule}]
This procedure adds a job specification to the list of all jobs to
run.  @var{time-proc} should be a procedure taking exactly one argument
which will be a UNIX time.  This procedure must compute the next time
that the job should run, and return the result.  @var{action} should be
a procedure taking no arguments, and contains the instructions that
actually get executed whenever the job is scheduled to
run.  @var{displayable} should be a string, and is only for the use of
humans; it can be anything which identifies or simply gives a clue as
to the purpose or function of this job.  @var{configuration-time} is
the time from which the first invokation of this job should be
computed.  Finally, @var{configuration-user} should be the passwd entry
for the user under whose personality the job is to run.
@end deffn

@deffn{Scheme procedure} run-job-loop @var{fd-list} @
  [#:schedule @var{%global-schedule}]
@cindex file descriptors
@cindex interrupting the mcron loop
This procedure returns only under exceptional circumstances, but
usually loops forever waiting for the next time to arrive when a job
needs to run, running that job, recomputing the next run time, and
then waiting again.  However, the wait can be interrupted by data
becoming available for reading on one of the file descriptors in the
fd-list, if supplied.  Only in this case will the procedure return to
the calling program, which may then make modifications to the job list
before calling the @code{run-job-loop} procedure again to resume execution of
the mcron base.
@end deffn

@deffn{Scheme procedure} remove-user-jobs user @
  [#:schedule @var{%global-schedule}]
The argument @var{user} should be a string naming a user (their
login name), or an integer UID, or an object representing the user's passwd
entry.  All jobs on the current job list that are scheduled to be run
under this personality are removed from the job list.
@end deffn

@deffn{Scheme procedure} display-schedule @var{count} [@var{port}] @
  [#:schedule @var{%global-schedule}]
@cindex schedule of jobs
This procedure is used to display a textual list of the next COUNT jobs
to run.

The argument @var{count} must be an integer value giving the number
of time-points in the future to report that jobs will run as.  Note
that this procedure is disruptive; if @code{run-job-loop} is called
after this procedure, the first job to run will be the one after the
last job that was reported in the schedule report.  The report itself
is returned to the calling program as a string.
@end deffn

@defopt %date-format
@cindex parameters, date format
@cindex date format parameter
This parameter holds the @code{(srfi srfi-19)} format string used to produce
the timestamp found in output messages.  It defaults to @code{"~5"}.
@end defopt

@deffn{Scheme procedure} validate-date-format @var{fmt}
@cindex date format validator
This procedure is used to validate @var{fmt}, a @code{(srfi srfi-19)} format
string.  When @var{fmt} is invalid, an error message is displayed and the
program is aborted.
@end deffn

@defopt %log-format
@cindex parameters, log format
@cindex log format parameter
This parameter holds the @code{(ice-9 format)} format string used to produce
the output messages.  The four arguments applied to format are the timestamp,
the process PID, the job name and the message.  It defaults to @code{"~a
~2@*~a: ~a~%"}.
@end defopt

@deffn{Scheme procedure} validate-log-format @var{fmt}
@cindex log format validator
This procedure is used to validate @var{fmt}, a @code{(ice-9 format)} format
string.  When @var{fmt} is invalid, an error message is displayed and the
program is aborted.
@end deffn

@node The redirect module, The vixie-time module, The base module, Guile modules
@section The redirect module
@cindex redirect module
@cindex modules, redirect

This module is introduced to a program with the command
@code{(use-modules (mcron redirect))}.

This module provides the @code{with-mail-out} function, described
fully in @ref{Guile Syntax}.

@node The vixie-time module, The job-specifier module, The redirect module, Guile modules
@section The vixie-time module
@cindex vixie-time module
@cindex modules, vixie-time

This module is introduced to a program by @code{(use-modules (mcron
vixie-time))}.

This module provides a single method for converting a vixie-style time
specification into a procedure which can be used as the
@code{next-time-function} to the base @code{add-job} procedure, or to
the @code{job-specifier} @code{job} procedure.  See @ref{Vixie Syntax}
for full details of the allowed format for the time string.

@deffn{Scheme procedure} parse-vixie-time time-string
The single argument @var{time-string} should be a string containing a
vixie-style time specification, and the return value is the required
procedure.
@end deffn


@node The job-specifier module, The vixie-specification module, The vixie-time module, Guile modules
@section The job-specifier module
@cindex job-specifier module
@cindex modules, job-specifier

This module is introduced to a program by @code{(use-modules (mcron
job-specifier))}.

This module provides all the functions available to user's Guile
configuration files, namely @code{range}, @code{next-year-from},
@code{next-year}, @code{next-month-from}, @code{next-month},
@code{next-day-from}, @code{next-day}, @code{next-hour-from},
@code{next-hour}, @code{next-minute-from}, @code{next-minute},
@code{next-second-from}, @code{next-second},
 and last but not least, @code{job}.  See @ref{Guile Syntax} for full
 details.

Once this module is loaded, a scheme configuration file can be used to
put jobs onto the job list simply by @code{load}ing the file.

@node The vixie-specification module,  , The job-specifier module, Guile modules
@section The vixie-specification module
@cindex vixie-specification module
@cindex modules, vixie-specification

To use this module, put the command @code{(use-modules (mcron
vixie-specification))} into your program.

This module exports a couple of functions for adding jobs to the
internal job list according to a Vixie-style crontab file.

@deffn{Scheme procedure} read-vixie-port port . parse-line

This procedure reads a crontab from the given port, and adds jobs to
the job list accordingly, taking care of environment specifications
and comments which may appear in such a file.

@var{parse-line} should not normally be used, except that if you are
parsing a (deprecated) @code{/etc/crontab} file with a slightly
modified syntax, you may pass the value @var{parse-system-vixie-line}
as the optional argument.

@end deffn

@deffn{Scheme procedure} read-vixie-file name . parse-line

This procedure attempts to open the named file, and if it fails will
return silently.  Otherwise, the behaviour is identical to
@code{read-vixie-port} above.

@end deffn

Once this module has been declared in a program, a crontab file can be
used to augment the current job list with a call to
@code{read-vixie-file}.

@node GNU Free Documentation License
@appendix GNU Free Documentation License

@include fdl.texi

@node Index,  , Guile modules, Top
@unnumbered Index

@printindex cp

@bye