mcron 1.0.3

Table of Contents

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

--- The Detailed Node Listing ---

Simple examples

Full available syntax

Extended Guile examples


Detailed invoking

Guile modules

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1 Introducing 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).

A full discussion of the design and philosophy of mcron can be found in the white paper at

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2 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.

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2.1 Guile

You have an executable my-program in your home directory, which you want to run every hour. Create a file job.guile in directory ~/.cron with the following contents

     (job '(next-hour) "my-program")

then run the command mcron.

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

           (next-hour (range 0 24 2))

and run the command mcron.

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

     (job "15 */2 * * *" "my-program")

and run the 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 .cron directory with the .guile extension. Alternatively, you can use any file you want and pass it as an argument to mcron, or even pipe the commands into the standard input.

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2.2 Vixie

You have an executable my-program in your home directory, which you want to run every hour. Create a file job.vixie in directory ~/.cron with the following contents

     0 * * * * my-program

then run the command mcron.

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

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3 Full available syntax

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3.1 Guile Syntax

3.1.1 Job specification

In Guile-formatted configuration files each command that needs executing is introduced with the 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.

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:

(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). 1

Similarly to next-second-from, there are also next-minute-from, next-hour-from, next-day-from, next-week-from, next-month-from, next-year-from.

Furthermore, the optional argument can be fulfilled by the function (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 (range 0 10 2) will yield the list '(0 2 4 6 8).

If the first argument to the job function is a list, it is taken to be program code made up of the functions (next-second . args), (next-minute...), etc, where the optional arguments can be supplied with the (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 core when the list is eval'd).

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

The second argument to the (job) function can be either a string, a list, or a function. In all cases the command is executed in the user's home directory, under the user's own UID. 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.

3.1.2 Sending output as e-mail

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.

3.1.3 Setting environment variables

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.

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3.2 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.

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3.2.1 Synthesizing “at” commands

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

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

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3.2.2 Every second Sunday

To run 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!).

     (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)
                                      (- 14 first-day))))
              (+ next-month (* 24 60 60 second-sunday))))

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3.2.3 Two hours every day

Surprisingly perhaps, the following will not have the desired effect.

     (job '(next-hour-from (next-day) '(1 2))

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.

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

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3.2.4 Missing the first appointment

The command

     (job '(next-hour-from (next-day) '(16))

will run 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

     (job '(next-hour '(16))

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3.2.5 Penultimate day of every month

The following will run the my-program program on the second-to-last day of every month.

     (job '(- (next-month-from (next-month)) (* 48 3600))

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3.3 Vixie

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 his cron program, and his copyright notice is duly reproduced below.

There are three problems with this specification.

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).

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).

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.

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3.3.1 Paul Vixie's copyright

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.

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3.3.2 Crontab files

A crontab file contains instructions to the 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 su as part of a cron command.

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,

     name = value

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

Several environment variables are set up automatically by the 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.

(Another note: the LOGNAME variable is sometimes called USER on BSD systems... on these systems, USER will be set also.) 2

In addition to LOGNAME, HOME, and SHELL, 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 cron when the minute, hour, and month of year fields match the current time, and when at least one of the two day fields (day of month, or day of week) match the current time (see “Note” below). cron examines cron entries once every minute. The time and date fields are:

Field Allowed values
—— ——————–
minute 0-59
hour 0-23
day of month 0-31
month 0-12 (or names, see below)
day of week 0-7 (0 or 7 is Sun, or use names)

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

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.

Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: “1,2,5,9”, “0-4,8-12”.

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”.

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. 3

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.

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 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.


     # use /bin/sh to run commands, no matter what /etc/passwd says
     # mail any output to `paul', no matter whose crontab this is
     # 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"

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3.3.3 Extensions and incompatibilities

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.

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4 Detailed invoking

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.

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.

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4.1 Invoking mcron

Mcron should be run by the user who wants to schedule his jobs. It may be made a background job using the facilities of the shell. The basic command is 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 directory. In either 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 stdin option to specify the type of file.

The program accepts the following options.

-s [count]
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.

With this option the program will detach itself from the controlling terminal and run as a daemon process.

-i (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.

This option causes a message to be printed on the standard output with information about the version and copyright for the current program.

This causes a short but complete usage message to be displayed on standard output.

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4.2 Invoking cron or crond


If the program runs by the name of cron or crond, then it will read all the files in /var/cron/tabs (which should only be readable by root) and the file /etc/crontab, and then detaches itself from the terminal to live forever as a daemon process. Additionally, it creates a UNIX socket at /var/cron/socket, 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, if the --noetc option was not used, a job is scheduled to run every minute to check if /etc/crontab has been modified recently. If so, this file will also be re-read.

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

This option causes a message to be printed on the standard output with information about the version and copyright for the current program.

This causes a short but complete usage message to be displayed on standard output.
-s [count]
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.

This tells cron not to add a job to the system which wakes up every minute to check for modifications to /etc/crontab. It is recommended that this option be used (and further that the /etc/crontab file be taken off the system altogether!)

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4.3 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

crontab [-u user] file


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).

Print the user's crontab file to the standard output, and exit.

Delete the user's crontab file, and exit.

Using the editor specified in the user's VISUAL or EDITOR environment variables, allow the user to edit his 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.

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4.4 Behaviour on laptops

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.

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4.5 Exit codes

The following are the status codes returned to the operating system when the program terminates.

No problems.
An attempt has been made to start cron but there is already a /var/run/ 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.
In parsing a guile configuration file, a 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.
In parsing a guile configuration file, a 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.
An attempt to run cron has been made by a user who does not have permission to access the crontabs in /var/cron/tabs. These files should be readable only by root, and the cron daemon must be run as root.
An attempt to run mcron has been made, but there are no jobs to schedule!
The system administrator has blocked this user from using crontab with the files /var/cron/allow and /var/cron/deny.
Crontab has been run with more than one of the arguments -l, -r, -e. These are mutually exclusive options.
Crontab has been run with the -u option by a user other than root. Only root is allowed to use this option.
An invalid vixie-style time specification has been supplied.
An invalid vixie-style job specification has been supplied.
A bad line has been seen in /etc/crontab.
The last component of the name of the program was not one of mcron, cron, crond or crontab.
Either the ~/.cron directory does not exist, or there is a problem reading the files there.
Crontab has been run without any arguments at all. There is no default behaviour in this case.
Cron has been run by a user other than root.

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5 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 core 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.

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5.1 The core module

This module may be used by including (use-modules (mcron core)) in a program. The main functions are add-job and 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.

The environment is a set of name-value pairs which is built up incrementally. Each time the 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.

— Scheme procedure: append-environment-mods name value

When a job is run make sure the environment variable name has the value value.

— Scheme procedure: clear-environment-mods

This procedure causes all the environment modifiers that have been specified so far to be forgotten.

— Scheme procedure: add-job time-proc action displayable configuration-time configuration-user

This procedure adds a job specification to the list of all jobs to run. 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. action should be a procedure taking no arguments, and contains the instructions that actually get executed whenever the job is scheduled to run. 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. configuration-time is the time from which the first invokation of this job should be computed. Finally, configuration-user should be the passwd entry for the user under whose personality the job is to run.

— Scheme procedure: run-job-loop . fd-list

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 run-job-loop procedure again to resume execution of the mcron core.

— Scheme procedure: remove-user-jobs user

The argument user should be a string naming a user (his 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.

— Scheme procedure: get-schedule count

The argument count should 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 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.

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5.2 The redirect module

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

This module provides the with-mail-out function, described fully in Guile Syntax.

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5.3 The vixie-time module

This module is introduced to a program by (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 next-time-function to the core add-job procedure, or to the job-specifier job procedure. See Vixie Syntax for full details of the allowed format for the time string.

— Scheme procedure: parse-vixie-time time-string

The single argument time-string should be a string containing a vixie-style time specification, and the return value is the required procedure.

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5.4 The job-specifier module

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

This module provides all the functions available to user's Guile configuration files, namely range, next-year-from, next-year, next-month-from, next-month, next-day-from, next-day, next-hour-from, next-hour, next-minute-from, next-minute, next-second-from, next-second, and last but not least, job. See 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 loading the file.

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5.5 The vixie-specification module

To use this module, put the command (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.

— 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.

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

— 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 read-vixie-port above.

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 read-vixie-file.

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[1] 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.

[2] mcron has not been ported to BSD, so these notes are not relevant.

[3] Mcron allows any alphabetic characters after a name, so full names of days or months are also valid.