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10.8.2 Scheduled Job Execution

The (gnu services mcron) module provides an interface to GNU mcron, a daemon to run jobs at scheduled times (see GNU mcron). GNU mcron is similar to the traditional Unix cron daemon; the main difference is that it is implemented in Guile Scheme, which provides a lot of flexibility when specifying the scheduling of jobs and their actions.

The example below defines an operating system that runs the updatedb (see Invoking updatedb in Finding Files) and the guix gc commands (see Invoking guix gc) daily, as well as the mkid command on behalf of an unprivileged user (see mkid invocation in ID Database Utilities). It uses gexps to introduce job definitions that are passed to mcron (see G-Expressions). 

(use-modules (guix) (gnu) (gnu services mcron))
(use-package-modules base idutils)

(define updatedb-job
  ;; Run 'updatedb' at 3AM every day.  Here we write the
  ;; job's action as a Scheme procedure.
  #~(job '(next-hour '(3))
         (lambda ()
           (execl (string-append #$findutils "/bin/updatedb")
                  "updatedb"
                  "--prunepaths=/tmp /var/tmp /gnu/store"))
         "updatedb"))

(define garbage-collector-job
  ;; Collect garbage 5 minutes after midnight every day.
  ;; The job's action is a shell command.
  #~(job "5 0 * * *"            ;Vixie cron syntax
         "guix gc -F 1G"))

(define idutils-job
  ;; Update the index database as user "charlie" at 12:15PM
  ;; and 19:15PM.  This runs from the user's home directory.
  #~(job '(next-minute-from (next-hour '(12 19)) '(15))
         (string-append #$idutils "/bin/mkid src")
         #:user "charlie"))

(operating-system
  ;; …

  ;; %BASE-SERVICES already includes an instance of
  ;; 'mcron-service-type', which we extend with additional
  ;; jobs using 'simple-service'.
  (services (cons (simple-service 'my-cron-jobs
                                   mcron-service-type
                                   (list garbage-collector-job
                                         updatedb-job
                                         idutils-job))
                  %base-services)))

Tip: When providing the action of a job specification as a procedure, you should provide an explicit name for the job via the optional 3rd argument as done in the updatedb-job example above. Otherwise, the job would appear as “Lambda function” in the output of herd schedule mcron, which is not nearly descriptive enough!

For more complex jobs defined in Scheme where you need control over the top level, for instance to introduce a use-modules form, you can move your code to a separate program using the program-file procedure of the (guix gexp) module (see G-Expressions). The example below illustrates that. 

(define %battery-alert-job
  ;; Beep when the battery percentage falls below %MIN-LEVEL.
  #~(job
     '(next-minute (range 0 60 1))
     #$(program-file
        "battery-alert.scm"
        (with-imported-modules (source-module-closure
                                '((guix build utils)))
          #~(begin
              (use-modules (guix build utils)
                           (ice-9 popen)
                           (ice-9 regex)
                           (ice-9 textual-ports)
                           (srfi srfi-2))

              (define %min-level 20)

              (setenv "LC_ALL" "C")     ;ensure English output
              (and-let* ((input-pipe (open-pipe*
                                      OPEN_READ
                                      #$(file-append acpi "/bin/acpi")))
                         (output (get-string-all input-pipe))
                         (m (string-match "Discharging, ([0-9]+)%" output))
                         (level (string->number (match:substring m 1)))
                         ((< level %min-level)))
                (format #t "warning: Battery level is low (~a%)~%" level)
                (invoke #$(file-append beep "/bin/beep") "-r5")))))))

See mcron job specifications in GNU mcron, for more information on mcron job specifications. Below is the reference of the mcron service.

On a running system, you can use the schedule action of the service to visualize the mcron jobs that will be executed next: 

# herd schedule mcron

The example above lists the next five tasks that will be executed, but you can also specify the number of tasks to display: 

# herd schedule mcron 10
Scheme Variable: mcron-service-type

This is the type of the mcron service, whose value is an mcron-configuration object.

This service type can be the target of a service extension that provides additional job specifications (see Service Composition). In other words, it is possible to define services that provide additional mcron jobs to run.

Data Type: mcron-configuration

Data type representing the configuration of mcron.

mcron (default: mcron)

The mcron package to use.

jobs

This is a list of gexps (see G-Expressions), where each gexp corresponds to an mcron job specification (see mcron job specifications in GNU mcron).

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