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11.10.2 Execução de trabalho agendado

The (gnu services mcron) module provides an interface to GNU mcron, a daemon to run jobs at scheduled times (veja 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 (veja Invoking updatedb em Finding Files) and the guix gc commands (veja Invocando guix gc) daily, as well as the mkid command on behalf of an unprivileged user (veja mkid invocation em ID Database Utilities). It uses gexps to introduce job definitions that are passed to mcron (veja Expressões-G).

(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 ()
           (system* (string-append #$findutils "/bin/updatedb")
                    "--prunepaths=/tmp /var/tmp /gnu/store"))

(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"))

  ;; …

  ;; %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
                                   (list garbage-collector-job

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!

Tip: Avoid calling the Guile procedures execl, execle or execlp inside a job specification, else mcron won’t be able to output the completion status of the job.

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 (veja Expressões-G). The example below illustrates that.

(define %battery-alert-job
  ;; Beep when the battery percentage falls below %MIN-LEVEL.
     '(next-minute (range 0 60 1))
        (with-imported-modules (source-module-closure
                                '((guix build utils)))
              (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*
                                      #$(file-append acpi "/bin/acpi")))
                         (output (get-string-all input-pipe))
                         (m (string-match "Discharging, ([0-9]+)%" output))
                         (level (string->number (match:substring m 1)))
                         ((< level %min-level)))
                (format #t "warning: Battery level is low (~a%)~%" level)
                (invoke #$(file-append beep "/bin/beep") "-r5")))))))

Veja mcron job specifications em 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
Variável: 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 (veja Composição de serviço). In other words, it is possible to define services that provide additional mcron jobs to run.

Data Type: mcron-configuration

Available mcron-configuration fields are:

mcron (default: mcron) (type: file-like)

The mcron package to use.

jobs (default: '()) (type: list-of-gexps)

This is a list of gexps (veja Expressões-G), where each gexp corresponds to an mcron job specification (veja mcron job specifications em GNU mcron).

log? (default: #t) (type: boolean)

Log messages to standard output.

log-file (default: "/var/log/mcron.log") (type: string)

Log file location.

log-format (default: "~1@*~a ~a: ~a~%") (type: string)

(ice-9 format) format string for log messages. The default value produces messages like ‘pid name: message’ (veja Invoking em GNU mcron). Each message is also prefixed by a timestamp by GNU Shepherd.

date-format (type: maybe-string)

(srfi srfi-19) format string for date.

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