(define-module (cronjobs)
  #:use-module (ice-9 match)
  #:use-module (guix monads)
  #:use-module (guix store)
  #:use-module (guix packages))

(define (battery-check)
  (display "Checking BAT0!")
  (let ((ex (execl (string-append #$upower "/bin/upower")
                   "upower"
                   "-i"
                   "/org/freedesktop/UPower/devices/line_power_ADP1")))
    (display ex)))

;; yields:
scheme@(guile-user)> (battery-check)
Checking BAT0!<unnamed port>:264:2: In procedure battery-check:
error: ungexp: unbound variable

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.