Votre utilisateur doit etre autoriser à lancer une commande avec sudo sans taper de mot de passe pour pouvoir lancer cette commande en automatique dans un fichier crontab

Modifier le fichier /etc/sudoers

  sudo nano /etc/sudoers

ou avec

 sudo visudo

avec la ligne à la fin ( si votre utilisateur est pi )

 pi ALL=(ALL) NOPASSWD: /sbin/shutdown -h now

sudoers.txt

# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
# See the man page for details on how to write a sudoers file.
#
Defaults        env_reset
Defaults        mail_badpass
Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
 
# Host alias specification
 
# User alias specification
 
# Cmnd alias specification
 
# User privilege specification
root    ALL=(ALL:ALL) ALL
 
# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL
 
# See sudoers(5) for more information on "@include" directives:
 
@includedir /etc/sudoers.d
pi ALL=(ALL) NOPASSWD: /sbin/shutdown -h now

   crontab  -e     

en ajoutant la ligne ( pour arret à 23h50 chaque jour ) :

  50  23 * * * sudo shutdown -h now

Pour lire le fichier crontab

  crontab -l

crontab.txt

# Edit this file to introduce tasks to be run by cron.
# 
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
# 
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').
# 
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
# 
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
# 
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
# 
# For more information see the manual pages of crontab(5) and cron(8)
# 
# m h  dom mon dow   command
 
50  23 * * * sudo shutdown -h now