If you’re using the fantastic Nextcloud and like to feel comfortable browsing all your auto-uploaded photos like once upon a time you did in some proprietary Picasa or so, you can benefit from Preview Generator, which will take care of pre-rendering the thumbnails.

As it states you must add some cron magic to get it working and the Nextcloud folks asked for a tutorial, here’s how to do it with cron or (my preference) with systemd timers.

Prerequisites

  • we need to setup the job as the system user running Nextcloud (e.g. www-data, nginx, httpd, … - depends on your distribution). You can find out by looking at the owner of the nextcloud files using ls -la
  • we need to use the absolut path to the php executable (which php)
  • we need to use the absolut path to Nextclouds occ file, depends in which folder your Nextcloud has been installed (e.g. /var/www/nextcloud/occ)

Cron

Add or create a new cronjob for the Nextcloud user:

crontab -e -u www-data

Using your favourite editor, add something like this, which will run the job at 04:00:

0 4 * * * /usr/bin/php -f /var/www/nextcloud/occ preview:pre-generate

Systemd timers

Systemd also has the possibility to run specific tasks at specific times or events. called Timers. You need to create 2 unit files:

/etc/systemd/system/nextcloud-preview-generator.service

[Unit]
Description=Nextcloud Preview Generator

[Service]
Type=oneshot
User=www-data
ExecStart=/usr/bin/php -f /var/www/nextcloud/occ preview:pre-generate

[Install]
WantedBy=basic.target

/etc/systemd/system/nextcloud-preview-generator.timer

[Unit]
Description=Run Nextcloud Preview Generator daily at 04:00

[Timer]
OnCalendar=*-*-* 4:00:00
Persistent=true

[Install]
WantedBy=timers.target
  • systemd reload systemctl daemon-reload
  • activate the timer systemctl enable nextcloud-preview-generator.timer
  • start the timer systemctl start nextcloud-preview-generator.timer