Linking is a process whereby templates are applied to hosts, whereas unlinking removes the association with the template from a host.
Templates are linked directly to individual hosts and not to host groups. Simply adding a template to a host group will not link it. Host groups are used only for logical grouping of hosts and templates.
To link a template to the host, do the following:
The host will now have all the entities (items, triggers, graphs, etc) of the template.
Linking multiple templates to the same host will fail if in those templates there are items with the same item key. And, as triggers and graphs use items, they cannot be linked to a single host from multiple templates either, if using identical item keys.
When entities (items, triggers, graphs etc.) are added from the template:
In the lists, all entities from the template now are prefixed by the template name, indicating that these belong to the particular template. The template name itself (in gray text) is a link allowing to access the list of those entities on the template level.
If some entity (item, trigger, graph etc.) is not prefixed by the template name, it means that it existed on the host before and was not added by the template.
When adding entities (items, triggers, graphs etc.) from a template it is important to know what of those entities already exist on the host and need to be updated and what entities differ. The uniqueness criteria for deciding upon the sameness/difference are:
To update template linkage of many hosts, in Configuration → Hosts select some hosts by marking their checkboxes, then click on Mass update below the list and then in the Templates tab select to link additional templates:
Select Link templates and start typing the template name in the auto-complete field until a dropdown appears offering the matching templates. Just scroll down to select the template to link.
The Replace option will allow to link a new template while unlinking any template that was linked to the hosts before. The Unlink option will allow to specify which templates to unlink. The Clear when unlinking option will allow to not only unlink any previously linked templates, but also remove all elements inherited from them (items, triggers, etc.).
Zabbix offers a sizable set of predefined templates. You can use these for reference, but beware of using them unchanged in production as they may contain too many items and poll for data too often. If you feel like using them, finetune them to fit you real needs.
If you try to edit an item or trigger that was linked from the template, you may realize that many key options are disabled for editing. This makes sense as the idea of templates is that things are edited in one-touch manner on the template level. However, you still can, for example, enable/disable an item on the individual host and set the update interval, history length and some other parameters.
If you want to edit the entity fully, you have to edit it on the template level (template level shortcut is displayed in the form name), keeping in mind that these changes will affect all hosts that have this template linked to them.
To unlink a template from a host, do the following:
Choosing the Unlink option will simply remove association with the template, while leaving all its entities (items, triggers, graphs etc.) with the host.
Choosing the Unlink and clear option will remove both the association with the template and all its entities (items, triggers, graphs etc.).