What variable can be used in Techniques and Directives and with what syntax?

In Rudder, you have several types of variables that can be used in Technique and Directive and expanded when the policy are generated or by the agent on the target node depending of your need. Of course, the context is not the same in each case, and you won't have access to exactly the same variable.

The main types of variables are:

• Parameters: they are global variables
  • https://www.rudder-project.org/doc-4.2/_variables.html#_user_defined_parameters
• A tiny subset of node information (node ID, hostname, policy server ID, root user)
  •  https://www.rudder-project.org/doc-4.2/_variables.html#_system_variables
• Node properties, which are defined by node (with optionnaly a default value) and can be overriden on the node
  • https://www.rudder-project.org/doc-4.2/_node_properties.html#_node_properties
• Techniques that define variables, like Generic Variable Defintion. That technique can be use with an override semantic to define more specific values on some groups
  •  https://www.rudder-project.org/doc-4.2/_directives_ordering.html#_special_use_case_overriding_generic_variable_definition

A must-have cheatsheet with the syntax for each case is available here: https://github.com/Normation/rudder-tools/blob/master/documents/cheatsheet-advanced/rudder-cheatsheet-advanced.pdf