emqx/changes/ce/feat-13507.en.md

Added a new builtin function getenv in the rule engine and variform expression to access the environment variables with below limitations.

• Prefix EMQXVAR_ is added before reading from OS environment variables. i.e. getenv('FOO_BAR') is to read EMQXVAR_FOO_BAR.
• The values are immutable once loaded from the OS environment.