system.env docs

COMMAND-System.md

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+# The `system.*` Command Group
+
+
+## system.env=NAME (since 0.9.7)
+
+Query the value of an environment variable, returns an empty string if `NAME` is not defined. Example:
+
+```ini
+session.path.set=(cat,(system.env,RTORRENT_HOME),"/.session")
+```