Returns the value of a system environment variable, if it exists.
The set of available environment variables is a set of (name, value) pairs forming part of the dynamic context, in which the name is unique within the set of pairs. The name and value are arbitrary strings.
$name argument matches the name of one of these pairs, the function
returns the corresponding value.
If there is no environment variable with a matching name, the function returns the empty sequence.
The collation used for matching names is implementation-defined, but must be the same as the collation used to ensure that the names of all environment variables are unique.
The function is deterministic, which means that if it is called several times within the same execution scope, with the same arguments, it must return the same result.
On many platforms, the term "environment variable" has a natural meaning in terms of facilities provided by the operating system. This interpretation of the concept does not exclude other interpretations, such as a mapping to a set of configuration parameters in a database system.
Environment variable names are usually case sensitive. Names are usually of the form
(letter|_) (letter|_|digit)*, but this varies by platform.
On some platforms, there may sometimes be multiple environment variables with the same name; in this case, it is implementation-dependent as to which is returned; see for example (Chapter 8, Environment Variables). Implementations may use prefixes or other naming conventions to disambiguate the names.
The requirement to ensure that the function is deterministic means in practice that the implementation must make a snapshot of the environment variables at some time during execution, and return values obtained from this snapshot, rather than using live values that are subject to change at any time.
Operating system environment variables may be associated with a particular process, while queries and stylesheets may execute across multiple processes (or multiple machines). In such circumstances implementations may choose to provide access to the environment variables associated with the process in which the query or stylesheet processing was initiated.
Security advice: Queries from untrusted sources should not be permitted unrestricted
access to environment variables. For example, the name of the account under which the
query is running may be useful information to a would-be intruder. An implementation may
therefore choose to restrict access to the environment, or may provide a facility to
fn:environment-variable always return the empty sequence.