The class of attributes which correspond to environment variables.
Environment variables are defined outside of a ComputerProgram, unlike
ordinary variables that are defined in the source code of the
ComputerProgram. Typically, the environment variable stores some value
that many if not all ComputerProgams will need when they execute. An
example is the environment variable PATH under Unix-like operating
systems that stores the ComputerDirectories where executable
ComputerPrograms can be found. Another example is the environment
variable CLASSPATH for Java programs, which stores the directory where
Java class files can be found that will be needed by any
ComputerProgram written in Java.