ENVIRON$ returns a DOS environment string.
ENVIRON changes or adds an environment string in the DOS environment table.
 
ENVIRON$ (env-variable$)
ENVIRON$ (n%)
ENVIRON stringexpression$
 
    - env-variable$        The name of a DOS environment variable.
    - n%                   Specifies that ENVIRON$ returns the nth string
                           from the environment string table.
    - stringexpression$    The name and setting of a DOS environment
                           variable (such as PATH or PROMPT) in one of the
                           following forms:
 
                           env-variable$=env-string$
                           env-variable$ env-string$
 
    - Changes made by the ENVIRON statement are erased when the program ends.
 
Example:
    ENVIRON "PATH=TEST"
    PRINT ENVIRON$("PATH")