A batch file might have the following line:
%PATH% is an environment variable whose value is taken from the
current environment and appended to the new PATH definition. If this
is done at the command line by typing the following line, the PATH
environment variable's value is not substituted, so the existing PATH
will not get appended to the new PATH ("%PATH%" would be appended
The ability to use environment variable substitution is not restricted
to existing MS-DOS environment variable names. Any variable that is
defined in the environment can be extended using the method above.
If the variable specified by the SET command is not defined in the
environment, the variable stays the same if used at the prompt, and is
replaced by a null expression if used in a batch file.
For example, type the following command in which XYZZY is an undefined
The commands result in the following if used at the command-line
The commands result in the following if used in a batch file: