SET
Displays, sets, or removes cmd.exe environment variables.
SET [variable=[string]]
variable Specifies the environment-variable name.
string Specifies a series of characters to assign to the variable.
Type SET without parameters to display the current environment variables.
SET command invoked with just a variable name, no equal sign or value
will display the value of all variables whose prefix matches the name
given to the SET command. For example:
SET P
would display all variables that begin with the letter 'P'
SET command will set the ERRORLEVEL to 1 if the variable name is not
found in the current environment.
SET command will allow an equal sign (=) in the value of an environment
variable in any position other than the first character.