1) View:
the SET - lists all environment variables and their values;
the SET XX-- view only XX variables and their values;
2) Modify:
the SET xxx = XX - XX variable to the value xxx;
the SET XX =% XX% yy - this is another convenient way of modification, the variable value to XX 'of the original value after the additional yy'. For example, for the path variable, its original value is very long, usually we modify its value is increased when a string on the basis of its original value, so in this way more convenient, without the need to copy and then modify the original value.
3) Delete:
the SET XX = - ie given a null value, put the XX variable deleted.
4) add:
the SET XX = xxx - in fact, the same modification is the same, no variable XX XX, then you add the variable, modify the value of some words of XX.
All modifications to the environment variables only for the current active window in cmd command line, not permanent changes. That is when the close the cmd command line window will no longer work.