1- unix of course. The simple fact of compiling and linking a program, or writing a script, changes the unix environment since you can then on use it as a new c

1- unix of course. The simple fact of compiling and linking a program, or writing a script, changes the unix environment since you can then on use it as a new command. 2- MPW, the Macintosh Programmer Workshop, which was influenced by the unix design and also by emacs did let you write new commands, and install them under menus. 3- any application with some kind of scriptability feature where you can add new scripts that you can use as new command would fit the bill.