implement builtins when one of us is available

[faboussard]

Your shell must implement the following builtins: ◦ echo with option -n ◦ cd with only a relative or absolute path ◦ pwd with no options ◦ export with no options ◦ unset with no options (setenv PATH /bin put back the env) ◦ env with no options or arguments ◦ exit with no options

[faboussard]

retirer la variable denvironnement speciale ? de export et env ( elle est contenue a la fin de la hashmap des variables denvirommenet)