* approach to tackling the assignment

    !!!! do it incrementally, write unit tests as you go,
         rerun all of your unit tests when you add anything new !!!!

    > design as you go (incrementally) and ask for input about your design
      before implementation if you've got questions/uncertainty

* the environment

** the process environment

    extern char **environ;

    a null terminated array (i.e. vector) of pointers to char
    each element in the array is a name=value string
    functions putenv, getenv, and setenv all use and/or modify environ

** the shell environment is different

    varlib.h/c provides a data structure for maintaining the shell env
    it's also an array of ptrs to string with name=value
    it knows which variables are exported
    an exported variable is included in the environment when executing a
    child process
    on shell startup, you call VLenviron2table to copy the process env into
    the shell env
    before calling exec, you call VLtable2environ to load the exported
    variables into environ

** on fork, the child has a copy of the parent's memory (with some exceptions)
   on exec, the memory is cleared (mostly) when the new process is loaded
   the process environment (environ) _isn't_ cleared, so if it has all of
   the exported variables from varlib, the child can still see them after
   the exec call

*