Re: request for review: Doing direct file I/O in Emacs Lisp

[Richard Stallman]

    Requiring all data to pass through a buffer, in order to save it to
    disk, is difficult to handle efficiently.  Because the whole file must
    be stored in the buffer before being saved, 

write-region can append to a file.  That's probably the easiest
way to handle eshell builtins.

write-region can also append the contents of a string.  So you don't
have to construct the builtin's output in a buffer--you can do it
in a string.  However, a buffer will be more efficient in most
cases.