Re: [TUHS] moving directories in svr2

[jnc@mercury.lcs.mit.edu (Noel Chiappa)]

    > From: Clem Cole

    > Try it on V6 or V7 and you will get 'directory exists' as an error.

The V6 'mv' is prepared to move a directory _within_ a directory (i.e.
'rename' functionality). I'm not sure why it's not prepared to move within
a partition; probably whoever wrote it wasn't prepared to deal with all the
extra work for that (unlink from the old '..', then link to the '..' in the
new directory, etc, etc).

(The MIT PWB1 had a 'mvdir' written by Jeff Schiller, so PWB1 didn't have
'move directory' functionality before that. MIT must have been using the PWB1
system for 6.031, which I didn't recall; the comments in 'mvdir' refer to it
being used there.)

The V6 'mv' is fairly complicated (as I found out when I tried to modify it
to use 'smdate()', so that moving a file didn't change its 'last write'
date). Oddly enough, it is prepared to do cross-partition 'moves' (it forks a
'cp' to do the move). Although on V6, 'cp' only does one file; 'cp *.c
{dest}' was not supported, there was 'cpall' for that. (Why no 'mvall', I
wonder? It would have been trivial to clone 'cpall'.)

Run fact; the V6 'mv' is the place that has the famous (?)  "values of B will
 give rise to dom!" error message (in the directory-moing section).

    > if the BSD mv command for 4.1 supported it. If it did then it was not
    > atomic -- it would have had to create the new directory, move the
    > contents independently and then remove the old one.

Speaking of atomic operation, in V6 'mkdir' (not being a system call) was
not atomic, so if interrupted at 'just the right time', it could leave
the FS in an inconsistent state. That's the best reason I've come across
to make 'mkdir' a system call - it can be made atomic that way.

	Noel