Re: The -d and -D options

[Paul Hoffman]

On Fri, Mar 30, 2007 at 07:32:31PM +0100, JCA wrote:

>   I wonder if someone could please explain the meaning of the
> following options the screen command:
> 
>       -d -r   Reattach a session and if necessary detach it first.
> 
>       -d -R   Reattach a session and if necessary detach or  even  create  
>       it
>               first.
> 
>       -d -RR  Reattach  a  session  and if necessary detach or create it. 
>       Use
>               the first session if more than one session is available.
> 
>       -D -r   Reattach a session. If necessary  detach  and  logout  
>       remotely
>               first.
> 
>       -D -R   Attach here and now. In detail this means: If a session is 
>       run-
>               ning, then reattach. If necessary detach  and  logout  
>               remotely
>               first.   If  it  was not running create it and notify the 
>               user.
>               This is the author's favorite.
> 
>       -D -RR  Attach here and now. Whatever that means, just do it.
> 
> I am enclosing them with their description in the screen man page
> because, quite frankly, I do not understand the differences (they all
> sound very similar), and under what circumstances I would want to use
> one of them rather than the others.

I was confused, too, but figured them out (more or less) by careful
re-reading and (more importantly) by experimenting with different
combinations.

The difference between -d and -D is that -D may cause a session to be
logged out remotely, whereas -d will not.  This is the only thing I'm
not sure I understand -- if you create a session on host A and then use
screen -d on host B, will it not detach the session that's running on
host A?  Do you have to use -D in this case?

The difference between -r and -R is that -R will create a session if no
session can be reattached.  The difference between -R and -RR is that
-RR will reattach the first session it finds, whereas -R will throw up
its hands if it finds more than one (i.e., it won't reattach but will
instead list all detached sessions so you can then use screen -r
<session> to specify the one you want to reattach).

> The "if necessary" qualifier is particularly galling, for it is not at
> all clear (to me) how to determine the necessity requirement.

Try re-reading them one at a time.  First one:

>       -d -r   Reattach a session and if necessary detach it first.

Translation: Try to reattach a session. If it's already attached, detach
it and then try again.  This way you can start a session in one terminal
and reattach it in another terminal *without having first detached it in
the first terminal*.

Next:

>       -d -R   Reattach a session and if necessary detach or  even
>               create  it first.

Translation: Try to reattach a session. If it's already attached, detach
it and and then try again. If it's neither attached nor detached --
i.e., if it doesn't even exist -- then create it and attach to it.

Next:

>       -d -RR  Reattach  a  session  and if necessary detach or create it. 
>               Use the first session if more than one session is available.

See above.

The remaining combinations (-D -r, -D -R, and -D -RR) are the same
except they will detach *and logout remotely*, whatever that means -- I
always use -D rather than -d.

> The description of -D -RR is the very worst, for it really is a
> useless explanation if there has ever been one (and there has.)

Yikes!  Relax; the world won't end if you don't understand this.

Paul.

-- 
Paul Hoffman <address@hidden>