This is from the 'examples/example.aliases.drush.php' file that ships with drush:

 * Aliases are commonly used to define short names for
 * local or remote Drupal installations; however, an alias
 * is really nothing more than a collection of options.
 * A canonical alias named "dev" that points to a local
 * Drupal site named "dev.mydrupalsite.com" looks like this: 
 *
 *   $aliases['dev'] = array(
 *     'root' => '/path/to/drupal',
 *     'uri' => 'dev.mydrupalsite.com',
 *   );
 *
 * With this alias definition, then the following commands
 * are equivalent:
 *
 *   $ drush @dev status
 *   $ drush --root=/path/to/drupal --uri=dev.mydrupalsite.com status
 *
 * Any option that can be placed on the drush commandline
 * can also appear in an alias definition.
 *
 * There are several ways to create alias files.
 *
 *   + Put each alias in a separate file called ALIASNAME.alias.drushrc.php
 *   + Put multiple aliases in a single file called aliases.drushrc.php
 *   + Put groups of aliases into files called GROUPNAME.aliases.drushrc.php

check out the example file for more details :) (with aliases, you can "be" anywhere on your system and run drush commands)

-Benj


On Wed, Sep 14, 2011 at 10:21 PM, Roger <arelem@bigpond.com> wrote:
On 15/09/11 13:06, support-request@drupal.org wrote:
> I 2nd this opinion - drush isn't sure which of your multi-sites you are
> trying to target. You'll either need to set up drush aliases or 'cd' to the
> specific multi-site you want to affect.
>
> -Benj
Thanks Benj.  How do I set up drush aliases? Maybe I could use such a
thing to flush without having to change diretories
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