h1. Installing Drush for working with Drupal 6-8 and Backdrop

Because Palante is still supporting a number of Drupal 6 sites, and we've fallen victim to the "weird gotcha where Drush destroys your git repo and other non-Drupal files":https://github.com/drush-ops/drush/issues/1581 that seems to happen more often with Drush 8 than other versions, we've decided to stick with Drush 7 as the default version in use at Palante. But since we are beginning to build Drupal 8 sites, we also need Drush 8 installed; Drush 8 is also better for Backdrop, which we're also working with, because with the "Backdrop Drush commands":https://github.com/backdrop-contrib/drush installed with Drush 7, "Drush stops working with Drupal 7.":https://github.com/backdrop-contrib/drush/issues/9 Finally, because we're interacting with some remote servers that can't support versions of Drush above 6, we also need to be able to use Drush 6 on occasion.

Below is a solution that allows us to use Drush 6, 7 and 8 in the same environment.

h2. Install Drush 6, 7 and 8 for all users

I've followed the "To install for all users on the server" instructions under "Composer - One Drush for all Projects" in the "Drush 7 installation documentation:":http://docs.drush.org/en/7.x/install/.

To install Drush 7, run the following commands as root or using @sudo@.

<pre>
git clone https://github.com/drush-ops/drush.git -b 7.2.0 /usr/local/src/drush7
cd /usr/local/src/drush7
composer install
ln -s /usr/local/src/drush7/drush /usr/bin/drush7
</pre>

Replace "7.2.0" with the tag of the most recent stable 7.x release (see https://github.com/drush-ops/drush/releases).

Then install Drush 8, again running these commands as root or using @sudo@.

<pre>
git clone https://github.com/drush-ops/drush.git -b 8.0.5 /usr/local/src/drush8
cd /usr/local/src/drush8
composer install
ln -s /usr/local/src/drush8/drush /usr/bin/drush8
</pre>

Replace "8.0.5" with the tag of the most recent stable 8.x release (see https://github.com/drush-ops/drush/releases)

If you also need to work with Drush 6--for example, when you need to run commands like @drush sql-sync@ against remote servers that can't run Drush 7 or later--you can install it in the same manner.

<pre>
git clone https://github.com/drush-ops/drush.git -b 6.6.0 /usr/local/src/drush6
cd /usr/local/src/drush6
composer install
ln -s /usr/local/src/drush6/drush /usr/bin/drush6
</pre>

You can now use the @drush6@, @drush7@ or @drush8@ commands to run each specific version of Drush.

Because I currently want to use Drush 8 when working with Backdrop, I'll also create a @drushbd@ alias pointing to Drush 8; later, if Backdrop needs to use a _different_ version of Drush, I can change where the @drushbd@ alias points and continue to use that command.

<pre>
ln -s /usr/local/src/drush8/drush /usr/bin/drushbd
</pre>

*EXTRA TIP:* To update version of Drush installed as outlined above, run the following commands within the @/usr/local/src/<drush-version>@ directory:

<pre>
git pull origin master
git checkout tags/7.2.0 -b 7.2.0
composer install
</pre>

Replace "7.2.0" with the most recent stable release of the Drush version you're updating.

h2. Install Backdrop Drush commands

In order for Drush to work with Backdrop, you must install the "Drush Backdrop commands":https://github.com/backdrop-contrib/drush within the Drush 8 installation itself.

Assuming that Drush 8 is installed in @/usr/local/src/drush8@, as specified above:

<pre>
git clone https://github.com/backdrop-contrib/drush.git /usr/local/src/drush8/commands/backdrop
</pre>

h2. Create script to automatically switch Drush versions based on git configuration setting

As per "this excellent guide from Marc van Gend of Triquanta":https://www.triquanta.nl/blog/automatically-switch-drush-versions-project, I've set up a script to let us switch between versions of Drush on a project-by-project basis using git configuration settings.

The script, which lives at @/usr/bin/drush@ and is therefore called whenever someone runs the @drush@ command:

<pre>
#!/bin/bash
version=$(git config --get drush.version)
if [ "$version" = '8' ];
then
    drush8 "$@"
elif [ "$version" = 'backdrop' ];
then
    drushbd "$@"
elif [ "$version" = '6' ];
  then
  drush6 "$@"
else
    drush7 "$@"
fi
</pre>

h2. Set drush.version git variable on a per-project basis

For the script to automatically switch to the right version of Drush when working with Drupal 8 or Backdrop, set the @drush.version@ value in the git repo for the project by running @git config drush.version <version>@. For example:

<pre>
$ drush --version
 Drush Version   :  7.2.0
$ git config drush.version 8
$ drush --version
 Drush Version   :  8.0.5
</pre>

For Backdrop, run @git config drush.version backdrop@.

h2. Update Drush remote site aliases to use correct Drush scripts

If you've got Drush site aliases set up for remote sites on servers where multiple versions of Drush are available, update your @aliases.drushrc.php@ file to include the specific drush script that should be used on the remote site. See the "example.aliases.drushrc.php":https://github.com/drush-ops/drush/blob/master/examples/example.aliases.drushrc.php file if you need help creating your @aliases.drushrc.php@ file or learning about <code>%drush-script</code>.

An example @aliases.drushrc.php@ file with <code>%drush-script</code> specified:

<pre>
$aliases['drupal8'] = array(
  'remote-host' => 'fake-dev-server.palantetech.coop',
  'remote-user' => 'jack',
  'root' => '/var/www/dev/drupal8',
  'uri' => 'drupal8.palantetech.coop',
  'path-aliases' => array(
    '%drush-script' => '/usr/bin/drush8',
  )
);
$aliases['backdrop'] = array(
  'remote-host' => 'fake-dev-server.palantetech.coop',
  'remote-user' => 'jack',
  'root' => '/var/www/dev/backdrop',
  'uri' => 'backdrop.palantetech.coop',
  'path-aliases' => array(
    '%drush-script' => '/usr/bin/drushbd',
  )
);
</pre>