How do I convert a pre-existing site to a Composer-managed application?

Question

The latest Drush 9.x version recommends that Drupal 8 sites will be built using Composer, with Drush listed as a dependency. However, when you install Composer and Drush globally per step #1 on http://docs.drush.org/en/master/install and then proceed to step #2 Installing Drush Launcher and then try to run Drush on a pre-existing Drupal website (e.g. drush --version), then you will get:

Drush Launcher Version: 0.6.0
The Drush launcher could not find a Drupal site to operate on. Please do one of the following:

• Navigate to any where within your Drupal project and try again.
• Add --root=/path/to/drupal so Drush knows where your site is located.

How do I get around this problem, and make Drush work with pre-existing Drupal sites?

Answer 1

• Follow
  https://www.drupal.org/docs/develop/using-composer/using-composer-to-manage-drupal-site-dependencies#managing-existing-site

• Copy-paste method:

  1. Install Composerize Drupal with composer global require grasmash/composerize-drupal command;
  2. Run cd path/to/drupal/project/repo && composer composerize-drupal --composer-root=[repo-root] --drupal-root=[drupal-root] where [composer-root] is the root directory of your project where .git is located and [drupal-root] is the Drupal root where index.php is located.

• Manually restructure pre-existing Drupal projects per discussion between Norman and Moshe on https://github.com/drush-ops/drush-launcher/issues/33

• Rebuild the code base from scratch, see https://www.jeffgeerling.com/blog/2018/converting-non-composer-drupal-codebase-use-composer

Answer 2

In case you use Drupal7, I did

drush pm-list --status="enabled" --no-core --fields=name,version > modules.txt
while read line; do comp=`echo $line | sed 's/.*(\(.*\))/\1/' - | sed 's/7\.x\-//' - | awk '$1=$1'`; echo \"drupal\/`echo $comp | awk '{ print $1 }'`\"\:\ \"`echo $comp | awk '{ print $2 }'`\"; done < modules.txt

You can cut&paste your modules list directly into your composer :D