We’re making a new site based on Drupal 8. This site needs to display addresses, and some quick research leads us to the Address module. However, after downloading and trying to enable Address, we get the following message: Address requires the commerceguys/addressing library. – Apr 19, 2018

In the old days a requirement like this would be satisfied by manually downloading and extracting the library into sites/all/libraries/addressing. Now-a-days that’s no longer possible, PHP libraries must be installed using Composer. Composer is a tool for managing dependencies on the project level, a project being your site or web application.

Due to its power, more and more modules are choosing to separate pieces of their logic into separate packages that can be used by the wider PHP community. They’re also choosing to depend on other people’s libraries in order to reduce their own workload. There is also no doubt that Composer will replace Drush Make, and make itself the preferred way of assembling sites. Advanced Drupal hosting services like Platform.sh (wink wink) already support it.

How to install composer in Linux?

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"

php -r "if (hash_file('sha384', 'composer-setup.php') === 'a5c698ffe4b8e849a443b120cd5ba38043260d5c4023dbf93e1558871f1f07f58274fc6f4c93bcfd858c6bd0775cd8d1') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"

php composer-setup.php
php -r "unlink('composer-setup.php');"
chmod +x composer.phar
mv composer.phar /usr/local/bin/composer
mv composer.phar /usr/bin/composer