Pre-requisites: subscription with physical hosting, allow SSH access for system user, install Drupal from Plesk catalog.
Login under system user by SSH
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-bash-4.2$ pwd
/var/www/vhosts/d4.a10-52-31-63.qa.plesk.ru/httpdocs
-bash-4.2$ which composer
/usr/bin/composer
-bash-4.2$ rpm -qf /usr/bin/composer
plesk-service-node-utilities-18.0-2.centos.7+p18.0.20.0+t191004.1750.x86_64
-bash-4.2$ php -v
PHP 7.2.23 (cli) (built: Oct 2 2019 17:47:22) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies
with the ionCube PHP Loader (enabled) + Intrusion Protection from ioncube24.com (unconfigured) v10.2.1, Copyright (c) 2002-2018, by ionCube Ltd.
with Zend OPcache v7.2.23, Copyright (c) 1999-2018, by Zend Technologies
-bash-4.2$ ls modules/
README.txt
-bash-4.2$
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Try to install any extension, for example
drupal/Token
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-bash-4.2$ composer require drupal/Token
Using version ^1.5 for drupal/token
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 35 installs, 0 updates, 0 removals
- Installing ircmaxell/password-compat (v1.0.4): Downloading (100%)
> Drupal\Core\Composer\Composer::vendorTestCodeCleanup
- Installing drupal/token (1.5.0): Downloading (100%)
> Drupal\Core\Composer\Composer::vendorTestCodeCleanup
- Installing symfony/css-selector (v3.4.26): Downloading (100%)
...
phpunit/phpunit suggests installing phpunit/php-invoker (~1.1)
Package phpunit/phpunit-mock-objects is abandoned, you should avoid using it. No replacement was suggested.
Writing lock file
Generating autoload files
> Drupal\Core\Composer\Composer:
reAutoloadDump
> Drupal\Core\Composer\Composer::ensureHtaccess
-bash-4.2$
Looks like really installed:
-bash-4.2$ ls modules/contrib/token/
css LICENSE.txt src tests token.info.yml token.libraries.yml token.module token.routing.yml token.tokens.inc
js README.md templates token.drush.inc token.install token.links.task.yml token.pages.inc token.services.yml
-bash-4.2$