Step 2: Install dependencies using composer
This step is required only if you installed (or updated) the codebase using git.
If you don't have Composer installed, see the official Composer installation instructions.
To get a working codebase, you need to run composer install --no-dev from the top level of the repository. This will install Symfony and other packages required by Drupal into the vendor directory.
If you skip this step, you will likely encounter an error message like this when trying to run the installer:
Warning: require(.../drupal/vendor/autoload.php): failed to open stream: No such file or directory in .../drupal/autoload.php on line 14
When installing packages for your production server, it's important to include the --no-dev flag. Some packages, such as phpunit/*, are inherently insecure and should never be installed on a production server. If you're installing Drupal on a local or development server and want to include development packages, then you may omit the --no-dev option.
You can also add the -o option to generate optimized autoload files. For a full list of options for the composer install subcommand, use composer help install.
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