You need to restart your Apache service for any changes you made to the config file to be effective. It is then very important for your to test your Apache config for errors before restarting as it could cause the Apache service to fail to start and disrupt the availability of your hosted websites.
Apache has built-in feature to test the validity of the config file that you can use to test your config files for errors or warnings. You could repeatedly check the configuration files until all the errors and warnings are gone before restarting the service.
You can use both apachectl and httpd tool to test your Apache configuration from the terminal.
$ sudo vi /etc/apache2/apache2.conf
$ sudo apachectl configtest AH00543: apache2: bad user name username Action 'configtest' failed. The Apache error log may have more information.
Use -t option instead of configtest if you're using httpd or apache and not apachectl
$ sudo apachectl configtest Syntax OK
$ sudo systemctl restart apache2 # Ubuntu, Debian, openSUSE and SLES $ sudo systemctl restart httpd # CentOS and Red Hat
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