With CentOS 7, how to install wordpress withouth elimininating Joomla?

What is your directory structure for /var/www/html now? You can play around it and set up both Joomla and WordPress correctly.

Now, that really depends on what you want. One of two possibilities are:

1. Joomla is main and you want to call WordPress with example.com/blog.
2. Joomla is main and you want to call WordPress using subdomain, e.g. blog.example.com. I’m assuming you want Joomla as main website, as you already used it. Same is if you want WordPress as main and Joomla as subsite.

For first, store Joomla in /var/www/html. You can store WP in /var/www/html/blog. Create blog directory, apply needed permissions and install WordPress there. You can basically follow WordPress tutorial as it is. Just to note that you don’t change /var/www/html (where Joomla lives), you change /var/www/html/blog! So when you see something saying do it on /var/www/html, you are doing on /var/www/html/blog.

Now, for second… Let’s create directory for WP - /var/www/blog.example.com/html. This requires you playing around Virtual Hosts and DNS configuration. It’s harder solution, but can be more suitable depending on usage. First create CNAME record for blog pointing to @, then create needed Virtual Host for that subdomain. How To Set Up Nginx Server Blocks (Virtual Hosts) on Ubuntu 16.04 should help you set up needed Virtual Host. And you are ready to follow WordPress tutorial. Just to note again, when you see /var/www/html use /var/www/blog.example.com/html.

This is two possible solutions. I think one of it is best way to set up it. You can’t store both in same directory, e.g. /var/www/html. In that case WordPress (if installed after Joomla) will overwrite some of Joomla files, and Joomla will stop working. That’s way you need to use special directory for it.