wp-cli, a command line interface for WordPress, just saved me a big headache.
It all began when I updated my server to the latest ubuntu LTS. That removed PHP5 support and replaced it with PHP7. That meant several of my sites stopped working as the nginx configuration had to change. With that done I thought the blog as the pages were still displaying OK but for some reason I could no longer access the admin interface to make new posts or moderate comments. Instead, when I logged in I would see a message: Sorry, you are not allowed to access this page
I decided the most likely cause of this would be a plugin. However, I couldn't use the admin interface to disable all the plugins so I needed to find another way. I read that I could start editing the database directly but then I stumbled upon wp-cli. Installing that allowed me to use an SSH shell to check, disable, enable and update the plugins. I quickly discovered that there was an update to cloudflare that hadn't been applied. I applied it and it failed. I'm not entirely sure if it failed due to the background change of PHP5 to PHP7, or if my earlier fault finding had changed a file owner (I tried removing the plugins by moving the directory to a new name), but once I used wp-cli to update cloudflare everything started working perfectly, including the ability to preview theme changes which had stopped working a long time ago.
If I didn't find wp-cli I would have been checking my backups, creating a test server in my office to install and test backups and assuming they worked removing and reinstalling wordpress.