Before WP-CLI, WordPress maintenance meant: browser open, log in to each site, click Update All, check if anything broke, log out, repeat. With 8 clients that was a half-day of clicking.
Now it's one command per site, or one command for all sites. Here's exactly how.
What WP-CLI actually is
WP-CLI is the command-line interface for WordPress. Everything you can do in the WordPress dashboard, you can do faster from the terminal -- plus things the dashboard can't do at all.
Install once on your server:
