Updated June 10th, 2019.
Some Mac keyboard shortcuts are so useful, and show up so many places, that you owe it to yourself to use them. ⌘-A for “Select All” is one of them. It works in the Finder, Safari, Mail, Notes, Messages, Pages, Numbers, Word, Excel, and many, many more. Look for “Select All” in the Edit menu– then notice the keyboard shortcut ⌘-A. Use the shortcut next time.