Backstory

I was setting up Time Machine on my laptop and figured I should look for big useless files and avoid backing them up. I like GrandPerspective for scanning directories to find disk space hogs. This showed me that I had a bunch of huge Rails logs (from development and testing).

The Syntax

The following command truncates all the log files in all my projects.

: > ~/Projects/*/log/*.log

That colon at the beginning is part of the command.
I think Bash is the same without the colon, but don’t take my word for it.

Postscript

I could have just deleted the files.

rm ~/Projects/*/log/*.log

Rails would simply recreate them as needed.