rclone cat z: > nul
that command will read every file from z:, and as a result, preload the vfs file cache.
using > nul, just a trick to prevent rclone from saving the files to local storage
https://ss64.com/nt/nul.html
rclone cat z:\path\to\dir > nul
not understanding what you mean?
do not use null, as the output of rclone cat will be saved to a local file named null.