qlmanage

QuickLook server tool. More information: https://ss64.com/osx/qlmanage.html.

Display QuickLook for one or multiple files:

qlmanage -p {{filename}} {{filename2}}

Compute 300px wide PNG thumbnails of all JPEGs in the current directory and put them in a directory:

qlmanage {{*.jpg}} -t -s {{300}} {{path/to/directory}}

Reset QuickLook:

qlmanage -r