./console exp:contentcheck --help
./console exp:content-check --help
php bin/php/contentcheck.php --help

Content checks (report only; --fix, --quarantine and --fix-permissions are a dry run unless --apply)

./console exp:content-check --build-index           build the reference index (also the cronjob part contentcheck)
./console exp:content-check --unreferenced --node=2  the unused assets below node 2
./console exp:content-check --broken --node=2        the broken relations below node 2, and the layouts'
./console exp:content-check --why=123,456           what uses objects 123 and 456
./console exp:content-check --storage               scan the storage files: orphans and file problems
./console exp:content-check --storage --quarantine            which orphans would be quarantined
./console exp:content-check --storage --quarantine --apply    move them to the quarantine
./console exp:content-check --storage --fix-permissions [--apply]   set owner and mode of used files
./console exp:content-check --missing-files --node=2  files missing from the storage, stale image aliases
./console exp:content-check --missing-files --fix=regenerate   what regenerating would do
./console exp:content-check --fix=restore --items=123-4,125-2 --apply  restore two field versions
./console exp:content-check --fix=replace --items=123-4 --replace-with=photo.jpg --apply

--fix (regenerate, restore, replace, remove, review), --quarantine and --fix-permissions are the only
options that change anything, and only with --apply: without it they show the plan and change nothing.
--dry-run means the same as no --apply; --apply together with --dry-run is a usage error (exit 2).
Without --items --fix takes every finding of --missing-files.
The lists are those of the user given by --login (default admin): what that user may read.
Exit code: 0 nothing found, 1 something found, 2 error.