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File permission confusion [Debian 9.1 with MATE]




As user "richard" I created 3 files.
I later wanted to protect them totally from accidental change.
For each file, I went to Properties->Permissions and changed Access for Owner, Group, and Others to "Read Only".
As user "richard" I was able to delete them with Caja.
*UNDESIRABLE*
As "root" I changed Owner and Group to "root" leaving Access for all as "Read Only".

User "richard" could still *DELETE THEM*!
"Read Only" evidently does not mean what it implies.

What's happening?
TIA