ability to delete a file from inspecting the ls-l output

Question

Down below there are four outputs of the ls -l command for the file and it's parent directory.

In which of the four scenarios student1 can delete file1? (the answer is in red)

I don't understand why the answer is the red one, moreover what does it mean in the ls -l output that there are only - and not anything else in the permissions part? Is it just saying that no one has permission for this file, and if so why is it still the answer?

Answer 1

Deleting a file is not an operation on the file, but on the directory. This is because the "file" entry in the directory is not a file; it is just a reference to the file (semantics are odd because of the overloaded meaning of the word "file" and the imprecision in common usage.) In order to delete a file (eg, remove a reference to it), you just need execute and read permission on the directory the file is in. Hence scenario 1 in your case.

Note that removing a reference (a "link") to a file in one directory only results in the deletion of that file if that is the last reference in the file system. That reference count is given in column 2 of the output of ls -l, so in your case the file linked to by the name "file1" in the directory "directory1" will get garbage collected by the filesystem. (eg, the data will be deleted.)

Of course, the data can also be deleted if the file is overwritten or truncated, so my entire answer is based on the assumption that you use "deleted" to mean "unlinked" or "removed". Imprecise language is rampant!