I want to grant a single user write permissions to a directory owned by root over command line. With chmod, it seems that I can only change the permissions for owner, group and other, not for a specific user.
If you mount the filesystem with the "acl" option (add it to
would give user USERNAME read access to FILENAME.
By default in Ubuntu every user belongs to a group with the same name, i.e. hannesh:hannesh.
You can make the folder owned by root:hannesh, and give it permissions like 770 though I see no reason for it to be owned by root if you want the user to be able to write there - why doesn't hannesh:hannesh suit you? Root would be able to write in that folder anyway.
Also have a read about the sticky bit if root creates files in that directory which need to be writeable by the user.