mv overwrites read-only file without prompting

Question

The GNU Coreutils manual for mv says:

If a destination file exists but is normally unwritable, standard input is a terminal, and the -f or --force option is not given, mv prompts the user for whether to replace the file. (You might own the file, or have write permission on its directory.) If the response is not affirmative, the file is skipped.

However, the version of mv I am using (GNU coreutils 8.21 on Ubuntu 14.04.3 LTS) exhibits unexpected behaviour:

$ which mv
/bin/mv
$ ls -l
total 0
$ echo foo > 1; chmod -w 1; cp 1 2; ls -l | cut -d' ' -f 1-5,9
-r-x------ 1 me me 4 1
-r-x------ 1 me me 4 2
$ echo bar > 2
-bash: 2: Permission denied
$ mv 1 2
$ ls -l | cut -d' ' -f 1-5,9
-r-x------ 1 me me 4 2

Based upon the manual excerpt quoted above, I would have expected the mv 1 2 command to have prompted the user before overwriting file 2.

Is there a bug in my version of mv, or a bug in my understanding? If the latter, then what does the manual mean?

Answer 1

Turns out, the server was using a file system of type "cifs" (presumably the Common Internet File System, aka CIFS). This was discovered by running the command df -T.

CIFS apparently exhibits unconventional behaviour with respect to permissions.

Running the same commands on a machine with an ext4 file system and a recent version of Coreutils yielded:

$ mv 1 2
mv: replace ‘2’, overriding mode 0444 (r--r--r--)?

as expected.