So anyway, for want of anything better to do I have been messing around with file permissions and sooner or later ended up with a file that had the following permissions:

--w--w--w-

I'll bet no one's seen that before. :-)
Is this just the dumbest thing ever, or is there a legitimate reason to have a file that can be written to by everyone but read by no one? Just curious....

Not quite the same thing but Yes.....
http://www.national.com/rap/Story/WOMorigin.html
http://en.wikipedia.org/wiki/Write-only_memory

Certainly not the only example of something on a computer being totally useless......

the good news: All combinations of permissions are available to you. You are in CONTROL!!

Actually, it could be worse I guess:

----------

Only Root can fix that. :-)

this might be a suitable directory to store the file in:

d-w-------

A write-only file does actually have a use. It could be an output device file, which could be mapped to a kernel device driver for a piece of hardware that was only an output and provided no feedback.

In practice, all output devices in common use have some kind of feedback, and so most device files are read/write, but that isn't necessarily the case.

Of course, such a file would normally be b-w--w--w- or c-w--w--w- , but since the advent of procfs, it would be perfectly possible to create such a file under a virtual filesystem like /proc or /sys. On my system, for example, I have a file named /proc/sysrq-trigger with permissions of --w-------- (but none writeable by all users).

I just did a $ sudo ls -Rl /proc 2>/dev/null | grep \\\-w and saw a lot of links to pipes and some other files (like clear_refs) with only write access, although only the proc file owner could (as one would expect) write to the "file."