File Write Permission Rules

[Guy Harris]

>A pity (is it really?) that ordinary users are not allowed to set
>'sticky mode', this makes it hard to use by anyone but root.

Wrongo.  Any user can set the sticky bit on a *directory*, assuming they
own it; they're just disallowed from setting it on plain files.

>I fail to understand what the possibility of unlinking has to do with
>sticky bits

It has nothing to do with sticky bits.

>(but then, you can fill me in); I thought that a sticky bit keeps a
>file on the swap disk, once it is loaded.

That notion obviously means nothing for directories, so the "sticky bit"
could be overloaded to mean one thing on directories and another thing
on plain files.  Berkeley (and, subsequently, AT&T, in S5R3.2) decided
to do so, in order to permit you to make "/tmp", or other directories,
directories in which people can create files, but from which only the
owner can remove a file.