[lug] Changing files on-the-fly

[Jeff Schroeder]

I've been writing shell scripts for years, and as I write more complex 
ones I'm repeatedly annoyed by the fact that if I want to change a text 
file on-the-fly I have to redirect it to a temp file.  For example, 
let's say I have a four-line text file called numbers.txt:

one
two
three
four

I want to remove the line containing 'three' from the file, so I'm left 
with

one
two
four

In order to do this (AFAIK) I have to do this:

grep -v three numbers.txt > .tempfile && mv .tempfile numbers.txt

Because if I try to do it all at once, via

grep -v three numbers.txt > numbers.txt

The resulting file is empty.  Is there a way to NOT use a temporary file 
for this sort of operation?  I'm not just talking about grep here; I'm 
including sed or any other tool that would alter a file's contents.  
Maybe this is just The Unix Way, but I figured I'd ask anyway. :)

TIA,
Jeff