Extrait du FAQ (http://pureftpd.org/FAQ) :
* How to start Pure-FTPd in background.
-> I start 'pure-ftpd' from an X terminal, and the server properly
answers. However, as soon as I close the terminal, the server stops.
This is a shell dependent issue. Your shell is configured to close all
background jobs when leaving. You can change your shell options
(probably with a 'set' directive) or detach background jobs with the
'disown' keyword. Alternatively, you can just start pure-ftpd with the
-B switch in order to have it detach at startup time :
/usr/local/sbin/pure-ftpd -B