It depends on the daemon :-). A lot of daemons can be told to re-read their configuration files or do other things like that by sending them certain signals. Quite of a HUP signal is used for. For example, inetd will re-read inetd.conf when sent a HUP signal, so you could do this:
killall -HUP inetd
[ Note: Do *NOT* do this on Solaris. Killall means something entirely different there ;-). ]
Alternatively, if you have a system with a standard SysV init (most distributions aside from Slackware do), you can simply use the init script do your work for you. In the simplest form,
will do the trick. (Some distributions, such as Debian, just call it /etc/init.d.) Even handier,
will generally do this for you. However, it's often considered crude to kill and restart the daemon when simply sending it a signal would do the trick :-). (Although, less crude than rebooting the whole system.) So, many init scripts (such as most of the ones bundled with a Red Hat system) support yet another command:
This will generally send whatever signal is required to get the daemon to re-read its config file.