Signal: Hangup

This signal is generated by the Kernel when your controlling terminal goes away to the terminals ProcessGroup. Or, in simplier terms, when you close the Xterm, or hang up a modem. Since daemons run in the background and don't have a controlling terminal, they often use SIGHUP to signal that they should reread their configuration files. This can cause issues with some programs that work as both a daemon and an interactive program, such as fetchmail(1).

An example of a daemon that rereads its configuration file on SIGHUP is init(8), the first process created (which is responsible for creating all other processes, like getty for logging in). If you edit inittab(5), its configuration file, you can do

kill -HUP 1

and it will re-read the config file (note that the correct way to do this is to use telinit(8)).

To restart an inetd(8) service, you find its PID and send a SIGHUP to it
kill -HUP $inetd_pid
You can prevent a process from recieving a SIGHUP signal by using the command nohup(1), for example
nohup wget &

will run wget(1) detached from the terminal, so it won't recieve a SIGHUP when you disconnect. This is useful if a download is so large that you want to log out instead of waiting until it is complete. (The & is Shell lingo to put the command into the background so that you can do something else, such as, uh, logging out.)