This signal is generated by the kernel when your controlling terminal goes away. 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 it's configuration file on SIGHUP is init(1)?, the first process created (which is responsible for creating all other processes, like getty for logging in). If you edit /etc/inittab, its configuration file, you can do
kill -HUP 1
and it will re-read the config file.
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