Textbook of Linux

Also known as: signal 1

SIGHUP (signal 1) originally meant "the controlling terminal has hung up"—a carryover from the days when terminals were actual modems, and losing carrier would notify the shell and its children. When a login shell receives SIGHUP, it forwards it to its child jobs, which is why closing a terminal usually kills everything running in it.

Modern daemons repurpose SIGHUP as a "reload your configuration without restarting" signal. Sending kill -HUP nginx causes nginx to re-read its config, reopen log files, and start new worker processes without dropping existing connections. This is now the near-universal convention, which is ironic since the original hangup meaning has mostly faded.

kill -HUP $(pidof nginx)
systemctl reload nginx           # preferred; systemd knows the signal
nohup long-running-command &     # resist SIGHUP when terminal closes

The command nohup arranges for its child to ignore SIGHUP, so long-running jobs survive terminal disconnection. Modern alternatives include running work inside tmux or screen, which do not just ignore SIGHUP but keep the terminal session alive across disconnects.