Hup

HUP is an abbreviation for hang up and is most commonly associated with the Hang Up signal in Unix-like operating systems, typically referred to by its symbolic name SIGHUP. The name originates from telecommunication systems, where a terminal line drop would send a hang-up to the host.

In modern systems, SIGHUP is delivered to a process when its controlling terminal is closed, when the session leader changes, or when a program itself requests reinitialization. The default action for SIGHUP on many systems is to terminate the process, but it is widely repurposed by daemons and services to indicate a reinitialization, loading of configuration, or reopening of log files rather than a complete shutdown.

Usage and behavior can vary by program. Developers may register a signal handler to perform a clean reload of configuration, reopen log files, or gracefully restart components in response to SIGHUP. Administrators commonly trigger it with commands such as kill -HUP <pid> or by signaling a service manager that a restart should occur without stopping the process completely.

Common examples include web servers and syslog daemons, which often implement SIGHUP as a mechanism to re-read configuration files or to rotate logs without terminating the process. While SIGHUP is standard in POSIX, its interpretation is API and application dependent; some programs ignore it, while others perform specific reinitialization tasks.

See also: SIGHUP, Unix signals, process control.