DICTIONARY SIGHUP

SIGHUP, or Signal Hangup, is a signal used in Unix and Unix-like operating systems to indicate that a controlling terminal or process has been disconnected. The term "hangup" comes from the days of dial-up modems when a user would physically hang up the phone to disconnect from a remote system. In the context of modern operating systems, SIGHUP is used for various purposes, including process control, configuration updates, and the proper termination of processes when the controlling terminal is closed.

When a process receives a SIGHUP signal, it usually indicates that the controlling terminal or parent process has been closed or terminated. By default, when a process receives a SIGHUP signal, it will terminate itself. However, processes can be programmed to catch and handle the SIGHUP signal in a specific way, such as re-reading their configuration files or performing a graceful shutdown. This allows developers to ensure that important data is saved, resources are released, or any other necessary cleanup is performed before the process exits.

For example, many Unix daemons (background processes) use SIGHUP to trigger a reload of their configuration files. When the daemon receives a SIGHUP signal, it reads the updated configuration without needing to be restarted. This is useful for making changes to a running system without disrupting its operation.

To send a SIGHUP signal to a process, you can use the kill command with the -HUP or -1 option, followed by the process ID (PID) of the target process:

kill -HUP <PID>

In summary, SIGHUP is a signal in Unix and Unix-like operating systems used to notify a process that its controlling terminal or parent process has been disconnected. It can be used for process control, configuration updates, and proper termination of processes. While the default behavior is to terminate the process upon receiving a SIGHUP signal, developers can program processes to handle the signal in a custom manner.