The [unix signal](https://en.wikipedia.org/wiki/Unix_signal) is a method of sending messages between [processes](https://en.wikipedia.org/wiki/Process_(computing)). When a signal is sent, the operating system interrupts the target process's normal flow of execution. There are standard signals that are used to stop a process, but there are also custom signals that can be used for other purposes. This document is an attempt to list all supported signals that Puma will respond to. In general, signals need only be sent to the master process of a cluster.
If you are new to signals, it can be helpful to see how they are used. When a process starts in a *nix-like operating system, it will have a [PID - or process identifier](https://en.wikipedia.org/wiki/Process_identifier) that can be used to send signals to the process. For demonstration, we will create an infinitely running process by tailing a file:
Now you will see via `ps` that there is no more `tail` process. Sometimes when referring to signals, the `SIG` prefix will be used. For example, `SIGTERM` is equivalent to sending `TERM` via `Process.kill`.