pidfiles

A pidfile is a file used by many Unix-like operating systems to store the process ID (PID) of a running daemon or background process. When a daemon starts, it typically creates a pidfile and writes its own PID into it. This allows other processes to easily find and interact with the running daemon. For example, a system administrator might use the PID from the pidfile to send signals to the daemon, such as to gracefully shut it down or reload its configuration.

The location of pidfiles can vary, but they are often found in directories like /var/run or /run. The filename usually corresponds to the name of the daemon, for example, nginx.pid for the Nginx web server. This convention helps in organizing and managing pidfiles.

Using pidfiles helps prevent multiple instances of the same daemon from running simultaneously. Before starting, a daemon checks if its corresponding pidfile already exists. If it does, and the PID inside points to a running process, the daemon will refuse to start, thus avoiding conflicts. This is a common mechanism for ensuring a single, stable instance of a service.