define pid
A PID (i.e., process identification number) is an identification number that is automatically assigned to each process when it is created on a Unix-like operating system. A process is an executing (i.e., running) instance of a program. Each process is guaranteed a unique PID, which is always a non-negative integer. The process init is the only process that will always have the same PID on any session and on any system, and that PID is 1. This is because init is always the first process on the system and is the ancestor of all other processes.