PID (process ID)

The number the kernel gives a process when it starts, and the only handle kill, ps and renice take without ambiguity. It names a process, not a program: /usr/bin/node is a file on disk with no PID at all, while the copy answering requests since 09:31 is PID 1741. The number is unique at a given moment, not forever. Ubuntu 24.04 counts to 4194304 (/proc/sys/kernel/pid_max) and then goes back to the lowest free number, so a PID written into yesterday's ticket may belong to something else entirely today — which is how a kill -9 lands on an innocent process. Confirm before you signal. ps -p 1741 -o pid,ppid,user,comm says who it is, and sudo ls -l /proc/1741/exe says which binary is actually running.