pstree (display a tree of processes)
Like →ps, pstree lists the processes running on a system, but it formats its output as a tree. This is useful for getting a quick overview of the dependencies that exist between processes, which may have parent and child processes. When process A launches process B, we say that A becomes B's parent.
At the root of the process tree is init, the ancestor of all processes. When Linux starts up, init is the first process that is created. It always gets the process ID 1 and goes on to launch the processes that are necessary for the system's basic functioning. When a parent process dies, its children are adopted by init.