During kernel boot, a kernel thread called init is spawned, which in turn is configured to initialize the first user-mode process (with the same name). The init (pid 1) process is then configured to carry out various initialization operations specified through configuration files, creating multiple processes. Every child process further created (which may in turn create its own child process(es)) are all descendants of the init process. Processes thus created end up in a tree-like structure or a single hierarchy model. The shell, which is one such process, becomes the interface for users to create user processes, when programs are called for execution.

Fork, vfork, exec, clone, wait and exit are the core kernel interfaces for the creation and control of new process. These operations are invoked through corresponding user-mode APIs.