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On some systems, like MPE/iX, the lowest available PID is used, sometimes in an effort to minimize the number of process information kernel pages in memory. The current process ID is provided by a getpid() system call, [8] or as a variable $$ in shell. The process ID of a parent process is obtainable by a getppid() system call. [9]
Instead, the system simply redefines the "parent PID" field in the child process's data to be the process that is the "ancestor" of every other process in the system, whose PID generally has the value of 1 (one), and whose name is traditionally "init" (except in the Linux kernel 3.4 and above [more info below]).
pstree is a Linux command that shows the running processes as a tree [1] [2] [3]. It is used as a more visual alternative to the ps command. The root of the tree is either init or the process with the given pid. It can also be installed in other Unix systems. In BSD systems, a similar output is created using ps -d, in Linux ps axjf [4] produces ...
PID: Process ID number PPID: ID number of the process's parent process PRI: Priority of the process RSS: Resident set size: S or STAT: Process status code START or STIME: Time when the process started VSZ: Virtual memory usage TIME: The amount of CPU time used by the process TT or TTY: Terminal associated with the process UID or USER: Username ...
clone is a system call in the Linux kernel that creates a child process that may share parts of its execution context with the parent. Like FreeBSD's rfork and IRIX's sproc, Linux's clone was inspired by Plan 9's rfork and can be used to implement threads (though application programmers will typically use a higher-level interface such as ...
The fork() function returns the child's PID to the parent process. The fork() function returns 0 to the child process. This enables the two otherwise identical processes to distinguish themselves from each other. The parent process can either continue execution or wait for the child process to complete.
When a child process terminates before the parent has called wait, the kernel retains some information about the process, such as its exit status, to enable its parent to call wait later. [3] Because the child is still consuming system resources but not executing it is known as a zombie process .
The systemd daemon serves as the root of the user space's process tree; the first process (PID 1) has a special role on Unix systems, as it replaces the parent of a process when the original parent terminates. Therefore, the first process is particularly well suited for the purpose of monitoring daemons.