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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]
(For example, the "e" or "-e" option will display environment variables.) On such systems, ps commonly runs with the non-standard options aux , where "a" lists all processes on a terminal , including those of other users, "x" lists all processes without controlling terminals and "u" adds a column for the controlling user for each process.
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 ...
The proc filesystem (procfs) is a special filesystem in Unix-like operating systems that presents information about processes and other system information in a hierarchical file-like structure, providing a more convenient and standardized method for dynamically accessing process data held in the kernel than traditional tracing methods or direct access to kernel memory.
The process id is particularly relevant since it is often used to cross-reference the tables defined above, e.g. showing which process is using which I/O devices, or memory areas. Process state data define the status of a process when it is suspended, allowing the OS to restart it later. This always includes the content of general-purpose CPU ...
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]).
The first process created in a PID namespace is assigned the process ID number 1 and receives most of the same special treatment as the normal init process, most notably that orphaned processes within the namespace are attached to it. This also means that the termination of this PID 1 process will immediately terminate all processes in its PID ...
One of the earliest references to a fork concept appeared in A Multiprocessor System Design by Melvin Conway, published in 1962. [1] Conway's paper motivated the implementation by L. Peter Deutsch of fork in the GENIE time-sharing system, where the concept was borrowed by Ken Thompson for its earliest appearance [2] in Research Unix.