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In computing, the process identifier (a.k.a. process ID or PID) is a number used by most operating system kernels—such as those of Unix, macOS and Windows—to uniquely identify an active process. This number may be used as a parameter in various function calls, allowing processes to be manipulated, such as adjusting the process's priority or ...
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 ...
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 ...
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.
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]).
systemd tracks processes using the Linux kernel's cgroups subsystem instead of using process identifiers (PIDs); thus, daemons cannot "escape" systemd, not even by double-forking. systemd not only uses cgroups, but also augments them with systemd-nspawn and machinectl , two utility programs that facilitate the creation and management of Linux ...
It has since been available in illumos and reimplemented for the Linux and BSDs (DragonFly BSD, FreeBSD, NetBSD, and OpenBSD). It searches for all the named processes that can be specified as extended regular expression patterns, and—by default—returns their process ID. Alternatives include pidof (finds process ID given a program name) and ps.