Plan 9 from Bell Labs is a distributed operating system which originated from the Computing Science Research Center (CSRC) at Bell Labs in the mid-1980s and built on UNIX concepts first developed there in the late 1960s. Since 2000, Plan 9 has been free and open-source. The final official release was in early 2015.
Plan 9 from Bell Labs was originally developed, starting in the late 1980s,[19] by members of the Computing Science Research Center at Bell Labs, the same group that originally developed Unix and the C programming language.[20] The Plan 9 team was initially led by Rob Pike, Ken Thompson, Dave Presotto and Phil Winterbottom, with support from Dennis Ritchie as head of the Computing Techniques Research Department. Over the years, many notable developers have contributed to the project, including Brian Kernighan, Tom Duff, Doug McIlroy, Bjarne Stroustrup and Bruce Ellis.[21]
Plan 9 replaced Unix as Bell Labs's primary platform for operating systems research.[22] It explored several changes to the original Unix model that facilitate the use and programming of the system, notably in distributed multi-user environments. After several years of development and internal use, Bell Labs shipped the operating system to universities in 1992. Three years later, Plan 9 was made available for commercial parties by AT&T via the book publisher Harcourt Brace. With source licenses costing $350, AT&T targeted the embedded systems market rather than the computer market at large. Ritchie commented that the developers did not expect to do "much displacement" given how established other operating systems had become.[23]
By early 1996, the Plan 9 project had been "put on the back burner" by AT&T in favor of Inferno, intended to be a rival to Sun Microsystems' Java platform.[24] In the late 1990s, Bell Labs' new owner Lucent Technologies dropped commercial support for the project and in 2000, a third release was distributed under an open-source license.[25] A fourth release under a new free software license occurred in 2002.[26] In early 2015, the final official release of Plan 9 occurred.[25]
A user and development community, including current and former Bell Labs personnel, produced minor daily releases in the form of ISO images. Bell Labs hosted the development.[27] The development source tree is accessible over the 9P and HTTP protocols and is used to update existing installations.[28] In addition to the official components of the OS included in the ISOs, Bell Labs also hosts a repository of externally developed applications and tools.[29]
As Bell Labs has moved on to later projects in recent years, development of the official Plan 9 system had stopped. On March 23, 2021, development resumed following the transfer of copyright from Bell Labs to the Plan 9 Foundation.[10][30][31] Unofficial development for the system also continues on the 9front fork, where active contributors provide monthly builds and new functionality. So far, the 9front fork has provided the system Wi-Fi drivers, Audio drivers, USB support and built-in game emulator, along with other features.[32][33] Other recent Plan 9-inspired operating systems include Harvey OS[34] and Jehanne OS.[35]
Date
Release
Comment
1992
Plan 9 1st edition
Released by Bell Labs to universities
1995
Plan 9 2nd edition
Released by Bell Labs for non-commercial purposes[36]
2000
Plan 9 3rd ed. (Brazil)
Released by Lucent Technologies under an open source license
2002
Plan 9 4th edition
Released by Lucent Technologies under a new free software license
Design concepts
Plan 9 from Bell Labs is like the Quakers: distinguished by its stress on the 'Inner Light,' noted for simplicity of life, in particular for plainness of speech. Like the Quakers, Plan 9 does not proselytize.
—Sape J. Mullender, Pierre G. Jansen. Real Time in a Real Operating System[37]
Plan 9 is a distributed operating system, designed to make a network of heterogeneous and geographically separated computers function as a single system.[38] In a typical Plan 9 installation, users work at terminals running the window system rio, and they access CPU servers which handle computation-intensive processes. Permanent data storage is provided by additional network hosts acting as file servers and archival storage.[39]
Its designers state that,
[t]he foundations of the system are built on two ideas: a per-process name space and a simple message-oriented file system protocol.
The first idea (a per-process name space) means that, unlike on most operating systems, processes (running programs) each have their own view of the namespace, corresponding to what other operating systems call the file system; a single path name may refer to different resources for different processes. The potential complexity of this setup is controlled by a set of conventional locations for common resources.[41][42]
The second idea (a message-oriented filesystem) means that processes can offer their services to other processes by providing virtual files that appear in the other processes' namespace. The client process's input/output on such a file becomes inter-process communication between the two processes. This way, Plan 9 generalizes the Unix notion of the filesystem as the central point of access to computing resources. It carries over Unix's idea of device files to provide access to peripheral devices (mice, removable media, etc.) and the possibility to mount filesystems residing on physically distinct filesystems into a hierarchical namespace, but adds the possibility to mount a connection to a server program that speaks a standardized protocol and treat its services as part of the namespace.
For example, the original window system, called 8½, exploited these possibilities as follows. Plan 9 represents the user interface on a terminal by means of three pseudo-files: mouse, which can be read by a program to get notification of mouse movements and button clicks, cons, which can be used to perform textual input/output, and bitblt, writing to which enacts graphics operations (see bit blit). The window system multiplexes these devices: when creating a new window to run some program in, it first sets up a new namespace in which mouse, cons and bitblt are connected to itself, hiding the actual device files to which it itself has access. The window system thus receives all input and output commands from the program and handles these appropriately, by sending output to the actual screen device and giving the currently focused program the keyboard and mouse input.[39] The program does not need to know if it is communicating directly with the operating system's device drivers, or with the window system; it only has to assume that its namespace is set up so that these special files provide the kind of input and accept the kind of messages that it expects.
Plan 9's distributed operation relies on the per-process namespaces as well, allowing client and server processes to communicate across machines in the way just outlined. For example, the cpu command starts a remote session on a computation server. The command exports part of its local namespace, including the user's terminal's devices (mouse, cons, bitblt), to the server, so that remote programs can perform input/output using the terminal's mouse, keyboard and display, combining the effects of remote login and a shared network filesystem.[39][40]
All programs that wish to provide services-as-files to other programs speak a unified protocol, called 9P. Compared to other systems, this reduces the number of custom programming interfaces. 9P is a generic, medium-agnostic, byte-oriented protocol that provides for messages delivered between a server and a client.[43] The protocol is used to refer to and communicate with processes, programs, and data, including both the user interface and the network.[44] With the release of the 4th edition, it was modified and renamed 9P2000.[26]
Unlike most other operating systems, Plan 9 does not provide special application programming interfaces (such as Berkeley sockets, X resources or ioctl system calls) to access devices.[43] Instead, Plan 9 device drivers implement their control interface as a file system, so that the hardware can be accessed by the ordinary file input/output operations read and write. Consequently, sharing the device across the network can be accomplished by mounting the corresponding directory tree to the target machine.[17]
Union directories and namespaces
Plan 9 allows the user to collect the files (called names) from different directory trees in a single location. The resulting union directory behaves as the concatenation of the underlying directories (the order of concatenation can be controlled); if the constituent directories contain files having the same name, a listing of the union directory (ls or lc) will simply report duplicate names.[45] Resolution of a single path name is performed top-down: if the directories top and bottom are unioned into u with top first, then u/name denotes top/name if it exists, bottom/name only if it exists and top/name does not exist, and no file if neither exists. No recursive unioning of subdirectories is performed, so if top/subdir exists, the files in bottom/subdir are not accessible through the union.[46]
A union directory can be created by using a sequence of bind commands:
In the example above, /arm/bin is mounted at /bin, the contents of /arm/bin replacing the previous contents of /bin. Acme's bin directory is then union mounted after /bin, and Alice's personal bin directory is union mounted before. When a file is requested from /bin, it is first looked for in /usr/alice/bin, then in /arm/bin, and then finally in /acme/bin/arm.
The separate process namespaces thus usually replace the notion of a search path in the shell. A path environment variable ($path) still exists in the rc shell (the shell mainly used in Plan 9); however, rc's path environment variable conventionally only contains the /bin and . directories and modifying the variable is discouraged, instead, adding additional commands should be done by binding several directories together as a single /bin.[47][39] Unlike in Plan 9, the path environment variable of Unix shells should be set to include the additional directories whose executable files need to be added as commands.
Furthermore, the kernel can keep separate mount tables for each process,[37] and can thus provide each process with its own file system namespace. Processes' namespaces can be constructed independently, and the user may work simultaneously with programs that have heterogeneous namespaces.[40] Namespaces may be used to create an isolated environment similar to chroot, but in a more secure way.[43]
Plan 9's union directory architecture inspired 4.4BSD and Linuxunion file system implementations,[45] although the developers of the BSD union mounting facility found the non-recursive merging of directories in Plan 9 "too restrictive for general purpose use".[46]
Instead of having system calls specifically for process management, Plan 9 provides the /proc file system. Each process appears as a directory containing information and control files which can be manipulated by the ordinary file IO system calls.[8]
The file system approach allows Plan 9 processes to be managed with simple file management tools such as ls and cat; however, the processes cannot be copied and moved as files.[8]
/net
Plan 9 does not have specialised system calls or ioctls for accessing the networking stack or networking hardware. Instead, the /net file system is used. Network connections are controlled by reading and writing control messages to control files. Sub-directories such as /net/tcp and /net/udp are used as an interface to their respective protocols.[8]
To reduce the complexity of managing character encodings, Plan 9 uses Unicode throughout the system. The initial Unicode implementation was ISO/IEC 10646-1:1993. Ken Thompson invented UTF-8, which became the native encoding in Plan 9. The entire system was converted to general use in 1992.[49] UTF-8 preserves backwards compatibility with traditional null-terminated strings, enabling more reliable information processing and the chaining of multilingual string data with Unix pipes between multiple processes. Using a single UTF-8 encoding with characters for all cultures and regions eliminates the need for switching between code sets.[50]
Combining the design concepts
Though interesting on their own, the design concepts of Plan 9 were supposed to be most useful when combined. For example, to implement a network address translation (NAT) server, a union directory can be created, overlaying the router's /net directory tree with its own /net. Similarly, a virtual private network (VPN) can be implemented by overlaying in a union directory a /net hierarchy from a remote gateway, using secured 9P over the public Internet. A union directory with the /net hierarchy and filters can be used to sandbox an untrusted application or to implement a firewall.[43] In the same manner, a distributed computing network can be composed with a union directory of /proc hierarchies from remote hosts, which allows interacting with them as if they are local.
When used together, these features allow for assembling a complex distributed computing environment by reusing the existing hierarchical name system.[8]
As a benefit from the system's design, most tasks in Plan 9 can be accomplished by using ls, cat, grep, cp and rm utilities in combination with the rc shell (the default Plan 9 shell).
Factotum is an authentication and key management server for Plan 9. It handles authentication on behalf of other programs such that both secret keys and implementation details need only be known to Factotum.[51]
Unlike Unix, Plan 9 was designed with graphics in mind.[44] After booting, a Plan 9 terminal will run the rio windowing system, in which the user can create new windows displaying rc.[52] Graphical programs invoked from this shell replace it in its window.
Plan 9 supports the Kfs, Paq, Cwfs, FAT, and Fossil file systems. The last was designed at Bell Labs specifically for Plan 9 and provides snapshot storage capability. It can be used directly with a hard drive or backed with Venti, an archival file system and permanent data storage system.
Software development
The distribution package for Plan 9 includes special compiler variants and programming languages, and provides a tailored set of libraries along with a windowing user interface system specific to Plan 9.[54] The bulk of the system is written in a dialect of C (ANSI C with some extensions and some other features left out). The compilers for this language were custom built with portability in mind; according to their author, they "compile quickly, load slowly, and produce medium quality object code".[55]
Though Plan 9 was supposed to be a further development of Unix concepts, compatibility with preexisting Unix software was never the goal for the project. Many command-line utilities of Plan 9 share the names of Unix counterparts, but work differently.[48]
Plan 9 can support POSIX applications and can emulate the Berkeley socket interface through the ANSI/POSIX Environment (APE) that implements an interface close to ANSI C and POSIX, with some common extensions (the native Plan 9 C interfaces conform to neither standard). It also includes a POSIX-compatible shell. APE's authors claim to have used it to port the X Window System (X11) to Plan 9, although they do not ship X11 "because supporting it properly is too big a job".[58] Some Linux binaries can be used with the help of a "linuxemu" (Linux emulator) application; however, it is still a work in progress.[59] Vice versa, the Vx32 virtual machine allows a slightly modified Plan 9 kernel to run as a user process in Linux, supporting unmodified Plan 9 programs.[60]
Reception
Comparison to contemporary operating systems
In 1991, Plan 9's designers compared their system to other early nineties operating systems in terms of size, showing that the source code for a minimal ("working, albeit not very useful") version was less than one-fifth the size of a Machmicrokernel without any device drivers (5899 or 4622 lines of code for Plan 9, depending on metric, vs. 25530 lines). The complete kernel comprised 18000 lines of code.[39] (According to a 2006 count, the kernel was then some 150,000 lines, but this was compared against more than 4.8million in Linux.[43])
Within the operating systems research community, as well as the commercial Unix world, other attempts at achieving distributed computing and remote filesystem access were made concurrently with the Plan 9 design effort. These included the Network File System and the associated vnode architecture developed at Sun Microsystems, and more radical departures from the Unix model such as the Sprite OS from UC Berkeley. Sprite developer Brent Welch points out that the SunOS vnode architecture is limited compared to Plan 9's capabilities in that it does not support remote device access and remote inter-process communication cleanly, even though it could have, had the preexisting UNIX domain sockets (which "can essentially be used to name user-level servers") been integrated with the vnode architecture.[41]
One critique of the "everything is a file", communication-by-textual-message design of Plan 9 pointed out limitations of this paradigm compared to the typed interfaces of Sun's object-oriented operating system, Spring:
Plan 9 constrains everything to look like a file. In most cases the real interface type comprises the protocol of messages that must be written to, and read from, a file descriptor. This is difficult to specify and document, and prohibits any automatic type checking at all, except for file errors at run time. (...) [A] path name relative to a process' implicit root context is the only way to name a service. Binding a name to an object can only be done by giving an existing name for the object, in the same context as the new name. As such, interface references simply cannot be passed between processes, much less across networks. Instead, communication has to rely on conventions, which are prone to error and do not scale.
A later retrospective comparison of Plan 9, Sprite and a third contemporary distributed research operating system, Amoeba, found that
the environments they [Amoeba and Sprite] build are tightly coupled within the OS, making communication with external services difficult. Such systems suffer from the radical departure from the UNIX model, which also discourages portability of already existing software to the platform (...). The lack of developers, the very small range of supported hardware and the small, even compared to Plan 9, user base have also significantly slowed the adoption of those systems (...). In retrospect, Plan 9 was the only research distributed OS from that time which managed to attract developers and be used in commercial projects long enough to warrant its survival to this day.
Plan 9 demonstrated that an integral concept of Unix—that every system interface could be represented as a set of files—could be successfully implemented in a modern distributed system.[52] Some features from Plan 9, like the UTF-8 character encoding of Unicode, have been implemented in other operating systems. Unix-like operating systems such as Linux have implemented 9P2000, Plan 9's protocol for accessing remote files, and have adopted features of rfork, Plan 9's process creation mechanism.[64] Additionally, in Plan 9 from User Space, several of Plan 9's applications and tools, including the sam and acme editors, have been ported to Unix and Linux systems and have achieved some level of popularity. Several projects seek to replace the GNU operating system programs surrounding the Linux kernel with the Plan 9 operating system programs.[65][66] The 9wm window manager was inspired by 8½, the older windowing system of Plan 9;[67]wmii is also heavily influenced by Plan 9.[63] In computer science research, Plan 9 has been used as a grid computing platform[68][62] and as a vehicle for research into ubiquitous computing without middleware.[69] In commerce, Plan 9 underlies Coraid storage systems. However, Plan 9 has never approached Unix in popularity, and has been primarily a research tool:
[I]t looks like Plan 9 failed simply because it fell short of being a compelling enough improvement on Unix to displace its ancestor. Compared to Plan 9, Unix creaks and clanks and has obvious rust spots, but it gets the job done well enough to hold its position. There is a lesson here for ambitious system architects: the most dangerous enemy of a better solution is an existing codebase that is just good enough.
Other factors that contributed to low adoption of Plan 9 include the lack of commercial backup, the low number of end-user applications, and the lack of device drivers.[52][53]
Plan 9 proponents and developers claim that the problems hindering its adoption have been solved, that its original goals as a distributed system, development environment, and research platform have been met, and that it enjoys moderate but growing popularity.[citation needed]Inferno, through its hosted capabilities, has been a vehicle for bringing Plan 9 technologies to other systems as a hosted part of heterogeneous computing grids.[70][71][72]
Several projects work to extend Plan 9, including 9atom and 9front. These forks augment Plan 9 with additional hardware drivers and software, including an improved version of the Upas e-mail system, the Go compiler, Mercurialversion control system support (and now also a git implementation), and other programs.[19][73] Plan 9 was ported to the Raspberry Pi single-board computer.[74][75] The Harvey project attempts to replace the custom Plan 9 C compiler with GCC, to leverage modern development tools such as GitHub and Coverity, and speed up development.[76]
Inferno is a descendant of Plan 9, and shares many design concepts and even source code in the kernel, particularly around devices and the Styx/9P2000 protocol. Inferno shares with Plan 9 the Unix heritage from Bell Labs and the Unix philosophy. Many of the command line tools in Inferno were Plan 9 tools that were translated to Limbo.
9atom[78] augments the Plan 9 distribution with the addition of a 386 PAE kernel, an amd64 cpu and terminal kernel, nupas, extra pc hardware support, IL and Ken's fs.[79][80]
9front[81] is a fork of Plan 9. It was started to remedy a perceived lack of devoted development resources inside Bell Labs, and has accumulated various fixes and improvements.
9legacy[82] is an alternative distribution. It includes a set of patches based on the current Plan 9 distribution.
Akaros[83] is designed for many-core architectures and large-scale SMP systems.
Harvey OS[84] is an effort to get the Plan 9 code working with gcc and clang.
JehanneOS[85] is an experimental OS derived from Plan 9. Its userland and modules are mostly derived from 9front, its build system from Harvey OS, and its kernel is a fork of the Plan9-9k 64-bit Plan9 kernel.
NIX[86][bettersourceneeded] is a fork of Plan9 aimed at multicore systems and cloud computing.
node9[87] is a scripted derivative of Plan9/Inferno that replaces the Limbo programming language and DIS virtual machine with the Lua language and LuaJit virtual machine. It also replaces the Inferno per-platform hosted I/O with Node.js' libuv eventing and I/O for consistent, cross-platform hosting. It's a proof-of-concept that demonstrates that a distributed OS can be constructed from per-process namespaces and generic cloud elements to construct a single-system-image of arbitrary size.
Plan B[88] designed to work in distributed environments where the set of available resources is different at different points in time. Originally based on the third edition Plan 9 kernel, Plan B was moved into user space to run on current Plan 9 systems.[89][90][91]
On March 23, 2021, ownership of Plan 9 transferred from Bell Labs to the Plan 9 Foundation,[93] and all previous releases have been relicensed to the MIT License.[10]
A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language, and is used by the operating system to control the execution of the system using shell scripts.
Acme is a text editor and graphical shell from the Plan 9 from Bell Labs operating system, designed and implemented by Rob Pike. It can use the Sam command language. The design of the interface was influenced by Oberon. It is different from other editing environments in that it acts as a 9P server. A distinctive element of the user interface is mouse chording.
In Unix-like and some other operating systems, the pwd command writes the full pathname of the current working directory to the standard output.
A virtual file system (VFS) or virtual filesystem switch is an abstract layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way. A VFS can, for example, be used to access local and network storage devices transparently without the client application noticing the difference. It can be used to bridge the differences in Windows, classic Mac OS/macOS and Unix filesystems, so that applications can access files on local file systems of those types without having to know what type of file system they are accessing.
Unionfs is a filesystem service for Linux, FreeBSD and NetBSD which implements a union mount for other file systems. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system. Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new, virtual filesystem.
In computing, a hard link is a directory entry that associates a name with a file. Thus, each file must have at least one hard link. Creating additional hard links for a file makes the contents of that file accessible via additional paths. This causes an alias effect: a process can open the file by any one of its paths and change its content. By contrast, a soft link or “shortcut” to a file is not a direct link to the data itself, but rather a reference to a hard link or another soft link.
Inferno is a distributed operating system started at Bell Labs and now developed and maintained by Vita Nuova Holdings as free software under the MIT License. Inferno was based on the experience gained with Plan 9 from Bell Labs, and the further research of Bell Labs into operating systems, languages, on-the-fly compilers, graphics, security, networking and portability. The name of the operating system, many of its associated programs, and that of the current company, were inspired by Dante Alighieri's Divine Comedy. In Italian, Inferno means "hell", of which there are nine circles in Dante's Divine Comedy.
9P is a network protocol developed for the Plan 9 from Bell Labs distributed operating system as the means of connecting the components of a Plan 9 system. Files are key objects in Plan 9. They represent windows, network connections, processes, and almost anything else available in the operating system.
In computing, a named pipe is an extension to the traditional pipe concept on Unix and Unix-like systems, and is one of the methods of inter-process communication (IPC). The concept is also found in OS/2 and Microsoft Windows, although the semantics differ substantially. A traditional pipe is "unnamed" and lasts only as long as the process. A named pipe, however, can last as long as the system is up, beyond the life of the process. It can be deleted if no longer used. Usually a named pipe appears as a file, and generally processes attach to it for IPC.
Filesystem in Userspace (FUSE) is a software interface for Unix and Unix-like computer operating systems that lets non-privileged users create their own file systems without editing kernel code. This is achieved by running file system code in user space while the FUSE module provides only a bridge to the actual kernel interfaces.
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. Typically, it is mapped to a mount point named /proc at boot time. The proc file system acts as an interface to internal data structures about running processes in the kernel. In Linux, it can also be used to obtain information about the kernel and to change certain kernel parameters at runtime (sysctl).
In software development, time-of-check to time-of-use is a class of software bugs caused by a race condition involving the checking of the state of a part of a system and the use of the results of that check.
Extended file attributes are file system features that enable users to associate computer files with metadata not interpreted by the filesystem, whereas regular attributes have a purpose strictly defined by the filesystem. Unlike forks, which can usually be as large as the maximum file size, extended attributes are usually limited in size to a value significantly smaller than the maximum file size. Typical uses include storing the author of a document, the character encoding of a plain-text document, or a checksum, cryptographic hash or digital certificate, and discretionary access control information.
On Microsoft Windows, a special folder is a folder that is presented to the user through an interface as an abstract concept instead of an absolute folder path. Special folders make it possible for any application to ask the operating system where an appropriate location for certain kinds of files can be found; independently of which version or user language of Windows is being used.
In computer operating systems, union mounting is a way of combining multiple directories into one that appears to contain their combined contents. Union mounting is supported in Linux, BSD and several of its successors, and Plan 9, with similar but subtly different behavior.
In computer science, a synthetic file system or a pseudo file system is a hierarchical interface to non-file objects that appear as if they were regular files in the tree of a disk-based or long-term-storage file system. These non-file objects may be accessed with the same system calls or utility programs as regular files and directories. The common term for both regular files and the non-file objects is node.
Unix is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. Initially intended for use inside the Bell System, AT&T licensed Unix to outside parties in the late 1970s, leading to a variety of both academic and commercial Unix variants from vendors including University of California, Berkeley (BSD), Microsoft (Xenix), Sun Microsystems (SunOS/Solaris), HP/HPE (HP-UX), and IBM (AIX).
In Unix and operating systems inspired by it, the file system is considered a central component of the operating system. It was also one of the first parts of the system to be designed and implemented by Ken Thompson in the first experimental version of Unix, dated 1969.
Namespaces are a feature of the Linux kernel that partition kernel resources such that one set of processes sees one set of resources, while another set of processes sees a different set of resources. The feature works by having the same namespace for a set of resources and processes, but those namespaces refer to distinct resources. Resources may exist in multiple namespaces. Examples of such resources are process IDs, host-names, user IDs, file names, some names associated with network access, and Inter-process communication.
↑ "Plan9License". akaros.cs.berkeley.edu. Archived from the original on 13 February 2014. Retrieved 14 June 2021. The University of California, Berkeley, has been authorised by Alcatel-Lucent to release all Plan 9 software previously governed by the Lucent Public License, Version 1.02 under the GNU General Public License, Version 2.
↑ Hancock, Brian (2003). "Reinventing Unix: an introduction to the Plan 9 operating system". Library Hi Tech. 21 (4). MCB UP: 471–76. doi:10.1108/07378830310509772.
1 2 Welch, Brent (1994). "A comparison of three distributed file system architectures: Vnode, Sprite, and Plan 9". Computing Systems. 7 (2): 175–199. CiteSeerX10.1.1.46.2817.
1 2 Mirtchovski, Andrey; Simmonds, Rob; Minnich, Ron (2004). Plan 9—an integrated approach to grid computing. Proc. 18th Int'l Parallel and Distributed Processing Symp. IEEE. CiteSeerX10.1.1.97.122.
1 2 "window manager improved 2". suckless.org. Archived from the original on 2011-12-31. Retrieved 2012-01-02. [wmii] has a 9p filesystem interface and supports classic and tiling (acme-like) window management.
↑ "The 9wm Window Manager". 9wm. Retrieved 2012-01-02. 9wm is an X window manager which attempts to emulate the Plan 9 window manager 8-1/2 as far as possible within the constraints imposed by X.
↑ Ballesteros, Francisco J.; Guardiola, Gorka; Soriano, Enrique; Leal Algara, Katia (2005). Traditional systems can work well for pervasive applications. A case study: Plan 9 from Bell Labs becomes ubiquitous. IEEE Intl' Conf. on Pervasive Computing and Communications. CiteSeerX10.1.1.109.8131.
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