Editing Plan 9 from Bell Labs (section)

=== Union directories and namespaces ===
{{anchor|Union directory|Name}}
Plan 9 allows the user to collect the files (called ''names'') from different directory trees in a single location. The resulting ''[[Union (set theory)|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 ({{mono|ls}} or {{mono|lc}}) will simply report duplicate names.{{r|valerie-aurora}} Resolution of a single path name is performed top-down: if the directories {{mono|top}} and {{mono|bottom}} are unioned into {{mono|u}} with {{mono|top}} first, then {{mono|u/name}} denotes {{mono|top/name}} if it exists, {{mono|bottom/name}} only if it exists ''and {{mono|top/name}} does not exist'', and no file if neither exists. No recursive unioning of subdirectories is performed, so if {{mono|top/subdir}} exists, the files in {{mono|bottom/subdir}} are not accessible through the union.{{r|4.4bsd}}

A union directory can be created by using a sequence of {{mono|bind}} commands:

<pre>
bind /arm/bin /bin
bind -a /acme/bin/arm /bin
bind -b /usr/alice/bin /bin
</pre>

In the example above, {{mono|/arm/bin}} is mounted at {{mono|/bin}}, the contents of {{mono|/arm/bin}} replacing the previous contents of {{mono|/bin}}. [[Acme (text editor)|Acme]]'s {{mono|bin}} directory is then union mounted after {{mono|/bin}}, and Alice's personal {{mono|bin}} directory is union mounted before. When a file is requested from {{mono|/bin}}, it is first looked for in {{mono|/usr/alice/bin}}, then in {{mono|/arm/bin}}, and then finally in {{mono|/acme/bin/arm}}.

The separate process namespaces thus usually replace the notion of a [[PATH (variable)|search path]] in the shell. A path environment variable ({{code|$path}}) still exists in the [[Rc (Unix shell)|rc]] shell (the shell mainly used in Plan 9); however, rc's path environment variable conventionally only contains the {{code|/bin}} and {{code|.}} directories and modifying the variable is discouraged, instead, adding additional commands should be done by binding several directories together as a single {{code|/bin}}.<ref name="rc_the_plan_9_shell">{{cite book |last1=Duff |first1=Tom |title=Rc — The Plan 9 Shell |url=http://doc.cat-v.org/plan_9/4th_edition/papers/rc|section=18|work=Plan 9, 4th edition}} ([https://www.scs.stanford.edu/nyu/04fa/sched/readings/rc.pdf PDF]);</ref><ref name= "distributed-system" /> 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,<ref name="HerbertJones2004"/> 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.<ref name = "use-of-namespaces" /> Namespaces may be used to create an isolated environment similar to [[chroot]], but in a more secure way.<ref name="unix-spirit-set-free" />

Plan 9's union directory architecture inspired [[4.4BSD]] and [[Linux]] [[union mount|union file system]] implementations,{{r|valerie-aurora}} 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".{{r|4.4bsd}}
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