Lexical File Names in Plan 9, or, Getting Dot-Dot Right

Symbolic links make the Unix file system non-hierarchical, resulting in multiple valid path names for a given file. This ambiguity is a source of confusion, especially since some shells work overtime to present a con­ sistent view from programs such as pwd, while other programs and the kernel itself do nothing about the problem. Plan 9 has no symbolic links but it does have other mechanisms that pro­ duce the same difficulty. Moreover, Plan 9 is founded on the ability to control a program s environment by manipulating its name space. Ambiguous names muddle the result of operations such as copying a name space across the network. To address these problems, the Plan 9 kernel has been modified to main­ tain an accurate path name for every active file (open file, working direc­ tory, mount table entry) in the system. The definition of accurate is that the path name for a file is guaranteed to be the rooted, absolute name the program used to acquire it. These names are maintained by an effi­ cient method that combines lexical processing such as evaluating .. by just removing the last path name element of a directory with local oper­ ations within the file system to maintain a consistently, easily understood view of the name system. Ambiguous situations are resolved by examin­ ing the lexically maintained names themselves. A new kernel call, fd2path, returns the file name associated with an open file, permitting the use of reliable names to improve system ser­ vices ranging from pwd to debugging. Although this work was done in Plan 9, Unix systems could also benefit from the addition of a method to recover the accurate name of an open file or the current directory. Motivation Consider the following unedited transcript of a session running the Bourne shell on a modern Unix system: