Why did Unix use slash as the directory separator?

Question

The Unix designers came from the GE/MIT Multics project, and Multics inspired some Unix features. In particular, Multics has a hierarchical filesystem, and so does Unix.

On Multics, pathnames were of the form:

>dir1>dir2>dir3>filename 

but Unix uses

/dir1/dir2/dir3/filename 

Is there any documentation of why they chose that syntax instead of copying Multics?

I thought of this because > has become common in modern day displays of other hierarchical paths, such as breadcrumb lists in web applications and CSS child selectors.

Multics also used < to represent upward motion in relative pathnames, e.g. Unix

../../foo/bar 

would be

<<foo>bar 

This has a nice symmetry that ../ lacks, although .. has the advantage of being implemented in the filesystem itself (it's simply a link to the parent directory). On Multics, < is implemented in a user-mode library that canonicalizes pathnames before being passed to the kernel.

Answer 1

Primordial Unix on the PDP-7 was in many ways very different from what we know today. Directories existed but were very awkward to use; in practice most work was done in a single directory. Most importantly, paths did not yet exist. What was implemented early on though was a simple syntax for IO redirection on the commandline. Whereas on Multics one would have to say¹

iocall attach user_output file xx list iocall attach user_output syn user_i/o 

on Unix one could simply say

ls >xx 

STDIN could similarly be redirected.

When paths were eventually implemented (on PDP-11 Unix) < and > were already in use so a new syntax was needed. I am unaware of the precise reason / was chosen for this; it's likely that its convenient location on Teletype Model 37² terminals was a consideration.

¹iocall was an 'author-maintained' (i.e. non-standard) command.

² The Model 37 could easily be adapted to handle mathematical symbols; this was important since early Unix was intended to be used for technical writing (see e.g. roff). The / is next to right-shift on the Model 37, same as on most modern keyboards.

Answer 2

There were other hierarchical file systems around.
The Burroughs operating systems used (and as Unisys, still do) / Although in a someone clunky way:

(PHYS212)ACOUSTICS/RAYS/OUTPUT ON USERPACK ^^^^ usercode ^^^^^^^ device 

MS-DOS and Windows also accept / in file paths, in addition to \. It is the command language where this does not work; system calls are fine.

EMAS-3 used {username:]{groupname.}*localname{_partname} where username is up to 6 letters and digits and the others are up to 11, groups are basically directories except for not being files, and parts have to do with "partitioned data sets" (think ar(1) archives but usefully nested).

Answer 3

DTSS, developed around the same time as Multics, and influenced by it, used : as the directory separator. A leading : was used for absolute paths (starting at the root directory). There was no easy way to indicate "parent directory" (Unix/Linux ..) — programs that needed to walk the directory tree had to keep track of things themselves.