

Ask HN: Why is the dot/period character used as file extension separator? - zuck9

Why dot and not any other character? What&#x27;s the historical reason behind it?
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cafard
I don't know why. VMS also used it to indicate the revision number of a file,
as I recall--demo.c.1, demo.c.2 ...--but it has been a while.

Years ago I worked with Data General minis. The 16-bit ones ran the operating
system RDOS (real-time disk operating system), the 32-bit ones AOS/VS
(advanced operating system with virtual storage. All used the '.' to mark of
stem from extension, but all also had a file type associated in the directory.
This meant, among other things, the the assembler would balk at processing
something called urgent.asm if the directory showed that it was a generic
file. That was baffling the first time you encountered it.

~~~
mveety
VMS also used the semicolon to do version information if you're interfacing
with DCL or DECWindows (or NFS I think too). The dot was also used as the
directory separator. In VMS land your path would look something like (in the
case of my login.com): DSA0:[USERS.MVEETY]LOGIN.COM;5

~~~
productionQA
My company still uses VMS today. This:

DSA0:[USERS.MVEETY]LOGIN.COM;5

Looks all too familiar to me :-/ I even still have to write some DCL from time
to time!

I swear we are going to be the last one's off this system.

~~~
atsaloli
VMS isn't dead yet.

HP just licensed VMS to vmssoftware.com:

[http://www.vmssoftware.com/news/announcement/faq.pdf](http://www.vmssoftware.com/news/announcement/faq.pdf)

Roadmap:
[http://www.vmssoftware.com/news/announcement/RM/VMS_Software...](http://www.vmssoftware.com/news/announcement/RM/VMS_Software_Roadmap.pdf)

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mahmud
Unix has no concept of file extension. The dot is just like any other
constituent character. To distinguish between file types, Unix uses the
file(1) command and its database of manually curated magic strings and
heuristics.

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gjvc
because the underscore is evil

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lutusp
> Why is the dot/period character used as file extension separator?

Why not? Maybe it was realized that other candidate non-alphabetic symbols
were already reserved for other things, like '/' as a path delimiter on
Unices, '\' same purpose on Windows, '#' as a comment delimiter, '%' as the
modulo operator, '|' as the command pipe symbol, '^' to signify either power
or exclusive-or depending on context ... we're quickly running out of
candidate symbols on an ASCII keyboard.

~~~
mooism2
Windows/MS-DOS had file extensions (which it inherited from CP/M) before it
had directories and directory separators.

Did CP/M and Unix independently come up with file extensions with a dot
separator, or did they acquire it from the same source? I'm dimly aware of
other operating systems that use(d) a different character (AS/400?), but it's
outside my ken.

~~~
ja27
I believe Microsoft's file extensions and the 8.3 naming scheme are descended
from Digital's 6.3 naming in VMS, RT-11, etc. DEC even used a number of the
same extensions: EXE, COM, OBJ. Not sure where they came from prior to that. I
don't think IBM used file extensions like that in System/360, System/32, or
anything prior to PC-DOS. From what I can tell, Burroughs and Univac didn't
use them either.

