[TUHS] Command-line options
Greg 'groggy' Lehey
grog at lemis.com
Sat Mar 26 09:25:16 AEST 2016
On Saturday, 26 March 2016 at 7:29:25 +1000, Warren Toomey wrote:
> On Fri, Mar 25, 2016 at 05:43:08PM +0300, Aharon Robbins wrote:
>> One of Unix's signature hallmarks is its terseness: short command names
>> like mv, ln, cp, cc, ed; short options (a dash and a single letter),
>> programs with just a few, if any, options at all, and short path names:
>> "usr" instead of "user", "src" instead of "source" and so on.
>> I have long theorized that the reason for the short names is that since
>> typing was so physically demanding, it was natural to make the command
>> names (and all the rest) be short and easier to type. I don't know if
>> this was a conscious decision, but I suspect it more likely to have been
>> an unconscious / natural one.
> I'm going to throw in an aside at this point. PDP-7 Unix packed 2
> characters per 18-bit word.
Really? Most machines of that era used 6 bit characters, so you could
fit 3 in an 18 bit word. I had hypothesized that that was the reason
for word lengths in increments of 6 bits in those days.
> So, when comparing things, it's easy to compare one word against
> another. I believe this is why command-line options were 2
> characters (e.g. -l, -v, -c, -d) etc.
You've seen the code, of course. Is that how it was done?
It's interesting to note that large UNIVAC systems in those days had a
similar option syntax. The 1100 series would add them after a comma.
Here an example from a random command file:
ASG meant "assign", and the options were after the comma. I'm a bit
vague about the exact meaning of the options, but they both associated
files with the current job.
The 494 did things slightly differently: there was no comma, and the
options came as a group, so instead of "@ASG,T" you might write
"#ASG T". This is very reminiscent of some early Unix stuff, such as
tar. The options were parsed by the shell equivalent (forget the
name) and packed into a single word bitmap.
In passing, it's interesting to note that the initial @ or # were used
to identify a command. Other systems of the time did similar things.
My guess is that the command prompt in Unix started life as a "here's
your command leadin for free".
I don't think that Unix inherited anything from UNIVAC; it's more
likely that the similarities are indicative of common descent.
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