Re: [ -d (#i)Temp ]

[Alexis <flexibeast@xxxxxxxxx>]

Ray Andrews <rayandrews@xxxxxxxxxxx> writes:

> Yeah, it strikes me a very strange that '[' is a binary, guess 
> you'd
> have to have been there back when the shells were being
> written.

The first Unix shell was the Thompson shell, released in 
1971. (The Bourne shell appeared in 1979 as part of Version 7 
Unix.) A 1976 paper describes the reasoning behind how shell 
control structures were implemented:

> The more complex shell control structures are actually 
> implemented as
> independent commands that cooperate with the shell, but are not
> actually part of it. They are designed specifically for use in 
> shell
> procedures, but are treated as ordinary commands. This 
> separation of
> function allows the shell to remain a small program, efficient 
> for
> on-line use, but still able to support powerful control 
> structures in
> procedures.
-- https://grosskurth.ca/bib/1976/mashey-command.pdf, p. 5

In 1976, Version 6 Unix ran on PDP-11s whose bus width ranged from 
16 to 22 bits; 22 bits allows directly addressing at most 512k of 
memory. The Thompson shell was designed to be small to maximise 
the space available for the programs it was running.

Then issues of 'backwards compatibility' did the rest[a].


Alexis.

[a] Notoriously, the reason Makefiles today need to use tabs is 
because the author of the original `make`, Stuart Feldman, was 
concerned about backwards compatibility:

> Why the tab in column 1? Yacc was new, Lex was brand new. I 
> hadn't
> tried either, so I figured this would be a good excuse to 
> learn. After
> getting myself snarled up with my first stab at Lex, I just did
> something simple with the pattern newline-tab. It worked, it
> stayed. And then a few weeks later I had a user population of 
> about a
> dozen, most of them friends, and I didn't want to screw up my 
> embedded
> base. The rest, sadly, is history.