mak@mbf.UUCP (03/01/90)
At Date: 19 Feb 90 18:29:46 GMT , peter@ficc.uu.net (Peter da Silva) said:
--
--I just read in the recent /usr/group "white paper" that...
--
-- Earlier drafts de-emphasised the !test! command in favor of
-- a new ![[ ]]! command. This new invention was discarded in the
-- current draft (Draft 9), but a new shell keyword, !, was added
-- to negate the return value of a pipeline to make test easier
-- to use for complex expressions.
--
--I fail to understand what was wrong with the existing ![ ]! command.
--
--Could someone elaborate?
Peter,
I am not sure what all those !'s mean. But in re-reading the
first (included) paragraph, my guess is that I should refer you back
to Draft 8. Page 128, beginning with line 1063, reiterates
some of Dave's reasons for the [[ ]] vs. test. I remember
that he based this rationale on a study about shell programming
errors (internal to Bell Labs?).
Korn's data showed that a new, quoting syntax, ala
[[ ]], would be less prone to programming error, and it could be made to
have a richer set of tests. Unfortunately, the quoting itself is a
problem for standardization. (see my postscript.)
But there really is nothing there which explains why [[ ]] instead of
[ ], per se. I believe that it separates [ ] and [[ ]] semantics so
extra goodies can be put into the shell "language."
Also, I do note that [ ] is missing from Bourne's paper on the
UNIX shell, BLTJ, July/Aug '78. All that means (to me) is that [ ]
MAY not have been in bourne shell. (Please note the dichotomatic use
of the terms, bourne shell and /bin/sh. My friends at Summit tell me
that these are different shells.)
I just hope that I was able to shed some light on the subject.
By the way, tell me what is this "white paper!"
= Bob Makowski
P.S. Between Draft 8 and Draft 9, the [[ ]] and (( )) were removed to
resolve balloting objections.
The Balloting Group has realized that the [[ ]] and the (( )) syntax
break compatability with BOURNE, /bin/sh, csh: the problem is not so
much the [[ ]] or (( )), per se. It's what goes inside these delimiters.
o ksh required new semantics for shell meta-characters
Let's start with * : it has a new meaning inside the (( )).
If a ksh script is run by /bin/sh or csh, * could be filename
expanded; so for ksh to work these math expressions, (( )) has to
quote the file metacharacters. (In fact, Draft 8, line 721, p122,
says (( )) behaves as if (" ") quoting were applied.)
Obviously, (( )) does not have this semantic in csh or /bin/sh.
For [[ ]], the situation is more or less the same; e.g., in Draft
8, [[ ]], as well as (( )), support > and < for comparisions.
Again, ksh quotes inside [[ ]], whereas /bin/csh and csh would not.
o counterindications: a thought experiment
So SUPPOSE that a ksh script were run by /bin/sh. (There are only
a million binary copies, so it might be important case ;-)
So SUPPOSE that /bin/sh interpreted * , < , > from inside (( ))
and [[ ]].
With * in a math expression, there'd be filenames; With > in [[
]], a file would be clobbered to a null length.
o unwinding and implementations
I tried these on SVR3.2 and grammar does not "unwind" enough to
interpret > or < as re-direction. But this is an "accident" of the
implementation; I do not recall a specification requiring this to
happen for POSIX compliant systems. Moreover, there is a strong
predeliction against POSIX' requiring implementations.
(Thankfully, this has much to do with the IEEE, per se, but it's
also due to the DEC suit over the SVID in AFCAC.)
--
= Mak :.................................................................:
: [Americans] have a philosophical method ... To evade the bondage of :
: system and habit, of family maxims, class opinions, and in some :
: degree, of national prejudices; to accept tradition, only as a lesson :
: to be used in doing otherwise and doing better; ... :
: ............................................Alexis de Tocueville, 1790:henry@utzoo.uucp (Henry Spencer) (03/02/90)
In article <2567@mbf.UUCP> uunet!attmail!mak writes: >Also, I do note that [ ] is missing from Bourne's paper on the >UNIX shell, BLTJ, July/Aug '78. All that means (to me) is that [ ] >MAY not have been in bourne shell. (Please note the dichotomatic use >of the terms, bourne shell and /bin/sh... It depends on what you mean by "in". :-) The original Seventh Edition test(1), which was not part of the shell, was prepared to cope properly with being invoked under the name `['. However, as I recall it didn't actually exist under that name in /bin as distributed, so there is some room for doubt about whether this feature was fully operational. But it does seem to have come from Steve Bourne. -- MSDOS, abbrev: Maybe SomeDay | Henry Spencer at U of Toronto Zoology an Operating System. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
peter@ficc.uu.net (Peter da Silva) (03/02/90)
[Time for a mvgroup to comp.org.uniforum? :->] > I am not sure what all those !'s mean. Oh, it's a convention I picked up somewhere to represent "quoted program text". > But in re-reading the > first (included) paragraph, my guess is that I should refer you back > to Draft 8. Page 128, beginning with line 1063, ... Where can I get these drafts? > Korn's data showed that a new, quoting syntax, ala > [[ ]], would be less prone to programming error, and it could be made to > have a richer set of tests. Unfortunately, the quoting itself is a > problem for standardization. (see my postscript.) Good idea, and I can see why they would want to use [[ ]] instead of [ ] for that... nothing worse than a silent change. If that's the case then I guess that answers my question. [ ] isn't necessarily going away, it's just not relevent to this point. > I just hope that I was able to shed some light on the subject. Yes Thanks. > By the way, tell me what is this "white paper!" POSIX Update: Shell and Utilities/An Update on the IEEE P1003.2/Shell and Utilities/Working Group//December 1989//by/Hal Jespersen. I like the $[expr] syntax, by the way. Is there any interest in adding the korn shell $(command) syntax as a (nestable) synonym for grave accents? -- _--_|\ Peter da Silva. +1 713 274 5180. <peter@ficc.uu.net>. / \ \_.--._/ Xenix Support -- it's not just a job, it's an adventure! v "Have you hugged your wolf today?" `-_-'
skwu@boulder.Colorado.EDU (WU SHI-KUEI) (03/02/90)
My AT&T Release 5.0 (June 1982) manual page for 'test' reads in part:
WARNING
In the second form of the command (i.e., the one that uses [], rather
than the word *test*), the square brackets must be delimited by
blanks.
Some UNIX systems do not recognize the second form of the command.karl@haddock.ima.isc.com (Karl Heuer) (03/16/90)
In article <1990Mar8.171918.16011@mks.com> eric@mks.com (Eric Gisin) writes: >In article <16108@haddock.ima.isc.com>, karl@haddock.ima.isc.com (Karl Heuer) writes: >>My suggestion would be: $\ (dollar-backslash) begins a C-like escape, so we >>would have $\a $\b $\t $\n $\v $\f $\r, octal escapes like $\177, and hex >>escapes like $\x7F. [Also maybe $\^X for ctrl-X.] > >My idea for special strings is $"...", where "..." is a C-like string with >the ANSI C \ escapes. I like that one, too. More precisely, I think the semantics should be the same as a normal double-quoted string except that there are more backslash escapes (besides the four that currently exist: \$ \` \" \\). Is comp.std.unix a better place to discuss this? Karl W. Z. Heuer (karl@ima.ima.isc.com or harvard!ima!karl), The Walking Lint