Ksh93 has unexpected, undocumented, support for math functions

I have been thinking about whether I want to contribute to the maintenance of the Korn shell (ksh93) since it was open sourced in 2013. While trying to understand the organization of the project and how to build it I noticed that the math builtin (e.g., $(( ... )) ) supports a lot of functions I was not aware of despite having used the Korn shell for more than two decades. Consider these examples:

$ echo $(( nearbyint(12.5) ))
12
$ echo $(( round(12.5) ))
13
$ echo $(( rint(12.5) ))
12

Why in the world should a command line shell support multiple methods to round a floating point value to an integer? There are numerous other examples of that type and they all derive from the ksh authors thinking that exposing low-level APIs to a shell script was appropriate.

These functions are defined in src/cmd/ksh93/data/math.tab. None are documented outside of the source code other than a pro-forma mention in the documentation that they exist (see the section labeled “Arithmetic Evaluation” in the man page) as prefaced by this text:

Any of the following math library functions that are in the C math library can be used within an arithmetic expression:

These are the functions in that table:

# <return type: i:integer f:floating-point> [<typed -arg-bitmask> < #floating-point-args> <function -name> [
# </function><function -name>l and </function><function -name>f variants are handled by features/math.sh
# @(#)math.tab (AT&T Research) 2013-08-11
f 1 acos
f 1 acosh
f 1 asin
f 1 asinh
f 1 atan
f 2 atan2
f 1 atanh
f 1 cbrt
f 1 ceil
f 2 copysign
f 1 cos
f 1 cosh
f 1 erf
f 1 erfc
f 1 exp
f 1 exp10
f 1 exp2
f 1 expm1
f 1 fabs abs
f 2 fdim
i 1 finite
f 1 float
f 1 floor
f 3 fma
f 2 fmax
f 2 fmin
f 2 fmod
i 1 fpclassify
i 1 fpclass
f 2 hypot
i 1 ilogb
f 1 int
i 1 isfinite
i 2 isgreater
i 2 isgreaterequal
i 1 isinf
i 1 isinfinite
i 2 isless
i 2 islessequal
i 2 islessgreater
i 1 isnan
i 1 isnormal
i 1 issubnormal fpclassify=FP_SUBNORMAL
i 2 isunordered
i 1 iszero fpclassify=FP_ZERO fpclass=FP_NZERO|FP_PZERO {return a1==0.0||a1==-0.0;} j0
f 1 j1
f 2 jn
x 2 ldexp
f 1 lgamma
f 1 log
f 1 log10
f 1 log1p
f 1 log2
f 1 logb
f 1 nearbyint
f 1 2 nextafter
f 1 2 nexttoward
f 2 pow
f 2 remainder
f 1 rint
f 1 round {Sfdouble_t r;Sflong_t y;y=floor(2*a1);r=rint(a1);if(2*a1==y)r+=(r<a1 )-(a1<0);return r;}
f 2 scalb
f 2 scalbn
i 1 signbit
f 1 sin
f 1 sinh
f 1 sqrt
f 1 tan
f 1 tanh
f 1 tgamma {Sfdouble_t r=exp(lgamma(a1));return (signgam<0)?-r:r;}
f 1 trunc
f 1 y0
f 1 y1
f 2 yn

The only way I would contribute to the evolution of ksh93 is if this bogosity were eliminated. There is no reason that a CLI like ksh/ksh93 should support all of those math functions. In fact most of those functions have no business being available in a CLI. Consider what it means to execute $(( isfinite(1) )). In the context of a CLI shell script the isifinite() function has no meaning.

Is ksh93 still alive?

As I mentioned in my previous article I’m looking for a new shell since I’ve given up on the Fish project. For many years I used ksh88 then ksh93. After that I switched to zsh because it looked like ksh was a dead project. But two years ago the AT&T Software Technology (“AST”) toolkit was moved to Github and open sourced. In the past year an individual has committed some changes to the ksh source code. If it’s once again being improved it might be worth a look.

So in addition to elvish I think I’ll take another look at ksh. The Korn shell lacks many of the features people have come to expect from newer shells. Most notably a good command completion subsystem. But the ksh source code is pretty clean. It has a consistent style and good interfaces. There are things about its style I don’t like such as the use of single statement blocks that are not enclosed in braces since that pattern makes it too easy to introduce a bug and makes it harder to visually parse the code. Here’s an example from the getopts.c module:

        if(r<0)
                r = 0;

Still, at least the code is consistent in employing that pattern. It also omits whitespace around binary operators like minus and commas that separate parameters. At least most of time. Something I think hurts readability especially since it doesn’t do so 100% of the time. I’d probably want to run the code through clang-format and otherwise manually fix the remaining style inconsistencies before contributing more substantive changes. Much like I did for fish. The question is whether the people with commit privileges would accept such changes. And whether they would be open to the idea of implementing some of ideas from newer shells like fish.

Time to pick a new shell: fish, xonsh, elvish, bash, zsh, ksh93

Why aren’t there any good alternatives to bash or zsh? Specifically, a OS CLI shell that does not suffer from the problems inherent in being compliant with the POSIX.2 (aka POSIX 1003.2) standard? And also doesn’t suffer from the other problem that bash and zsh have due to all the configurable behaviors that make it effectively impossible to predict how those shells will behave?

Two years ago I got fed up with zsh and wrote a blog post why. That caused me to look for a saner alternative. I considered xonsh because it is based on Python which is one of my two favorite programming languages (the other being C). But I gave up on Xonsh fairly quickly. I eventually settled on Fish.

I had some misgivings about the Fish project because, as of Sep 2015, it had numerous open pull-requests. Including several more than a year old and quite a few more older than three months. The source code (both C++ and functions written in its native script language) was also a mess with no coherent style and hundreds of lint errors. At least two lint errors identified actual bugs and most of the remainder pointing out code that was hard to understand. Too, there were a couple hundred open issues more than three years old which even a cursory review suggested were no longer relevant. Nonetheless, I chose it as my new day to day shell and started contributing changes to the project. Primarily because its explicit non-adherence to the POSIX 1003.2 standard for shells meant it had far fewer surprising behaviors. It also had several innovative ideas such as every variable being an array even if it contained only a single element.

Near the end of 2015 I was offered commit privileges; that is, becoming a member of the core development team. I demurred at that time because I wasn’t sure I was going to use Fish long term. But in late January 2016 I accepted the invite to join the core dev team. Primarily because the development team expressed no objections to my requirement that the code style be standardized and changes to deal with oclint and cpplint errors would be merged without debate.

I spent several months doing things like running the code through clang-format and oclint and fixing problems they found. As well as making it easy for everyone else to do so by adding new build targets like make style and make lint. Once the code was in a better state I felt more comfortable making more substantive changes.

Fast-forward to today where I am looking for a different shell to be my day to day interactive shell. Why? There are several reasons. Such as the fact that the Fish model for I/O redirection and pipelines is broken and results in FAQs from people who expect saner behavior provided by nearly every other shell. Broken behavior I accepted till now because I hoped that such problems would be fixed sooner rather than later. I no longer believe that will happen. Again, why? Because there have been too many arguments over bike-shedding issues like whether all uppercase var names like FISH_HISTORY and FISH_VERSION should be renamed to lowercase. Or, in the case of FISH_VERSION, renamed to version because one developer is a fan of csh (actually tcsh although they never use that term). More importantly no one but myself seemed to be interested in setting a consistent vision for future releases with milestones and a roadmap.

Another example: Fish issue #478 was opened five years ago by xiaq to suggest improving how commands to options are handled. Keep in mind the ID of the person who opened that issue as it’s important to this blog post. I commented a year ago asking if we should implement a Fish compatible getopt command. Fast forward to today. I opened a new issue asking for feedback on a different approach to issue #478. Mostly because it seemed like the DocOpt based solution would never be implemented. Too, I was not convinced the DocOpt idea was sound. I implemented the argparse command and the feedback was uniformly positive and resulted in several subsequent improvements. The Fish developer I’ve clashed with from the day they were granted commit privileges then reopened issue #478 and the leader of the fish project stated they still wanted to try and implement that solution. I’m wondering if I’ll die of old age before that happens. Especially since it has been more than two years since the last substantive commit to the Python DocOpt project which has been open five years, and was the basis for the fish-shell implementation, and it still hasn’t reached v1.0 release status.

I have no intention of going back to either bash or zsh as my day to day interactive shell. They have too much baggage and broken behaviors. Zsh in particular suffers from a lack of a coherent vision which has lead to the project incorporating too many incompatible behaviors via configurable options.

So I’m once again looking for an innovative shell that I could use on a daily basis and would be willing to contribute non-trivial changes to the project. I took another look at Xonsh but was put off by the fact it has several pull-requests more than a year old and most of the others are more than a month old. Too, it has recently implemented things like the cat command as a builtin which seems rather pointless.

The current candidate for my new day to day shell is Elvish. It is even more extreme in its departure from traditional POSIX.2 shells than fish compared to shells like ksh, bash, and zsh. But therein lies its strength. Elvish is based on sound programming language principles rather than the adhoc mess that is the original Bourne shell and all subsequent, POSIX.2, based shells. My primary concern is how it handles external commands that exit with a non-zero status. At present it turns that into an exception. Which means Elvish behaves as if bash/ksh/zsh was running with set -o errexit in effect. This is great from a safety perspective. The problem is that there are a large number of commands which exit with a non-zero status for non-fatal situations. The grep command, for example, exits with status of one if no lines matched the pattern. That is rarely a fatal error that deserves raising an exception.