2005 lines
85 KiB
TeX
2005 lines
85 KiB
TeX
%&latex -*- latex -*-
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\documentstyle[code,11pt,lcs-note,boxedminipage,openbib,twoside,
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palatino,ct]{article}
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\def\ie{\mbox{\em i.e.}} % \mbox keeps the last period from
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\def\etc{\mbox{\em etc.}}
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\def\Lisp{{\sc Lisp}}
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\def\CommonLisp{{\sc Common Lisp}}
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\def\Ascii{{\sc Ascii}}
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\def\Unix{{Unix}} % No \sc, according to Bart.
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\def\Scheme{{Scheme}} % No \sc.
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\def\scm{{Scheme 48}}
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\def\R4RS{R4RS}
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\newcommand{\synteq}{{\rm ::=}}
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%% What you frequently want when you say \tt:
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\newcommand{\remark}[1]{\mbox{$<<$}{\bf #1}\mbox{$>>$}}
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% For use in code. The \llap magicness makes the lambda exactly as wide as
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% the other chars in \tt; the \hskip shifts it right a bit so it doesn't
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% crowd the left paren -- which is necessary if \tt is cmtt.
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% Note that (\l{x y} (+ x y)) uses the same number of columns in TeX form
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% as it produces when typeset. This makes it easy to line up the columns
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% in your input. \l is bound to some useless command in LaTeX, so we have to
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% define it w/renewcommand.
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\let\oldl\l %Save the old \l on \oldl
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\makeatother
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%%% End preamble
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\begin{document}
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\notenum{3}
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\project{Personal Information Architecture}
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\title{A {\Scheme} Shell}
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\author{Olin Shivers \\ {\ttt shivers@lcs.mit.edu}}
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\date{4/94}
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\maketitle
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\pagestyle{empty}
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\thispagestyle{empty}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\headingquote{
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Although robust enough for general use, adventures \\
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into the esoteric periphery of the C shell may reveal \\
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unexpected quirks.}
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{SunOS 4.1 csh(1) man page, 10/2/89}
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\vspace{-2em}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section*{Prologue}
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%\addcontentsline{toc}{section}{Prologue}
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Shell programming terrifies me. There is something about writing a simple
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shell script that is just much, much more unpleasant than writing a simple C
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program, or a simple {\CommonLisp} program, or a simple Mips assembler program.
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Is it trying to remember what the rules are for all the different quotes? Is
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it having to look up the multi-phased interaction between filename expansion,
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shell variables, quotation, backslashes and alias expansion? Maybe it's having
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to subsequently look up which of the twenty or thirty flags I need for my
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grep, sed, and awk invocations. Maybe it just gets on my nerves that I have to
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run two complete programs simply to count the number of files in a directory
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(\ex{ls | wc -l}), which seems like several orders of magnitude more cycles
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than was really needed.
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Whatever it is, it's an object lesson in angst. Furthermore, during late-night
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conversations with office mates and graduate students, I have formed the
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impression that I am not alone. In late February\footnote{February 1992, that
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is.}, I got embroiled in a multi-way email flamefest about just exactly what it
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was about Unix that drove me nuts. In the midst of the debate, I did a rash
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thing. I claimed that it would be easy and so much nicer to do shell
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programming from {\Scheme}. Some functions to interface to the OS and a few
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straightforward macros would suffice to remove the spectre of \cd{#!/bin/csh}
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from my life forever. The serious Unix-philes in the debate expressed their
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doubts. So I decided to go do it.
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Probably only take a week or two.
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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% Keywords page for the MIT TR
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{
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\clearpage
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\vspace*{\fill}
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\newcommand{\keywords}[1]%
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{\newlength{\kwlength}\settowidth{\kwlength}{\bf Keywords: }%
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\setlength{\kwlength}{-\kwlength}\addtolength{\kwlength}{\linewidth}%
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\noindent{\bf Keywords: }\parbox[t]{\kwlength}{\raggedright{}#1.}}
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\keywords{operating systems, programming languages, Scheme,
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Unix, shells, functional languages, systems programming}
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}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\cleardoublepage
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\tableofcontents
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\cleardoublepage
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\setcounter{page}{1}
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\pagestyle{plain}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Introduction}
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The central artifact of this paper is a new {\Unix} shell called scsh.
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However, I have a larger purpose beyond simply giving a description of
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the new system.
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It has become fashionable recently to claim that ``language doesn't matter.''
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After twenty years of research, operating systems and systems
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applications are still mainly written in C and its complex successor, C++.
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Perhaps advanced programming languages offer too little for the price they
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demand in efficiency and formal rigor.
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I disagree strongly with this position, and
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I would like to use scsh, in comparison to other {\Unix} systems programming
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languages, to make the point that language {\em does\/} matter.
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After presenting scsh in the initial sections of the paper,
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I will describe its design principles,
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and make a series of points concerning the effect language design has
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upon systems programming.
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I will use scsh, C, and the traditional shells as linguistic exemplars,
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and show how their various notational and semantic tradeoffs affect
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the programmer's task.
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In particular, I wish to show that a functional language such as Scheme is an
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excellent tool for systems programming.
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Many of the linguistic points I will make are well-known to the members of
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the systems programming community that employ modern programming
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languages, such as DEC SRC's Modula-3 \cite{Nelson}.
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In this respect, I will merely be serving to recast these ideas in
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a different perspective, and perhaps diffuse them more widely.
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The rest of this paper is divided into four parts:
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\begin{itemize}
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\item In part one, I will motivate the design of scsh
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(section~\ref{sec:shells}), and then give a brief
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tutorial on the system
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(\ref{sec:proc-forms}, \ref{sec:syscall-lib}).
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\item In part two, I discuss the design issues behind scsh,
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and cover some of the relevant implementation details
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(\ref{sec:zen}--\ref{sec:size}).
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\item Part three concerns systems programming with advanced languages.
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I will illustrate my points by comparing scsh to other {\Unix}
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programming systems (\ref{sec:scm-sysprog}, \ref{sec:opl}).
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\item Finally, we conclude, with some indication of future directions
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and a few final thoughts.
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\end{itemize}
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%\part{Shell Programming}
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Unix shells}
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\label{sec:shells}
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Unix shells, such as sh or csh, provide two things at once: an interactive
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command language and a programming language. Let us focus on the latter
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function: the writing of ``shell scripts''---interpreted programs
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that perform small tasks or assemble a collection of Unix tools into
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a single application.
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Unix shells are real programming languages. They have variables, if/then
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conditionals, and loops. But they are terrible programming languages. The
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data structures typically consist only of integers and vectors of strings.
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The facilities for procedural abstraction are non-existent to minimal. The
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lexical and syntactic structures are multi-phased, unprincipled, and baroque.
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If most shell languages are so awful, why does anyone use them?
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There are a few important reasons.
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\begin{itemize}
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\item
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A programming language is a notation for expressing computation. Shells
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have a notation that is specifically tuned for running Unix programs and
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hooking them together. For example, suppose you want to run programs
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\ex{foo} and \ex{bar} with \ex{foo} feeding output into \ex{bar}. If you do
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this in C, you must write: two calls to \ex{fork()}, two calls to
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\ex{exec()}, one call to \ex{pipe()}, several calls to \ex{close()},
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two calls to \ex{dup()}, and a lot of error checks (fig.~\ref{fig:C-pipe}).
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This is a lot of picky bookkeeping: tedious to write, tedious to read,
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and easy to get wrong on the first try. In sh, on the other hand,
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you simply write ``\ex{foo | bar}'' which is much easier to write and
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much clearer to read.
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One can look at this expression and instantly understand it;
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one can write it and instantly be sure that it is correct.
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\begin{figure}
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\begin{boxedminipage}{\linewidth}\vskip 1.5ex
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\footnotesize
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\begin{verbatim}
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int fork_foobar(void) /* foo | bar in C */
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{
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int pid1 = fork();
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int pid2, fds[2];
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if( pid1 == -1 ) {
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perror("foo|bar");
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return -1;
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}
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if( !pid1 ) {
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int status;
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if( -1 == waitpid(pid1, &status, 0) ) {
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perror("foo|bar");
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return -1;
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}
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return status;
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}
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if( -1 == pipe(fds) ) {
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perror("foo|bar");
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exit(-1);
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}
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pid2 = fork();
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if( pid2 == -1 ) {
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perror("foo|bar");
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exit(-1);
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}
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if( !pid2 ) {
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close(fds[1]);
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dup2(fds[0], 1);
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execlp("foo", "foo", NULL);
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perror("foo|bar");
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exit(-1);
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}
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close(fds[0]);
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dup2(fds[1], 0);
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execlp("bar", "bar", NULL);
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perror("foo|bar");
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exit(-1);
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}\end{verbatim}
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\caption{Why we program with shells.}
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\label{fig:C-pipe}
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\end{boxedminipage}
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\end{figure}
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\item
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They are interpreted. Debugging is easy and interactive; programs are small.
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On my workstation, the ``hello, world'' program is 16kb as a compiled C
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program, and 29 bytes as an interpreted sh script.
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In fact, \ex{/bin/sh} is just about the only language interpreter
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that a programmer can absolutely rely upon having available
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on the system, so this is just about the only reliable way to
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get interpreted-code density and know that one's program
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will run on any Unix system.
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\item
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Because the shell is the programmer's command language, the programmer
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is usually very familiar with its commonly-used command-language
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subset (this familiarity tails off rapidly, however, as the demands
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of shell programming move the programmer out into the dustier recesses
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of the language's definition.)
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\end{itemize}
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There is a tension between the shell's dual role as interactive command
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language and shell-script programming language. A command language should be
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terse and convenient to type. It doesn't have to be comprehensible. Users
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don't have to maintain or understand a command they typed into a shell a month
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ago. A command language can be ``write-only,'' because commands are thrown
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away after they are used. However, it is important that most commands fit on
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one line, because most interaction is through tty drivers that don't let the
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user back up and edit a line after its terminating newline has been entered.
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This seems like a trivial point, but imagine how irritating it would be if
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typical shell commands required several lines of input. Terse notation is
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important for interactive tasks.
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Shell syntax is also carefully designed to allow it to be parsed
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on-line---that is, to allow parsing and interpretation to be interleaved.
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This usually penalizes the syntax in other ways (for example, consider
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rc's clumsy if/then/else syntax \cite{rc}).
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Programming languages, on the other hand, can be a little more verbose, in
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return for generality and readability. The programmer enters programs into a
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text editor, so the language can spread out a little more.
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The constraints of the shell's role as command language are one of the
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things that make it unpleasant as a programming language.
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The really compelling advantage of shell languages over other programming
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languages is the first one mentioned above. Shells provide a powerful
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notation for connecting processes and files together. In this respect,
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shell languages are extremely well-adapted to the general paradigm of
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the Unix operating system.
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In Unix, the fundamental computational agents are programs, running
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as processes in individual address spaces.
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These agents cooperate and communicate among themselves to solve a problem
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by communicating over directed byte streams called pipes.
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Viewed at this level, Unix is a data-flow architecture.
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From this perspective, the shell serves a critical role
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as the language designed to assemble the individual computational
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agents to solve a particular task.
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As a programming language, this interprocess ``glue'' aspect of the
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shell is its key desireable feature.
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This leads us to a fairly obvious idea: instead of adding weak
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programming features to a Unix process-control language,
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why not add process invocation features to a strong programming language?
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What programming language would make a good base?
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We would want a language that was powerful and high-level.
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It should allow for implementations based on interactive interpreters, for
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ease of debugging and to keep programs small.
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Since we want to add new notation to the language, it would help if the
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language was syntactically extensible.
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High-level features such as automatic storage allocation would help keep
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programs small and simple.
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{\Scheme} is an obvious choice.
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It has all of the desired features, and its weak points, such as it lack of a
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module system or its poor performance relative to compiled C on certain
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classes of program, do not apply to the writing of shell scripts.
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I have designed and implemented a {\Unix} shell called scsh that is
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embedded inside {\Scheme}.
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I had the following design goals and non-goals:
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\begin{itemize}
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\item
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The general systems architecture of {\Unix} is cooperating computational
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agents that are realised as processes running in separate, protected address
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spaces, communicating via byte streams.
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The point of a shell language is to act as the glue to connect up these
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computational agents.
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That is the goal of scsh.
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I resisted the temptation to delve into other programming models.
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Perhaps cooperating lightweight threads communicating through shared memory
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is a better way to live, but it is not {\Unix}.
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The goal here was not to come up with a better systems architecture, but
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simply to provide a better way to drive {\Unix}.
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\note{Agenda}
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\item
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I wanted a programming language, not a command language, and I was
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unwilling to compromise the quality of the programming language to
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make it a better command language. I was not trying to replace use of
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the shell as an interactive command language. I was trying to provide
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a better alternative for writing shell scripts. So I did not focus
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on issues that might be important for a command language, such as job
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control, command history, or command-line editing. There are no write-only
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notational conveniences. I made no effort to hide the
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base {\Scheme} syntax, even though an interactive user might find all
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the necessary parentheses irritating.
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(However, see section \ref{sec:future-work}.)
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\item
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I wanted the result to fit naturally within {\Scheme}. For example,
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this ruled out complex non-standard control-flow paradigms,
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such as awk's or sed's.
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\end{itemize}
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The result design, scsh, has two dependent components, embedded
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within a very portable {\Scheme} system:
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\begin{itemize}
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\item A high-level process-control notation.
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\item A complete library of {\Unix} system calls.
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\end{itemize}
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The process-control notation allows the user to control {\Unix} programs
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with a compact notation.
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The syscall library gives the programmer full low-level access to the kernel
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for tasks that cannot be handled by the high-level notation.
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In this way, scsh's functionality spans a spectrum of detail that is
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not available to either C or sh.
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\section{Process notation}
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\label{sec:proc-forms}
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Scsh has a notation for controlling {\Unix} processes that takes the
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form of s-expressions; this notation can then be embedded inside of
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standard {\Scheme} code.
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The basic elements of this notation are {\em process forms},
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{\em extended process forms}, and {\em redirections}.
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\subsection{Extended process forms and i/o redirections}
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An {\em extended process form\/} is a specification of a {\Unix} process to
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run, in a particular I/O environment:
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\codex{\var{epf} {\synteq} (\var{pf} $\var{redir}_1$ {\ldots} $\var{redir}_n$)}
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where \var{pf} is a process form and the $\var{redir}_i$ are redirection specs.
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A {\em redirection spec} is one of:
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\begin{inset}
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\begin{tabular}{@{}l@{\qquad{\tt; }}l@{}}
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\ex{(< \var{[fdes]} \var{file-name})} & \ex{Open file for read.}
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|
\\\ex{(> \var{[fdes]} \var{file-name})} & \ex{Open file create/truncate.}
|
|
\\\ex{(<< \var{[fdes]} \var{object})} & \ex{Use \var{object}'s printed rep.}
|
|
\\\ex{(>> \var{[fdes]} \var{file-name})} & \ex{Open file for append.}
|
|
\\\ex{(= \var{fdes} \var{fdes/port})} & \ex{Dup2}
|
|
\\\ex{(- \var{fdes/port})} & \ex{Close \var{fdes/port}.}
|
|
\\\ex{stdports} & \ex{0,1,2 dup'd from standard ports.}
|
|
\end{tabular}
|
|
\end{inset}
|
|
The \var{fdes} file descriptors have these defaults:
|
|
\begin{center}
|
|
{\ttt
|
|
\begin{tabular}{|cccc|}\hline < & << & > & >> \\
|
|
0 & 0 & 1 & 1 \\ \hline
|
|
\end{tabular}
|
|
}
|
|
\end{center}
|
|
|
|
The subforms of a redirection are implicitly backquoted,
|
|
and symbols stand for their print-names.
|
|
So \ex{(> ,x)} means
|
|
``output to the file named by {\Scheme} variable \ex{x},''
|
|
and \ex{(< /usr/shivers/.login)} means ``read from \ex{/usr/shivers/.login}.''
|
|
This implicit backquoting is an important feature of the process notation,
|
|
as we'll see later (sections~\ref{sec:zen} and \ref{sec:sexp}).
|
|
|
|
Here are two more examples of i/o redirection:
|
|
%
|
|
\begin{center}
|
|
\begin{codebox}
|
|
(< ,(vector-ref fv i))
|
|
(>> 2 /tmp/buf)\end{codebox}
|
|
\end{center}
|
|
%
|
|
These two redirections cause the file \ex{fv[i]} to be opened on stdin, and
|
|
\ex{/tmp/buf} to be opened for append writes on stderr.
|
|
|
|
The redirection \ex{(<< \var{object})} causes input to come from the
|
|
printed representation of \var{object}.
|
|
For example,
|
|
\codex{(<< "The quick brown fox jumped over the lazy dog.")}
|
|
causes reads from stdin to produce the characters of the above string.
|
|
The object is converted to its printed representation using the \ex{display}
|
|
procedure, so
|
|
\codex{(<< (A five element list))}
|
|
is the same as
|
|
\codex{(<< "(A five element list)")}
|
|
is the same as
|
|
\codex{(<< ,(reverse '(list element five A))){\rm.}}
|
|
(Here we use the implicit backquoting feature to compute the list to
|
|
be printed.)
|
|
|
|
The redirection \ex{(= \var{fdes} \var{fdes/port})} causes \var{fdes/port}
|
|
to be dup'd into file descriptor \var{fdes}.
|
|
For example, the redirection
|
|
\codex{(= 2 1)}
|
|
causes stderr to be the same as stdout.
|
|
\var{fdes/port} can also be a port, for example:
|
|
\codex{(= 2 ,(current-output-port))}
|
|
causes stderr to be dup'd from the current output port.
|
|
In this case, it is an error if the port is not a file port
|
|
(\eg, a string port). \note{No port sync}
|
|
|
|
More complex redirections can be accomplished using the \ex{begin}
|
|
process form, discussed below, which gives the programmer full control
|
|
of i/o redirection from {\Scheme}.
|
|
|
|
\subsection{Process forms}
|
|
A {\em process form\/} specifies a computation to perform as an independent
|
|
{\Unix} process. It can be one of the following:
|
|
%
|
|
\begin{leftinset}
|
|
\begin{codebox}
|
|
(begin . \var{scheme-code})
|
|
(| \vari{pf}{\!1} {\ldots} \vari{pf}{\!n})
|
|
(|+ \var{connect-list} \vari{pf}{\!1} {\ldots} \vari{pf}{\!n})
|
|
(epf . \var{epf})
|
|
(\var{prog} \vari{arg}{1} {\ldots} \vari{arg}{n})
|
|
\end{codebox}
|
|
\qquad
|
|
\begin{codebox}
|
|
; Run \var{scheme-code} in a fork.
|
|
; Simple pipeline
|
|
; Complex pipeline
|
|
; An extended process form.
|
|
; Default: exec the program.
|
|
\end{codebox}
|
|
\end{leftinset}
|
|
%
|
|
The default case \ex{(\var{prog} \vari{arg}1 {\ldots} \vari{arg}n)}
|
|
is also implicitly backquoted.
|
|
That is, it is equivalent to:
|
|
%
|
|
\codex{(begin (apply exec-path `(\var{prog} \vari{arg}1 {\ldots} \vari{arg}n)))}
|
|
%
|
|
\ex{Exec-path} is the version of the \ex{exec()} system call that
|
|
uses scsh's path list to search for an executable.
|
|
The program and the arguments must be either strings, symbols, or integers.
|
|
Symbols and integers are coerced to strings.
|
|
A symbol's print-name is used.
|
|
Integers are converted to strings in base 10.
|
|
Using symbols instead of strings is convenient, since it suppresses the
|
|
clutter of the surrounding \ex{"{\ldots}"} quotation marks.
|
|
To aid this purpose, scsh reads symbols in a case-sensitive manner,
|
|
so that you can say
|
|
\codex{(more Readme)}
|
|
and get the right file.
|
|
(See section \ref{sec:lex} for further details on lexical issues.)
|
|
|
|
A \var{connect-list} is a specification of how two processes are to be wired
|
|
together by pipes.
|
|
It has the form \ex{((\vari{from}1 \vari{from}2 {\ldots} \var{to}) \ldots)}
|
|
and is implicitly backquoted.
|
|
For example,
|
|
%
|
|
\codex{(|+ ((1 2 0) (3 3)) \vari{pf}{\!1} \vari{pf}{\!2})}
|
|
%
|
|
runs \vari{pf}{\!1} and \vari{pf}{\!2}.
|
|
The first clause \ex{(1 2 0)} causes \vari{pf}{\!1}'s
|
|
stdout (1) and stderr (2) to be connected via pipe
|
|
to \vari{pf}{\!2}'s stdin (0).
|
|
The second clause \ex{(3 3)} causes \vari{pf}{\!1}'s file descriptor 3 to be
|
|
connected to \vari{pf}{\!2}'s file descriptor 3.
|
|
%---this is unusual, and not expected to occur very often.
|
|
|
|
%[Note that {\R4RS} does not specify whether or not | and |+ are readable
|
|
%symbols. Scsh does.]
|
|
|
|
\subsection{Using extended process forms in \Scheme}
|
|
Process forms and extended process forms are {\em not\/} {\Scheme}.
|
|
They are a different notation for expressing computation that, like {\Scheme},
|
|
is based upon s-expressions.
|
|
Extended process forms are used in {\Scheme} programs by embedding them inside
|
|
special Scheme forms.
|
|
\pagebreak
|
|
There are three basic {\Scheme} forms that use extended process forms:
|
|
\ex{exec-epf}, \cd{&}, and \ex{run}:
|
|
\begin{inset}
|
|
\begin{codebox}[t]
|
|
(exec-epf . \var{epf})
|
|
(& . \var{epf})
|
|
(run . \var{epf})
|
|
\end{codebox}
|
|
\quad
|
|
\begin{codebox}[t]
|
|
; Nuke the current process.
|
|
; Run \var{epf} in background; return pid.
|
|
; Run \var{epf}; wait for termination.
|
|
; Returns exit status.\end{codebox}
|
|
\end{inset}
|
|
These special forms are macros that expand into the equivalent
|
|
series of system calls.
|
|
The definition of the \ex{exec-epf} macro is non-trivial,
|
|
as it produces the code to handle i/o redirections and set up pipelines.
|
|
However, the definitions of the \cd{&} and \ex{run} macros are very simple:
|
|
\begin{leftinset}
|
|
\begin{tabular}{@{}l@{\quad$\Rightarrow$\quad}l@{}}
|
|
\cd{(& . \var{epf})} & \ex{(fork (\l{} (exec-epf . \var{epf})))} \\
|
|
\ex{(run . \var{epf})} & \cd{(wait (& . \var{epf}))}
|
|
\end{tabular}
|
|
\end{leftinset}
|
|
|
|
Figures \ref{fig:ex1} and \ref{fig:ex2} show a series of examples
|
|
employing a mix of the process notation and the syscall library.
|
|
Note that regular Scheme is used to provide the control structure,
|
|
variables, and other linguistic machinery needed by the script fragments.
|
|
%
|
|
\begin{figure}[bp]\footnotesize
|
|
\begin{boxedminipage}{\linewidth}\vskip 1.5ex
|
|
\begin{center}\begin{codebox}
|
|
;; If the resource file exists, load it into X.
|
|
(if (file-exists? f))
|
|
(run (xrdb -merge ,f)))
|
|
|
|
;; Decrypt my mailbox; key is "xyzzy".
|
|
(run (crypt xyzzy) (< mbox.crypt) (> mbox))
|
|
|
|
;; Dump the output from ls, fortune, and from into log.txt.
|
|
(run (begin (run (ls))
|
|
(run (fortune))
|
|
(run (from)))
|
|
(> log.txt))
|
|
|
|
;; Compile FILE with FLAGS.
|
|
(run (cc ,file ,@flags))
|
|
|
|
;; Delete every file in DIR containing the string "/bin/perl":
|
|
(with-cwd dir
|
|
(for-each (\l{file}
|
|
(if (zero? (run (grep -s /bin/perl ,file)))
|
|
(delete-file file)))
|
|
(directory-files)))\end{codebox}
|
|
\end{center}
|
|
\caption{Example shell script fragments (a)}
|
|
\label{fig:ex1}
|
|
\end{boxedminipage}
|
|
\end{figure}
|
|
|
|
\begin{figure}\footnotesize
|
|
\begin{boxedminipage}{\linewidth}\vskip 1.5ex
|
|
\begin{center}\begin{codebox}
|
|
;; M4 preprocess each file in the current directory, then pipe
|
|
;; the input into cc. Errlog is foo.err, binary is foo.exe.
|
|
;; Run compiles in parallel.
|
|
(for-each (\l{file}
|
|
(let ((outfile (replace-extension file ".exe"))
|
|
(errfile (replace-extension file ".err")))
|
|
(& (| (m4) (cc -o ,outfile))
|
|
(< ,file)
|
|
(> 2 ,errfile))))
|
|
(directory-files))
|
|
|
|
;; Same as above, but parallelise even the computation
|
|
;; of the filenames.
|
|
(for-each (\l{file}
|
|
(& (begin (let ((outfile (replace-extension file ".exe"))
|
|
(errfile (replace-extension file ".err")))
|
|
(exec-epf (| (m4) (cc -o ,outfile))
|
|
(< ,file)
|
|
(> 2 ,errfile))))))
|
|
(directory-files))
|
|
|
|
;; DES encrypt string PLAINTEXT with password KEY. My DES program
|
|
;; reads the input from fdes 0, and the key from fdes 3. We want to
|
|
;; collect the ciphertext into a string and return that, with error
|
|
;; messages going to our stderr. Notice we are redirecting Scheme data
|
|
;; structures (the strings PLAINTEXT and KEY) from our program into
|
|
;; the DES process, instead of redirecting from files. RUN/STRING is
|
|
;; like the RUN form, but it collects the output into a string and
|
|
;; returns it (see following section).
|
|
|
|
(run/string (/usr/shivers/bin/des -e -3)
|
|
(<< ,plaintext) (<< 3 ,key))
|
|
|
|
;; Delete the files matching regular expression PAT.
|
|
;; Note we aren't actually using any of the process machinery here --
|
|
;; just pure Scheme.
|
|
(define (dsw pat)
|
|
(for-each (\l{file}
|
|
(if (y-or-n? (string-append "Delete " file))
|
|
(delete-file file)))
|
|
(file-match #f pat)))\end{codebox}
|
|
\end{center}
|
|
\caption{Example shell script fragments (b)}
|
|
\label{fig:ex2}
|
|
\end{boxedminipage}
|
|
\end{figure}
|
|
|
|
|
|
\subsection{Procedures and special forms}
|
|
It is a general design principle in scsh that all functionality
|
|
made available through special syntax is also available in a
|
|
straightforward procedural form.
|
|
So there are procedural equivalents for all of the process notation.
|
|
In this way, the programmer is not restricted by the particular details of
|
|
the syntax.
|
|
Here are some of the syntax/procedure equivalents:
|
|
\begin{inset}
|
|
\begin{tabular}{@{}|ll|@{}}
|
|
\hline
|
|
Notation & Procedure \\ \hline \hline
|
|
\ex{|} & \ex{fork/pipe} \\
|
|
\ex{|+} & \ex{fork/pipe+} \\
|
|
\ex{exec-epf} & \ex{exec-path} \\
|
|
redirection & \ex{open}, \ex{dup} \\
|
|
\cd{&} & \ex{fork} \\
|
|
\ex{run} & $\ex{wait} + \ex{fork}$ \\
|
|
\hline
|
|
\end{tabular}
|
|
\end{inset}
|
|
%
|
|
Having a solid procedural foundation also allows for general notational
|
|
experimentation using Scheme's macros.
|
|
For example, the programmer can build his own pipeline notation on top of the
|
|
\ex{fork} and \ex{fork/pipe} procedures.
|
|
%Because the shell notation has {\Scheme} escapes
|
|
%(\eg, the \ex{begin} process form),
|
|
%the programmer can move back and forth easily, using the simple notation
|
|
%where possible, and escaping to general {\Scheme} only where necessary.
|
|
|
|
\begin{protos}
|
|
\protoitem{fork}{[thunk]}{procedure}
|
|
\ex{Fork} spawns a {\Unix} subprocess.
|
|
Its exact behavior depends on whether it is called with the optional
|
|
\var{thunk} argument.
|
|
|
|
With the \var{thunk} argument, \ex{fork} spawns off a subprocess that
|
|
calls \var{thunk}, exiting when \var{thunk} returns.
|
|
\ex{Fork} returns the subprocess' pid to the parent process.
|
|
|
|
Without the \var{thunk} argument, \ex{fork} behaves like the C \ex{fork()}
|
|
routine.
|
|
It returns in both the parent and child process.
|
|
In the parent, \ex{fork} returns the child's pid;
|
|
in the child, \ex{fork} returns \cd{#f}.
|
|
|
|
\protoitem{fork/pipe}{[thunk]}{procedure}
|
|
Like \ex{fork}, but the parent and child communicate via a pipe
|
|
connecting the parent's stdin to the child's stdout. This function
|
|
side-effects the parent by changing his stdin.
|
|
|
|
In effect, \ex{fork/pipe} splices a process into the data stream
|
|
immediately upstream of the current process.
|
|
This is the basic function for creating pipelines.
|
|
Long pipelines are built by performing a sequence of \ex{fork/pipe} calls.
|
|
\pagebreak
|
|
For example, to create a background two-process pipe \ex{a | b}, we write:
|
|
%
|
|
\begin{tightcode}
|
|
(fork (\l{} (fork/pipe a) (b)))\end{tightcode}
|
|
%
|
|
which returns the pid of \ex{b}'s process.
|
|
|
|
To create a background three-process pipe \ex{a | b | c}, we write:
|
|
%
|
|
\begin{code}
|
|
(fork (\l{} (fork/pipe a)
|
|
(fork/pipe b)
|
|
(c)))\end{code}
|
|
%
|
|
which returns the pid of \ex{c}'s process.
|
|
|
|
|
|
\protoitem{fork/pipe+}{conns [thunk]}{procedure}
|
|
Like \ex{fork/pipe}, but the pipe connections between the child and parent
|
|
are specified by the connection list \var{conns}.
|
|
See the
|
|
\codex{(|+ \var{conns} \vari{pf}{\!1} \ldots{} \vari{pf}{\!n})}
|
|
process form for a description of connection lists.
|
|
\end{protos}
|
|
|
|
\subsection{Interfacing process output to {\Scheme}}
|
|
\label{sec:io-interface}
|
|
There is a family of procedures and special forms that can be used
|
|
to capture the output of processes as {\Scheme} data.
|
|
Here are the special forms for the simple variants:
|
|
\\[2ex]%\begin{center}
|
|
\begin{codebox}
|
|
(run/port . \var{epf}) ; Return port open on process's stdout.
|
|
(run/file . \var{epf}) ; Process > temp file; return file name.
|
|
(run/string . \var{epf}) ; Collect stdout into a string and return.
|
|
(run/strings . \var{epf}) ; Stdout->list of newline-delimited strings.
|
|
(run/sexp . \var{epf}) ; Read one sexp from stdout with READ.
|
|
(run/sexps . \var{epf}) ; Read list of sexps from stdout with READ.\end{codebox}
|
|
\\[2ex]%\end{center}
|
|
%
|
|
\ex{Run/port} returns immediately after forking off the process;
|
|
other forms wait for either the process to die (\ex{run/file}),
|
|
or eof on the communicating pipe
|
|
(\ex{run/string}, \ex{run/strings}, \ex{run/sexps}).
|
|
These special forms just expand into calls to the following analogous
|
|
procedures:
|
|
%
|
|
\begin{center}
|
|
\begin{column}
|
|
\proto{run/port*} {thunk}{procedure} \\
|
|
\proto{run/file*} {thunk}{procedure} \\
|
|
\proto{run/string*} {thunk}{procedure} \\
|
|
\proto{run/strings*} {thunk}{procedure} \\
|
|
\proto{run/sexp*} {thunk}{procedure} \\
|
|
\proto{run/sexps*} {thunk}{procedure}
|
|
\end{column}
|
|
\end{center}
|
|
%
|
|
For example, \ex{(run/port . \var{epf})} expands into
|
|
\codex{(run/port* (\l{} (exec-epf . \var{epf}))).}
|
|
|
|
These procedures can be used to manipulate the output of {\Unix}
|
|
programs with {\Scheme} code. For example, the output of the \ex{xhost(1)}
|
|
program can be manipulated with the following code:
|
|
\begin{code}
|
|
;;; Before asking host REMOTE to do X stuff,
|
|
;;; make sure it has permission.
|
|
(while (not (member remote (run/strings (xhost))))
|
|
(display "Pausing for xhost...")
|
|
(read-char))\end{code}
|
|
|
|
The following procedures are also of utility for generally parsing
|
|
input streams in scsh:
|
|
%(port->string \var{port})
|
|
%(port->sexp-list \var{port})
|
|
%(port->string-list \var{port})
|
|
%(port->list \var{reader} \var{port})
|
|
\begin{center}
|
|
\begin{column}
|
|
\proto{port->string}{port}{procedure} \\
|
|
\proto{port->sexp-list}{port}{procedure} \\
|
|
\proto{port->string-list}{port}{procedure} \\
|
|
\proto{port->list}{reader port}{procedure}
|
|
\end{column}
|
|
\end{center}
|
|
\ex{Port->string} reads the port until eof,
|
|
then returns the accumulated string.
|
|
\ex{Port->sexp-list} repeatedly reads data from the port until eof,
|
|
then returns the accumulated list of items.
|
|
\ex{Port->string-list} repeatedly reads newline-terminated strings from the
|
|
port until eof, then returns the accumulated list of strings.
|
|
The delimiting newlines are not part of the returned strings.
|
|
\ex{Port->list} generalises these two procedures.
|
|
It uses \var{reader} to repeatedly read objects from a port.
|
|
It accumulates these objects into a list, which is returned upon eof.
|
|
The \ex{port->string-list} and \ex{port->sexp-list} procedures
|
|
are trivial to define, being merely \ex{port->list} curried with
|
|
the appropriate parsers:
|
|
\begin{code}\cddollar
|
|
(port->string-list \var{port}) $\equiv$ (port->list read-line \var{port})
|
|
(port->sexp-list \var{port}) $\equiv$ (port->list read \var{port})\end{code}
|
|
%
|
|
The following compositions also hold:
|
|
\begin{code}\cddollar
|
|
run/string* $\equiv$ port->string $\circ$ run/port*
|
|
run/strings* $\equiv$ port->string-list $\circ$ run/port*
|
|
run/sexp* $\equiv$ read $\circ$ run/port*
|
|
run/sexps* $\equiv$ port->sexp-list $\circ$ run/port*\end{code}
|
|
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{System calls}
|
|
\label{sec:syscall-lib}
|
|
We've just seen scsh's high-level process-form notation,
|
|
for running programs, creating pipelines, and performing I/O redirection.
|
|
This notation is at roughly the same level as traditional {\Unix} shells.
|
|
The process-form notation is convenient, but does not provide detailed,
|
|
low-level access to the operating system.
|
|
This is provided by the second component of scsh: its system-call library.
|
|
|
|
Scsh's system-call library is a nearly-complete set of {\sc Posix} bindings,
|
|
with some extras, such as symbolic links.
|
|
As of this writing, network and terminal i/o controls have still not yet
|
|
been implemented; work on them is underway.
|
|
Scsh also provides a convenient set of systems programming utility procedures,
|
|
such as routines to perform pattern matching on file-names and general strings,
|
|
manipulate {\Unix} environment variables, and parse file pathnames.
|
|
Although some of the procedures have been described in passing,
|
|
a detailed description of the system-call library is beyond the scope of
|
|
this note.
|
|
The reference manual \cite{ref-man} contains the full details.
|
|
|
|
%\part{Design Notes}
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{The Tao of {\Scheme} and {\Unix}}
|
|
\label{sec:zen}
|
|
Most attempts at embedding shells in functional programming languages
|
|
\cite{fsh,ellis}
|
|
try to hide the difference between running a program and calling a procedure.
|
|
That is, if the user tries
|
|
\codex{(lpr "notes.txt")}
|
|
the shell will first treat \ex{lpr} as a procedure to be called.
|
|
If \ex{lpr} isn't found in the variable environment, the shell will then
|
|
do a path search of the file system for a program.
|
|
This sort of transparency is in analogy to the function-binding mechanisms
|
|
of traditional shells, such as ksh.
|
|
|
|
This is a fundamental error that has hindered these previous designs.
|
|
Scsh, in contrast, is explicit about the distinction between
|
|
procedures and programs.
|
|
In scsh, the programmer must know which are which---the mechanisms
|
|
for invocation are different for the two cases
|
|
(procedure call {\em versus\/} the \ex{(run . \var{epf})} special form),
|
|
and the namespaces are different
|
|
(the program's lexical environment {\em versus\/}
|
|
\ex{\$PATH} search in the file system).
|
|
|
|
Linguistically separating these two mechanisms was an important design
|
|
decision in the language.
|
|
It was done because the two computational models are fundamentally different;
|
|
any attempt to gloss over the distinctions would have made the semantics
|
|
ugly and inconsistent.
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\begin{figure}
|
|
\begin{boxedminipage}{\linewidth}\vskip 1.5ex
|
|
\begin{center}
|
|
\begin{tabular}{ll}
|
|
\bf Unix: &
|
|
\begin{tabular}[t]{l}
|
|
Computational agents are processes, \\ communicate via byte streams.
|
|
\end{tabular} \\
|
|
\\
|
|
\bf Scheme: &
|
|
\begin{tabular}[t]{l}
|
|
Computational agents are procedures, \\ communicate via procedure call/return.
|
|
\end{tabular}
|
|
\end{tabular}
|
|
\end{center}
|
|
\caption{The Tao of {\Scheme} and {\Unix}}
|
|
\label{fig:tao}
|
|
\end{boxedminipage}
|
|
\end{figure}
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
|
|
There are two computational worlds here (figure~\ref{fig:tao}),
|
|
where the basic computational agents are procedures or processes.
|
|
These agents are composed differently.
|
|
In the world of applicative-order procedures, agents execute serially,
|
|
and are composed with function composition: \ex{(g (f x))}.
|
|
In the world of processes, agents execute concurrently
|
|
and are composed with pipes, in a data-flow network: \ex{f | g}.
|
|
A language with both of these computational structures, such as scsh,
|
|
must provide a way to interface them. \note{Normal order}
|
|
In scsh, we have ``adapters'' for crossing between these paradigms:
|
|
%(figure~\ref{fig:cross-connect}).
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
%\begin{figure}[bhp]
|
|
%\begin{center}
|
|
\begin{inset}
|
|
\def\foo{\rule[-1.5ex]{0in}{4ex}}
|
|
\begin{tabular}{l|c|c|}
|
|
\multicolumn{1}{l}{} & \multicolumn{1}{c}{Scheme}
|
|
& \multicolumn{1}{c}{Unix} \\ \cline{2-3}
|
|
\foo Scheme & \ex{(g (f x))} & \ex{(<< ,x)} \\ \cline{2-3}
|
|
\foo Unix & \ex{run/string},\ldots & \ex{f | g} \\ \cline{2-3}
|
|
\end{tabular}
|
|
\end{inset}
|
|
%\end{center}
|
|
%\caption{Scheme/Unix cross-connectors}
|
|
%\label{fig:cross-connect}
|
|
%\end{figure}
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
The \ex{run/string} form and its cousins (section~\ref{sec:io-interface})
|
|
map process output to procedure input;
|
|
the \ex{<<} i/o redirection maps procedure output to process input.
|
|
For example:
|
|
\begin{code}
|
|
(run/string (nroff -ms)
|
|
(<< ,(texinfo->nroff doc-string)))\end{code}
|
|
By separating the two worlds, and then providing ways for them to
|
|
cross-connect, scsh can cleanly accommodate the two paradigms within
|
|
one notational framework.
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{I/O}
|
|
\label{sec:io}
|
|
Perhaps the most difficult part of the design of scsh was the integration
|
|
of {\Scheme} ports and {\Unix} file descriptors.
|
|
Dealing with {\Unix} file descriptors in a {\Scheme} environment is difficult.
|
|
In {\Unix}, open files are part of the process state, and are referenced
|
|
by small integers called {\em file descriptors}.
|
|
Open file descriptors are the fundamental way i/o redirections are passed to
|
|
subprocesses, since file descriptors are preserved across \ex{fork()}
|
|
and \ex{exec()} calls.
|
|
|
|
{\Scheme}, on the other hand, uses ports for specifying i/o sources.
|
|
Ports are anonymous, garbage-collected Scheme objects, not integers.
|
|
When a port is collected, it is also closed. Because file
|
|
descriptors are just integers, it's impossible to garbage collect them---in
|
|
order to close file descriptor 3, you must prove that the process will never
|
|
again pass a 3 as a file descriptor to a system call doing I/O, and that it
|
|
will never \ex{exec()} a program that will refer to file descriptor 3.
|
|
|
|
This is difficult at best.
|
|
|
|
If a {\Scheme} program only used {\Scheme} ports, and never directly used
|
|
file descriptors, this would not be a problem.
|
|
But {\Scheme} code must descend to the file-descriptor level in at least two
|
|
circumstances:
|
|
\begin{itemize}
|
|
\item when interfacing to foreign code;
|
|
\item when interfacing to a subprocess.
|
|
\end{itemize}
|
|
This causes problems. Suppose we have a {\Scheme} port constructed
|
|
on top of file descriptor 2. We intend to fork off a C program that
|
|
will inherit this file descriptor. If we drop references to the port,
|
|
the garbage collector may prematurely close file 2 before we exec
|
|
the C program.
|
|
|
|
Another difficulty arising between the anonymity of ports
|
|
and the explicit naming of file descriptors arises when the
|
|
user explicitly manipulates file descriptors, as is required by
|
|
{\Unix}.
|
|
For example, when a file port is opened in {\Scheme}, the underlying run-time
|
|
{\Scheme} kernel must open a file and allocate an integer file descriptor.
|
|
When the user subsequently explicitly manipulates particular file descriptors,
|
|
perhaps preparatory to executing some {\Unix} subprocess, the port's
|
|
underlying file descriptor could be silently redirected to some new file.
|
|
|
|
Scsh's {\Unix} i/o interface is intended to fix this and
|
|
other problems arising from the mismatch between ports and file descriptors.
|
|
The fundamental principle is that in scsh, most ports are attached to files,
|
|
not to particular file descriptors.
|
|
When the user does an i/o redirection (\eg, with \ex{dup2()})
|
|
that must allocate a particular file descriptor \var{fd}, there is a chance
|
|
that \var{fd} has already been inadvertently allocated to a port by a prior
|
|
operation (\eg, an \ex{open-input-file} call).
|
|
If so, \var{fd}'s original port will be shifted to some new file descriptor
|
|
with a \ex{dup(\var{fd})} operation, freeing up \var{fd} for use.
|
|
The port machinery is allowed to do this as it does not in general
|
|
reveal which file descriptors are allocated to particular {\Scheme} ports.
|
|
Not revealing the particular file descriptors allocated to {\Scheme}
|
|
ports allows the system two important freedoms:
|
|
\begin{itemize}
|
|
\item When the user explicitly allocates a particular file descriptor,
|
|
the run-time system is free to shuffle around the port/file-descriptor
|
|
associations as required to free up that descriptor.
|
|
\item When all pointers to an unrevealed file port have been dropped,
|
|
the run-time system is free to close the underlying file descriptor.
|
|
If the user doesn't know which file descriptor was associated with the
|
|
port, then there is no way he could refer to that i/o channel by its
|
|
file-descriptor name.
|
|
This allows scsh to close file descriptors during gc or when
|
|
performing an \ex{exec()}.
|
|
\end{itemize}
|
|
Users {\em can\/} explicitly manipulate file descriptors, if so desired.
|
|
In this case, the associated ports are marked by the run time as ``revealed,''
|
|
and are no longer subject to automatic collection.
|
|
The machinery for handling this is carefully marked in the documentation,
|
|
and with some simple invariants in mind, follow the user's intuitions.
|
|
This facility preserves the transparent close-on-collect property
|
|
for file ports that are used in straightforward ways, yet allows
|
|
access to the underlying {\Unix} substrate without interference from
|
|
the garbage collector. This is critical, since shell programming
|
|
absolutely requires access to the {\Unix} file descriptors, as their
|
|
numerical values are a critical part of the process interface.
|
|
|
|
Under normal circumstances, all this machinery just works behind the scenes
|
|
to keep things straightened out. The only time the user has to think about
|
|
it is when he starts accessing file descriptors from ports, which he should
|
|
almost never have to do. If a user starts asking what file descriptors
|
|
have been allocated to what ports, he has to take responsibility for managing
|
|
this information.
|
|
|
|
Further details on the port mechanisms in scsh are beyond the scope of
|
|
this note; for more information, see the reference manual \cite{ref-man}.
|
|
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{Lexical issues}
|
|
\label{sec:lex}
|
|
|
|
Scsh's lexical syntax is not fully {\R4RS}-compliant in two ways:
|
|
\begin{itemize}
|
|
\item In scsh, symbol case is preserved by \ex{read} and is significant on
|
|
symbol comparison. This means
|
|
\codex{(run (less Readme))}
|
|
displays the right file.
|
|
|
|
\item ``\ex{-}'' and ``\ex{+}'' are allowed to begin symbols.
|
|
So the following are legitimate symbols:
|
|
\codex{-O2 -geometry +Wn}
|
|
\end{itemize}
|
|
%
|
|
Scsh also extends {\R4RS} lexical syntax in the following ways:
|
|
\begin{itemize}
|
|
\item ``\ex{|}'' and ``\ex{.}'' are symbol constituents.
|
|
This allows \ex{|} for the pipe symbol, and \ex{..} for the parent-directory
|
|
symbol. (Of course, ``\ex{.}'' alone is not a symbol, but a
|
|
dotted-pair marker.)
|
|
|
|
\item A symbol may begin with a digit.
|
|
So the following are legitimate symbols:
|
|
\codex{9x15 80x36-3+440}
|
|
|
|
\item Strings are allowed to contain the {\sc Ansi} C escape sequences
|
|
such as \verb|\n| and \verb|\161|.
|
|
|
|
\item \cd{#!} is a comment read-macro similar to \ex{;}.
|
|
This is important for writing shell scripts.
|
|
\end{itemize}
|
|
|
|
The lexical details of scsh are perhaps a bit contentious.
|
|
Extending the symbol syntax remains backwards compatible
|
|
with existing correct {\R4RS} code.
|
|
Since flags to {\Unix} programs always begin with a dash,
|
|
not extending the syntax would have required the user to explicitly
|
|
quote every flag to a program, as in
|
|
\codex{(run (cc "-O" "-o" "-c" main.c)).}
|
|
This is unacceptably obfuscatory, so the change was made to cover
|
|
these sorts of common {\Unix} flags.
|
|
|
|
More serious was the decision to make symbols read case-sensitively,
|
|
which introduces a true backwards incompatibility with {\R4RS} {\Scheme}.
|
|
This was a true case of clashing world-views:
|
|
{\Unix}'s tokens are case-sensitive; {\Scheme}'s, are not.
|
|
|
|
It is also unfortunate that the single-dot token, ``\ex{.}'', is both
|
|
a fundamental {\Unix} file name and a deep, primitive syntactic token
|
|
in {\Scheme}---it means the following will not parse correctly in scsh:
|
|
\codex{(run/strings (find . -name *.c -print))}
|
|
You must instead quote the dot:
|
|
\codex{(run/strings (find "." -name *.c -print))}
|
|
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{Implementation}
|
|
\label{sec:impl}
|
|
|
|
Scsh is currently implemented on top of {\scm}, a freely-available
|
|
{\Scheme} implementation written by Kelsey and Rees \cite{S48}.
|
|
{\scm} uses a byte-code interpreter for portability, good code density,
|
|
and medium efficiency. It is {\R4RS}-compliant, and includes a module
|
|
system designed by Rees.
|
|
|
|
The scsh design is not {\scm}-specific, although the current implementation
|
|
is necessarily so. Scsh is intended to be implementable in other {\Scheme}
|
|
implementations---although such a port may require some work. (I would
|
|
be very interested to see scsh ported to some of the {\Scheme} systems designed
|
|
to serve as embedded command languages---\eg, elk, esh, or any of the other
|
|
C-friendly interpreters.)
|
|
|
|
Scsh scripts currently have a few problems owing to the current
|
|
{\scm} implementation technology.
|
|
\begin{itemize}
|
|
\item Before running even the smallest shell script, the {\scm} vm must first
|
|
load in a 1.4Mb heap image. This i/o load adds a few seconds to the startup
|
|
time of even trivial shell scripts.
|
|
|
|
\item Since the entire {\scm} and scsh runtime is in the form of byte-code
|
|
data in the {\Scheme} heap, the heap is fairly large. As the {\scm} vm
|
|
uses a non-generational gc, all of this essentially permanent data
|
|
gets copied back and forth by the collector.
|
|
|
|
\item The large heap size is compounded by {\Unix} forking.
|
|
If you run a
|
|
four-stage pipeline, \eg,
|
|
\begin{code}
|
|
(run (| (zcat paper.tex.Z)
|
|
(detex)
|
|
(spell)
|
|
(enscript -2r)))\end{code}
|
|
then, for a brief instant, you could have up to five copies of scsh
|
|
forked into existence. This would briefly quintuple the virtual memory
|
|
demand placed by a single scsh heap, which is fairly large to begin with.
|
|
Since all the code is actually in the data pages of the process, the OS
|
|
can't trivially share pages between the processes. Even if the OS is clever
|
|
enough to do copy-on-write page sharing, it may insist on reserving enough
|
|
backing store on disk for worst-case swapping requirements. If disk space
|
|
is limited, this may overflow the paging area, causing the \ex{fork()}
|
|
operations to fail.
|
|
\end{itemize}
|
|
%
|
|
Byte-coded virtual machines are intended to be a technology
|
|
that provides memory savings through improved code density.
|
|
It is ironic that the straightforward implementation of such a byte-code
|
|
interpreter actually has high memory cost through bad interactions with
|
|
{\Unix} \ex{fork()} and the virtual memory system.
|
|
|
|
The situation is not irretrievable, however. A recent release of {\scm}
|
|
allows the pure portion of a heap image to be statically linked with the
|
|
text pages of the vm binary. Putting static data---such as all the code for
|
|
the runtime---into the text pages should drastically shorten start-up time,
|
|
move a large amount of data out of the heap, improve paging,
|
|
and greatly shrink the dynamic size. This should all lessen
|
|
the impact of \ex{fork()} on the virtual memory system.
|
|
|
|
Arranging for the garbage collector to communicate with the virtual memory
|
|
system with the near-standard \ex{madvise()} system call would further improve
|
|
the system. Also, breaking the system run-time into separate modules (\eg,
|
|
bignums, list operations, i/o, string operations, scsh operations, compiler,
|
|
\etc), each of which can be demand-loaded shared-text by the {\scm} vm
|
|
(using \ex{mmap()}), will allow for a full-featured system with a surprisingly
|
|
small memory footprint.
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{Size}
|
|
\label{sec:size}
|
|
Scsh can justifiably be criticised for being a florid design.
|
|
There are a lot of features---perhaps too many.
|
|
The optional arguments to many procedures, the implicit backquoting, and
|
|
the syntax/procedure equivalents are all easily synthesized by the user.
|
|
For example, \ex{port->strings}, \ex{run/strings*}, \ex{run/sexp*},
|
|
and \ex{run/sexps*} are all trivial compositions and curries of other base
|
|
procedures.
|
|
The \ex{run/strings} and \ex{run/sexps} forms are easily
|
|
written as macros, or simply written out by hand.
|
|
Not only does scsh provide the basic \ex{file-attributes} procedure
|
|
(\ie, the \ex{stat()} system call),
|
|
it also provides a host of derived procedures: \ex{file-owner}, \ex{file-mode},
|
|
\ex{file-directory?}, and so forth.
|
|
Still, my feeling is that it is easier and clearer to read
|
|
\codex{(filter file-directory? (directory-files))}
|
|
than
|
|
\begin{code}
|
|
(filter (\l{fname}
|
|
(eq? 'directory
|
|
(fileinfo:type (file-attributes fname))))
|
|
(directory-files))\end{code}
|
|
A full library can make for clearer user code.
|
|
|
|
One measure of scsh's design is that the source code consists of
|
|
a large number of small procedures: the source code for scsh has 448
|
|
top-level definitions; the definitions have an average length of 5 lines of
|
|
code.
|
|
That is, scsh is constructed by connecting together a lot of
|
|
small, composable parts, instead of designing one inflexible monolithic
|
|
structure.
|
|
These small parts can also be composed and abstracted by the programmer
|
|
into his own computational structures.
|
|
Thus the total functionality of scsh is greater than more traditional
|
|
large systems.
|
|
|
|
|
|
%\part{Systems Programming}
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{Systems programming in {\Scheme}}
|
|
\label{sec:scm-sysprog}
|
|
{\Unix} systems programming in {\Scheme} is a much more pleasant experience
|
|
than {\Unix} systems programming in C.
|
|
Several features of the language remove a lot of the painful or error-prone
|
|
problems C systems programmers are accustomed to suffering.
|
|
The most important of these features are:
|
|
\begin{itemize}
|
|
\item exceptions
|
|
\item automatic storage management
|
|
\item real strings
|
|
\item higher-order procedures
|
|
\item S-expression syntax and backquote
|
|
\end{itemize}
|
|
%
|
|
Many of these features are available in other advanced programming languages,
|
|
such as Modula-3 or ML. None are available in C.
|
|
|
|
\subsection{Exceptions and robust error handling}
|
|
In scsh, system calls never return the error codes that make careful
|
|
systems programming in C so difficult. Errors are signaled by raising
|
|
exceptions.
|
|
Exceptions are usually handled by default handlers that either abort the
|
|
program or invoke a run-time debugger; the programmer can override these when
|
|
desired by using exception-handler expressions.
|
|
Not having to return error codes frees up procedures to return useful values,
|
|
which encourages procedural composition.
|
|
It also keeps the programmer from cluttering up his code with
|
|
(or, as is all too often the case, just forgetting to include)
|
|
error checks for every system call.
|
|
In scsh, the programmer can assume that if a system call returns at all, it
|
|
returns successfully.
|
|
This greatly simplifies the flow of the code from the programmer's point
|
|
of view, as well as greatly increasing the robustness of the program.
|
|
|
|
\subsection{Automatic storage management}
|
|
Further, {\Scheme}'s automatic storage allocation removes the
|
|
``result'' parameters from the procedure argument lists.
|
|
When composite data is returned, it is simply returned in a
|
|
freshly-allocated data structure.
|
|
Again, this helps make it possible for procedures to return useful values.
|
|
|
|
For example, the C system call \ex{readlink()}
|
|
dereferences a symbolic link in the file system.
|
|
A working definition for the system call is given in
|
|
figure~\ref{fig:symlink}b.
|
|
It is complicated by many small bookkeeping details,
|
|
made necessary by C's weak linguistic facilities.
|
|
|
|
In contrast, scsh's equivalent procedure, \ex{read-symlink},
|
|
has a much simpler definition (fig.~\ref{fig:symlink}a).
|
|
%
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\begin{figure}\fboxsep=1.5em
|
|
\renewcommand{\subcaption}[1]
|
|
{\unskip\begin{center}\unskip\em#1\end{center}}
|
|
|
|
\begin{boxedminipage}{\linewidth} \vskip 1.5 ex
|
|
\ex{(read-symlink fname)}\\[1.5ex]
|
|
\ex{read-symlink} returns the filename referenced by symbolic link
|
|
\ex{fname}.
|
|
An exception is raised if there is an error.
|
|
\subcaption{(a) {\Scheme} definition of \ex{readlink}}
|
|
\end{boxedminipage}
|
|
|
|
\vskip 3ex plus 1fil
|
|
|
|
\begin{boxedminipage}{\linewidth}\vskip 1.5ex
|
|
\ex{readlink(char *path, char *buf, int bufsiz)}\\[1.5ex]
|
|
\ex{readlink} dereferences the symbolic link \ex{path}.
|
|
If the referenced filename is less than or equal to \ex{bufsiz} characters
|
|
in length,
|
|
it is written into the \ex{buf} array, which we fondly hope the
|
|
programmer has arranged to be at least of size \ex{bufsiz} characters.
|
|
If the referenced filename is longer than \ex{bufsiz} characters,
|
|
the system call returns an error code;
|
|
presumably the programmer should then reallocate a larger buffer and try
|
|
again.
|
|
If the system call succeeds, it returns the length of the result filename.
|
|
When the referenced filename is written into \ex{buf}, it is {\em not\/}
|
|
nul-terminated; it is the programmer's responsibility to leave space
|
|
in the buffer for the terminating nul
|
|
(remembering to subtract one from the actual buffer length when passing it to
|
|
the system call), and deposit the terminal nul after the system call returns.
|
|
|
|
If there is a real error,
|
|
the procedure will, in most cases, return an error code.
|
|
(We will gloss over the error-code mechanism for the sake of
|
|
brevity.)
|
|
% I will gloss over the -1/\ex{errno} mechanism involved, with its
|
|
% dependency upon a global, shared variable, for the sake of
|
|
% brevity.
|
|
However, if the length of \ex{buf} does not actually match the argument
|
|
\ex{bufsiz},
|
|
the system call may either%
|
|
\begin{itemize}%
|
|
\item succeed anyway,
|
|
\item dump core,
|
|
\item overwrite other storage and silently proceed,
|
|
\item report an error,
|
|
\item or perform some fifth action.
|
|
\end{itemize}%
|
|
It all depends.
|
|
\subcaption{(b) C definition of \ex{readlink}}
|
|
\end{boxedminipage}
|
|
|
|
\caption{Two definitions of \protect\ex{readlink}}
|
|
\label{fig:symlink}
|
|
\end{figure}
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
%
|
|
With the scsh version, there is no possibility that the result buffer will be
|
|
too small.
|
|
There is no possibility that the programmer will misrepresent the
|
|
size of the result buffer with an incorrect \ex{bufsiz} argument.
|
|
These sorts of issues are completely eliminated by the {\Scheme} programming
|
|
model.
|
|
Instead of having to worry about seven or eight trivial but potentially
|
|
fatal issues, and write the necessary 10 or 15 lines of code to correctly
|
|
handle the operation, the programmer can write a single function call and
|
|
get on with his task.
|
|
|
|
\subsection{Return values and procedural composition}
|
|
Exceptions and automatic storage allocation make it easier for
|
|
procedures to return useful values.
|
|
This increases the odds that the programmer can use the compact notation
|
|
of function composition---\ex{f(g(x))}---to connect producers and consumers
|
|
of data, which is surprisingly difficult in C.
|
|
%Making it possible for procedures to return useful values is quite
|
|
%useful, as it encourages programmers to use the compact notation of function
|
|
%composition---\ex{f(g(x))}---to indicate data flow, which is surprisingly
|
|
%difficult in C.
|
|
|
|
In C, if we wish to compose two procedure calls, we frequently must write:
|
|
\begin{code}
|
|
/* C style: */
|
|
g(x,&y);
|
|
{\ldots}f(y)\ldots\end{code}
|
|
Procedures that compute composite data structures for a result
|
|
commonly return them by storing them into a data structure passed
|
|
by-reference as a parameter.
|
|
If \ex{g} does this, we cannot nest calls, but must write the code as shown.
|
|
|
|
In fact, the above code is not quite what we want; we forgot to check \ex{g}
|
|
for an error return.
|
|
What we really wanted was:
|
|
\begin{code}
|
|
/* Worse/better: */
|
|
err=g(x,&y);
|
|
if( err ) \{
|
|
<{\it{handle error on {\tt{g}} call}}>
|
|
\}
|
|
{\ldots}f(y)\ldots\end{code}
|
|
The person who writes this code has to remember to check for the error;
|
|
the person who reads it has to visually link up the data flow by
|
|
connecting \ex{y}'s def and use points.
|
|
% puzzle out the data flow that goes from \ex{g}'s output value \ex{y} to
|
|
% \ex{f}'s input value.
|
|
% This is the data-flow equivalent of puzzling out the control flow
|
|
% of a program by tracing its \ex{goto}'s.
|
|
This is the data-flow equivalent of \ex{goto}'s,
|
|
with equivalent effects on program clarity.
|
|
|
|
In {\Scheme}, none of this is necessary. We simply write
|
|
\codex{(f (g x)) ; Scheme}
|
|
Easy to write; easy to read and understand.
|
|
Figure \ref{fig:stat-file} shows an example of this problem, where the
|
|
task is determining if a given file is owned by root.
|
|
\begin{figure}[bthp]
|
|
\begin{boxedminipage}{\linewidth}\vskip 1.5ex
|
|
\begin{tightcode}
|
|
(if (zero? (fileinfo:owner (file-attributes fname)))
|
|
\ldots)\end{tightcode}
|
|
\subcaption{\Scheme}
|
|
|
|
\medskip
|
|
|
|
\begin{tightinset}
|
|
\begin{verbatim}
|
|
if( stat(fname,&statbuf) ) {
|
|
perror(progname);
|
|
exit(-1);
|
|
}
|
|
if( statbuf.st_uid == 0 ) ...\end{verbatim}
|
|
\end{tightinset}
|
|
\subcaption{C}
|
|
\caption{Why we program with Scheme.}
|
|
\label{fig:stat-file}
|
|
\end{boxedminipage}
|
|
\end{figure}
|
|
|
|
\subsection{Strings}
|
|
Having a true string datatype turns out to be surprisingly valuable
|
|
in making systems programs simpler and more robust.
|
|
The programmer never has to expend effort to make sure that a string
|
|
length kept in a variable matches the actual length of the string;
|
|
never has to expend effort wondering how it will affect his program if
|
|
a nul byte gets stored into his string.
|
|
This is a minor feature, but like garbage collection, it eliminates a whole
|
|
class of common C programming bugs.
|
|
|
|
\subsection{Higher-order procedures}
|
|
Scheme's first-class procedures are very convenient for systems programming.
|
|
Scsh uses them to parameterise the action of procedures that create
|
|
{\Unix} processes.
|
|
The ability to package up an arbitrary computation as a thunk turns
|
|
out to be as useful in the domain of {\Unix} processes as it is in the domain
|
|
of {\Scheme} computation.
|
|
Being able to pass computations in this way to the procedures that create
|
|
{\Unix} processes, such as \ex{fork}, \ex{fork/pipe} and \ex{run/port*} is a
|
|
powerful programming technique.
|
|
|
|
First-class procedures allow us to parameterise port readers over different
|
|
parsers, with the
|
|
\codex{(port->list \var{parser} \var{port})}
|
|
procedure.
|
|
This is the essential {\Scheme} ability to capture abstraction in a procedure
|
|
definition.
|
|
If the user wants to read a list of objects written in some syntax from an
|
|
i/o source, he need only write a parser capable of parsing a single
|
|
object.
|
|
The \ex{port->list} procedure can work with the user's parser as easily as it
|
|
works with \ex{read} or \ex{read-line}.
|
|
\note{On-line streams}
|
|
|
|
First-class procedures also allow iterators such as \ex{for-each} and
|
|
\ex{filter} to loop over lists of data.
|
|
For example, to build the list of all my files in \ex{/usr/tmp}, I write:
|
|
\begin{code}
|
|
(filter (\l{f} (= (file-owner f) (user-uid)))
|
|
(glob "/usr/tmp/*"))\end{code}
|
|
To delete every C file in my directory, I write:
|
|
\codex{(for-each delete-file (glob "*.c"))}
|
|
|
|
\subsection{S-expression syntax and backquote}
|
|
\label{sec:sexp}
|
|
In general, {\Scheme}'s s-expression syntax is much, much simpler to
|
|
understand and use than most shells' complex syntax, with their embedded
|
|
pattern matching, variable expansion, alias substitution, and multiple
|
|
rounds of parsing.
|
|
This costs scsh's notation some compactness, at the gain of comprehensibility.
|
|
|
|
\subsubsection*{Recursive embeddings and balls of mud}
|
|
Scsh's ability to cover a high-level/low-level spectrum of expressiveness
|
|
is a function of its uniform s-expression notational framework.
|
|
Since scsh's process notation is embedded within Scheme,
|
|
and Scheme escapes are embedded within the process notation,
|
|
the programmer can easily switch back and forth as needed,
|
|
using the simple notation where possible,
|
|
and escaping to system calls and general {\Scheme} where necessary.
|
|
This recursive embedding is what gives scsh its broad-spectrum coverage
|
|
of systems functionality not available to either shells or traditional
|
|
systems programming languages;
|
|
it is essentially related to the ``ball of mud'' extensibility of the
|
|
Lisp and Scheme family of languages.
|
|
|
|
\subsubsection*{Backquote and reliable argument lists}
|
|
Scsh's use of implicit backquoting in the process notation is a particularly
|
|
nice feature of the s-expression syntax.
|
|
%Most {\Unix} shells provide the user with a way to compute a list of strings
|
|
%and use these strings as arguments to a program.
|
|
Most {\Unix} shells provide the user with a way to take a computed string,
|
|
split it into pieces, and pass them as arguments to a program.
|
|
This usually requires the introduction of some sort of \ex{\$IFS} separator
|
|
variable to control how the string is parsed into separate arguments.
|
|
This makes things error prone in the cases where a single argument
|
|
might contain a space or other parser delimiter.
|
|
Worse than error prone, \ex{\$IFS} rescanning is in fact the source of a
|
|
famous security hole in {\Unix} \cite{Reeds}.
|
|
|
|
In scsh, data are used to construct argument lists using the implicit backquote
|
|
feature of process forms, \eg:
|
|
\begin{tightcode}
|
|
(run (cc ,file -o ,binary ,@flags)).\end{tightcode}
|
|
Backquote completely avoids the parsing issue because it deals
|
|
with pre-parsed data: it constructs expressions from lists, not character
|
|
strings.
|
|
When the programmer computes a list of arguments, he has complete
|
|
confidence that they will be passed to the program exactly as is,
|
|
without running the risk of being re-parsed by the shell.
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{Other programming languages}
|
|
\label{sec:opl}
|
|
Having seen the design of scsh,
|
|
we can now compare it to other approaches in some detail.
|
|
|
|
\subsection{Functional languages}
|
|
The design of scsh could be ported without much difficulty
|
|
to any language that provides first-class procedures, GC, and exceptions,
|
|
such as {\CommonLisp} or ML.
|
|
However, {\Scheme}'s syntactic extensibility (macros) plays an important
|
|
role in making the shell features convenient to use.
|
|
In this respect, {\Scheme} and {\CommonLisp} are better choices than ML.
|
|
Using the \ex{fork/pipe} procedure with a series of closures
|
|
involves more low-level detail than
|
|
using scsh's \ex{(| \vari{pf}{\!1} {\ldots} \vari{pf}{\!n})}
|
|
process form with the closures implied.
|
|
Good notations suppress unnecessary detail.
|
|
|
|
The payoff for using a language such as ML would come not with small
|
|
shell scripts, but with larger programs, where the power provided by the
|
|
module system and the static type checking would come into play.
|
|
|
|
\subsection{Shells}
|
|
Traditional {\Unix} shells, such as sh, have no advantage at all as
|
|
scripting languages.
|
|
|
|
\subsubsection*{Escaping the least common denominator trap}
|
|
One of the attractions of scsh is that it is a {\Unix} shell that isn't
|
|
constrained by the limits of {\Unix}'s uniform ``least common denominator''
|
|
representation of data as a text string.
|
|
Since the standard medium of interchange at the shell level is {\Ascii}
|
|
byte strings, shell programmers are forced to parse and reparse data, often
|
|
with tools of limited power.
|
|
For example, to determine the number of files in a directory, a shell
|
|
programmer typically uses an expression of the form \ex{ls | wc -l}.
|
|
This traditional idiom is in fact buggy: {\Unix} files are allowed to contain
|
|
newlines in their names, which would defeat the simple \ex{wc} parser.
|
|
Scsh, on the other hand, gives the programmer direct access to the system
|
|
calls, and employs a much richer set of data structures.
|
|
Scsh's \ex{directory-files} procedure returns a {\em list\/} of strings,
|
|
directly taken from the system call.
|
|
There is no possibility of a parsing error.
|
|
|
|
As another example, consider the problem of determining if a file has its
|
|
setuid bit set.
|
|
The shell programmer must grep the text-string output of \ex{ls -l}
|
|
for the ``s'' character in the right position.
|
|
Scsh gives the programmer direct access to the \ex{stat()} system call,
|
|
so that the question can be directly answered.
|
|
|
|
\subsubsection*{Computation granularity and impedance matching}
|
|
Sh and csh provide minimal computation facilities on the assumption that all
|
|
real computation will happen in C programs invoked from the shell.
|
|
This is a granularity assumption.
|
|
As long as the individual units of computation are large, then the cost of
|
|
starting up a separate program is amortised over the actual computation.
|
|
However, when the user wants to do something simple---\eg, split an X
|
|
\verb|$DISPLAY| string at the colon,
|
|
count the number of files in a directory,
|
|
or lowercase a string---then the overhead of program invocation
|
|
swamps the trivial computation being performed.
|
|
One advantage of using a real programming language for the shell language is
|
|
that we can get a wider-range ``impedance match'' of computation to process
|
|
overhead.
|
|
Simple computations can be done in the shell;
|
|
large grain computations can still be spawned off
|
|
to other programs if necessary.
|
|
|
|
\subsection{New-generation scripting languages}
|
|
A newer generation of scripting languages has been supplanting sh in {\Unix}.
|
|
Systems such as perl and tcl provide many of the advantages of scsh for
|
|
programming shell scripts \cite{perl, tcl}.
|
|
However, they are still limited by weak linguistic features.
|
|
Perl and tcl still deal with the world primarily in terms of strings,
|
|
which is both inefficient and expressively limiting.
|
|
Scsh makes the full range of Scheme data types available to the programmer:
|
|
lists, records, floating point numbers, procedures, and so forth.
|
|
Further, the abstraction mechanisms in perl and tcl are also much more limited
|
|
than Scheme's lexically scoped, first-class procedures and lambda expressions.
|
|
As convenient as tcl and perl are, they are in no sense full-fledged
|
|
general systems-programming languages: you would not, for example, want
|
|
to write an optimizing compiler in tcl.
|
|
Scsh is Scheme, hence a powerful, full-featured general programming tool.
|
|
|
|
It is, however, instructive to consider the reasons for the popular success of
|
|
tcl and perl.
|
|
I would argue that good design is necessary but insufficient for
|
|
a successful tool.
|
|
Tcl and perl are successful because they are more than just competently
|
|
designed;
|
|
critically, they are also available on the Net in turn-key forms,
|
|
with solid documentation.
|
|
A potential user can just down-load and compile them.
|
|
Scheme, on the other hand, has existed in multiple mutually-incompatible
|
|
implementations that are not widely portable, do not portably address
|
|
systems issues, and are frequently poorly documented.
|
|
A contentious and standards-cautious Scheme community has not standardised
|
|
on a record datatype or exception facility for the language,
|
|
features critical for systems programming.
|
|
Scheme solves the hard problems, but punts the necessary, simpler ones.
|
|
This has made Scheme an impractical systems tool,
|
|
banishing it to the realm of pedagogical programming languages.
|
|
Scsh, together with Scheme 48, fills in these lacunae.
|
|
Its facilities may not be the ultimate solutions,
|
|
but they are useable technology: clean, consistent, portable and documented.
|
|
|
|
%\part{Conclusion}
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{Future work}
|
|
\label{sec:future-work}
|
|
Several extensions to scsh are being considered or implemented.
|
|
\subsection{Command language features}
|
|
The primary design effort of scsh was for programming.
|
|
We are now designing and implementing features to make scsh
|
|
a better interactive command language, such as job control.
|
|
A top-level parser for an sh-like notation has been designed;
|
|
the parser will allow the user to switch back to {\Scheme} notation
|
|
when desired.
|
|
|
|
We are also considering a display-oriented interactive shell,
|
|
to be created by merging the edwin screen editor and scsh.
|
|
The user will interact with the operating system using single-keystroke
|
|
commands, defining these commands using scsh, and reverting to
|
|
{\Scheme} when necessary for complex tasks.
|
|
Given a reasonable set of GUI widgets, the same trick could be played
|
|
directly in X.
|
|
|
|
\subsection{Little languages}
|
|
Many {\Unix} tools are built around the idea of ``little languages,'' that is,
|
|
custom, limited-purpose languages that are designed to fit the area of
|
|
application. The problem with the little-languages approach is that these
|
|
languages are usually ugly, idiosyncratic, and limited in expressiveness.
|
|
The syntactic quirks of these little languages are notorious.
|
|
The well-known problem with \ex{make}'s syntax distinguishing tab and
|
|
space has been tripping up programmers for years.
|
|
Because each little language is different
|
|
from the next, the user is required to master a handful of languages,
|
|
unnecessarily increasing the cognitive burden to use these tools.
|
|
|
|
An alternate approach is to embed the tool's primitive operations inside
|
|
{\Scheme},
|
|
and use the rest of {\Scheme} as the procedural glue to connect the
|
|
primitives into complex systems. This sort of approach doesn't require the
|
|
re-invention of all the basic functionality needed by a language---{\Scheme}
|
|
provides variables, procedures, conditionals, data structures, and so
|
|
forth. This means there is a greater chance of the designer ``getting it
|
|
right'' since he is really leveraging off of the enormous design effort that
|
|
was put into designing the {\Scheme} language. It also means the user doesn't
|
|
have to learn five or six different little languages---just {\Scheme} plus the
|
|
set of base primitives for each application. Finally, it means the base
|
|
language is not limited because the designer didn't have the time or resources
|
|
to implement all the features of a real programming language.
|
|
|
|
With the scsh {\Unix} library, these ``little language'' {\Unix} tools could
|
|
easily be redesigned from a {\Scheme} perspective and have their interface and
|
|
functionality significantly improved.
|
|
Some examples under consideration are:
|
|
\begin{itemize}
|
|
\item The awk pattern-matching language can be implemented in
|
|
scsh by adding a single record-input procedure to the existing code.
|
|
|
|
\item Expect is a scripting language used for automating the
|
|
use of interactive programs, such as ftp. With the exception of the tty
|
|
control syscalls currently under construction, all the pieces needed to
|
|
design an alternate scsh-based {\Unix} scripting tool already exist in scsh.
|
|
|
|
\item A dependency-directed system for controlling recompilation such
|
|
as make could easily be implemented on top of scsh. Here, instead of
|
|
embedding the system inside of {\Scheme}, we embed {\Scheme} inside
|
|
of the system. The dependency language would use s-expression notation,
|
|
and the embedded compilation actions would be specified as {\Scheme}
|
|
expressions, including scsh notation for running {\Unix} programs.
|
|
\end{itemize}
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{Conclusion}
|
|
Scsh is a system with several faces.
|
|
From one perspective,
|
|
it is not much more than a system-call library and
|
|
a few macros.
|
|
Yet, there is power in this minimalist description---it points up the
|
|
utility of embedding systems in languages such as {\Scheme}.
|
|
{\Scheme} is at core what makes scsh a successful design.
|
|
Which leads us to three final thoughts on the subject of scsh and
|
|
systems programming in {\Unix}:
|
|
\begin{itemize}
|
|
\item A Scheme shell wins because it is broad-spectrum.
|
|
\item A functional language is an excellent tool for systems programming.
|
|
\item Hacking Unix isn't so bad, actually, if you don't have to use C.
|
|
\end{itemize}
|
|
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\section{Acknowledgements}
|
|
John Ellis' 1980 {\em SIGPLAN Notices\/} paper \cite{ellis} got me thinking
|
|
about this entire area.
|
|
Some of the design for the system calls was modeled after
|
|
Richard Stallman's emacs \cite{emacs},
|
|
Project MAC's MIT {\Scheme} \cite{c-scheme}, and {\CommonLisp} \cite{cltl2}.
|
|
Tom Duff's {\Unix} shell, rc, was also inspirational;
|
|
his is the only elegant {\Unix} shell I've seen \cite{rc}.
|
|
Flames with Bennet Yee and Scott Draves drove me to design scsh in the
|
|
first place;
|
|
polite discussions with John Ellis and Scott Nettles subsequently improved it.
|
|
Douglas Orr was my private {\Unix} kernel consultant.
|
|
Richard Kelsey and Jonathan Rees provided me with twenty-four hour
|
|
turnaround time on requested modifications to {\scm}, and
|
|
spent a great deal of time explaining the internals of the implementation
|
|
to me.
|
|
Their elegant {\Scheme} implementation was a superb platform for development.
|
|
The design and the major portion of the implementation of scsh were completed
|
|
while I was visiting on the faculty of the University of Hong Kong
|
|
in 1992.
|
|
It was very pleasant to work in such a congenial atmosphere.
|
|
Doug Kwan was a cooperative sounding-board during the design phase.
|
|
Hsu Suchu has patiently waited quite a while for this document to
|
|
be finished.
|
|
Members of the MIT LCS and AI Lab community encouraged me to polish
|
|
the research prototype version of the shell into something releasable
|
|
to the net.
|
|
Henry Minsky and Ian Horswill did a lot of the encouraging;
|
|
my students Dave Albertz and Brian Carlstrom did a lot of the polishing.
|
|
|
|
Finally,
|
|
the unix-haters list helped a great deal to maintain my perspective.
|
|
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\cleardoublepage
|
|
\begin{thebibliography}{MIT Scheme}
|
|
\addcontentsline{toc}{section}{References}
|
|
\sloppy
|
|
\def\\{\newblock}
|
|
|
|
\renewcommand{\=}{\discretionary{/}{}{/}}
|
|
\renewcommand{\.}{\discretionary{.}{}{.}}
|
|
\newcommand{\ob}{\linebreak[0]}
|
|
|
|
\itemsep= 2ex plus 1fil
|
|
\let\Bibitem=\bibitem
|
|
|
|
\Bibitem[CLtL2]{cltl2} Guy L.~Steele Jr. \\
|
|
{\em Common Lisp: The Language.} \\
|
|
Digital Press, Maynard, Mass., second edition 1990.
|
|
|
|
\Bibitem[Ellis]{ellis} John R.~Ellis. \\ A {\sc Lisp} shell. \\
|
|
{\em SIGPLAN Notices}, 15(5):24--34, May 1980.
|
|
|
|
\Bibitem[emacs]{emacs} Bil Lewis, Dan LaLiberte, Richard M.~Stallman,
|
|
{\em et al.} \\
|
|
{\em The GNU Emacs Lisp Reference Manual, vol.~2.} \\
|
|
Free Software Foundation, Cambridge, Mass., edition 2.1 September 1993.
|
|
(Also available from many ftp sites.)
|
|
|
|
\Bibitem[fsh]{fsh} Chris S.~McDonald. \\
|
|
{\em fsh}---A functional {\Unix} command interpreter. \\
|
|
{\em Software---Practice and Experience}, 17(10):685--700,
|
|
October 1987.
|
|
|
|
\Bibitem[MIT Scheme]{c-scheme} Chris Hanson. \\
|
|
{\em MIT Scheme Reference Manual.} \\
|
|
MIT Artificial Intelligence Laboratory Technical Report 1281,
|
|
January 1991.
|
|
(Also URL
|
|
` {\tt http://zurich\.ai\.mit\.edu\=emacs-html\.local\=scheme\_toc.html})
|
|
|
|
\Bibitem[Nelson]{Nelson} Greg Nelson, ed. \\
|
|
{\em Systems Programming with Modula-3.} \\
|
|
Prentice Hall, Englewood Cliffs, New Jersey, 1991.
|
|
|
|
\Bibitem[perl]{perl} Larry Wall and Randal Schwartz. \\
|
|
{\em Programming Perl.} \\
|
|
O'Reilly \& Associates.
|
|
|
|
\Bibitem[rc]{rc} Tom Duff. \\ Rc---A shell for Plan 9 and {\Unix} systems. \\
|
|
In {\em Proceedings of the Summer 1990 UKUUG Conference},
|
|
pages 21--33, July 1990, London.
|
|
(A revised version is reprinted in ``Plan 9: The early papers,''
|
|
Computing Science Technical Report 158, AT\&T Bell Laboratories.
|
|
Also available in Postscript form as URL
|
|
\ex{ftp:{\ob}/\=research.att.com/dist/plan9doc/7}.)
|
|
|
|
\Bibitem[Reeds]{Reeds} J.~Reeds. \\
|
|
\ex{/bin/sh}: the biggest UNIX security loophole. \\
|
|
11217-840302-04TM, AT\&T Bell Laboratories (1988).
|
|
|
|
\Bibitem[refman]{ref-man} Olin Shivers. \\ Scsh reference manual. \\
|
|
In preparation.
|
|
|
|
\Bibitem[S48]{S48} Richard A.~Kelsey and Jonathan A.~Rees. \\
|
|
A tractable Scheme implementation. \\
|
|
To appear, {\em Lisp and Symbolic Computation},
|
|
Kluwer Academic Publishers, The Netherlands.
|
|
(Also URL {\tt ftp:/\=altdorf\.ai\.mit\.edu\=pub\=jar\=lsc.ps})
|
|
|
|
\Bibitem[tcl]{tcl} John~K.~Ousterhout. \\
|
|
Tcl: An embeddable command language. \\
|
|
In {\em The Proceedings of the 1990 Winter USENIX Conference},
|
|
pp.~133--146.
|
|
(Also URL
|
|
{\tt ftp:{\ob}/\=ftp\.cs\.berkeley\.edu\=ucb\=tcl\=tclUsenix90.ps})
|
|
\vfill
|
|
\end{thebibliography}
|
|
|
|
\appendix
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\cleardoublepage
|
|
\section*{Notes}
|
|
\addcontentsline{toc}{section}{Notes}
|
|
\newcommand{\notetext}[1]{\subsection*{\{Note #1\}}}
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\notetext{Agenda}
|
|
In fact, I have an additional hidden agenda.
|
|
I do believe that computational agents should be expressed as procedures
|
|
or procedure libraries, not as programs.
|
|
Scsh is intended to be an incremental step in this direction, one that
|
|
is integrated with {\Unix}.
|
|
Writing a program as a Scheme 48 module should allow the user to make it
|
|
available as a both a subroutine library callable from other Scheme 48
|
|
programs or the interactive read-eval-print loop, and, by adding a small
|
|
top-level, as a standalone {\Unix} program.
|
|
So {\Unix} programs written this way will also be useable as linkable
|
|
subroutine libraries---giving the programmer module interfaces superior
|
|
to {\Unix}'s ``least common denominator'' of {\sc Ascii} byte streams
|
|
sent over pipes.
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\notetext{No port sync}
|
|
\begin{sloppypar}
|
|
In scsh, {\Unix}' stdio file descriptors and {\Scheme}'s standard i/o ports
|
|
(\ie, the values of \ex{(current-input-port)}, \ex{(current-output-port)} and
|
|
\ex{(error-output-port)}) are not necessarily synchronised.
|
|
This is impossible to do in general, since some {\Scheme} ports are
|
|
not representable as {\Unix} file descriptors.
|
|
For example, many Scheme implementations provide ``string ports,''
|
|
that is, ports that collect characters sent to them into memory buffers.
|
|
The accumulated string can later be retrieved from the port as a string.
|
|
If a user were to bind \ex{(current-output-port)} to such a port, it would
|
|
be impossible to associate file descriptor 1 with this port, as it
|
|
cannot be represented in {\Unix}.
|
|
So, if the user subsequently forked off some other program as a subprocess,
|
|
that program would of course not see the Scheme string port as its standard
|
|
output.
|
|
\end{sloppypar}
|
|
|
|
To keep stdio synced with the values of {\Scheme}'s current i/o ports,
|
|
use the special redirection \ex{stdports}.
|
|
This causes 0, 1, 2 to be redirected from the current {\Scheme} standard ports.
|
|
It is equivalent to the three redirections:
|
|
\begin{code}
|
|
(= 0 ,(current-input-port))
|
|
(= 1 ,(current-output-port))
|
|
(= 2 ,(error-output-port))\end{code}
|
|
%
|
|
The redirections are done in the indicated order. This will cause an error if
|
|
the one of current i/o ports isn't a {\Unix} port (\eg, if one is a string
|
|
port).
|
|
This {\Scheme}/{\Unix} i/o synchronisation can also be had in {\Scheme} code
|
|
(as opposed to a redirection spec) with the \ex{(stdports->stdio)}
|
|
procedure.
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\notetext{Normal order}
|
|
Having to explicitly shift between processes and functions in scsh is in part
|
|
due to the arbitrary-size nature of a {\Unix} stream.
|
|
A better, more integrated approach might be to use a lazy, normal-order
|
|
language as the glue or shell language.
|
|
Then files and process output streams could be regarded as first-class values,
|
|
and treated like any other sequence in the language.
|
|
However, I suspect that the realities of {\Unix}, such as side-effects, will
|
|
interfere with this simple model.
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\notetext{On-line streams}
|
|
The \ex{(port->list \var{reader} \var{port})} procedure is a batch processor:
|
|
it reads the port all the way to eof before returning a value.
|
|
As an alternative, we might write a procedure to take a port and a reader,
|
|
and return a lazily-evaluated list of values,
|
|
so that I/O can be interleaved with element processing.
|
|
A nice example of the power of Scheme's abstraction facilities is the
|
|
ease with which we can write this procedure:
|
|
it can be done with five lines of code.
|
|
\begin{code}
|
|
;;; A <lazy-list> is either
|
|
;;; (delay '()) or
|
|
;;; (delay (cons data <lazy-list>)).
|
|
|
|
(define (port->lazy-list reader port)
|
|
(let collector ()
|
|
(delay (let ((x (reader port)))
|
|
(if (eof-object? x) '()
|
|
(cons x (collector)))))))\end{code}
|
|
|
|
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|
|
\notetext{Tempfile example}
|
|
For a more detailed example showing the advantages of higher-order procedures
|
|
in {\Unix} systems programming, consider the task of making random temporary
|
|
objects (files, directories, fifos, \etc) in the file system.
|
|
Most {\Unix}'s simply provide a function such as \ex{tmpnam()} that creates a
|
|
file with an unusual name, and hope for the best.
|
|
Other {\Unix}'s provide functions that avoid the race condition between
|
|
determining the temporary file's name and creating it, but they do not
|
|
provide equivalent features for non-file objects, such as directories or
|
|
symbolic links.
|
|
\pagebreak
|
|
This functionality is easily generalised with the procedure
|
|
\codex{(temp-file-iterate \var{maker} \var{[template]})}
|
|
This procedure can be used to perform atomic transactions on
|
|
the file system involving filenames, \eg:
|
|
\begin{itemize}
|
|
\item Linking a file to a fresh backup temporary name.
|
|
\item Creating and opening an unused, secure temporary file.
|
|
\item Creating an unused temporary directory.%
|
|
\end{itemize}
|
|
%
|
|
The string \var{template} is a \ex{format} control string used to generate
|
|
a series of trial filenames; it defaults to
|
|
%
|
|
\begin{tightinset}\verb|"/usr/tmp/<pid>.~a"|\end{tightinset}\ignorespaces
|
|
%
|
|
where \ex{<pid>} is the current process' process id.
|
|
Filenames are generated by calling \ex{format} to instantiate the
|
|
template's \verb|~a| field with a varying string.
|
|
(It is not necessary for the process' pid to be a part of the filename
|
|
for the uniqueness guarantees to hold. The pid component of the default
|
|
prefix simply serves to scatter the name searches into sparse regions, so
|
|
that collisions are less likely to occur. This speeds things up, but does
|
|
not affect correctness.)
|
|
|
|
The \ex{maker} procedure is serially called on each filename generated.
|
|
It must return at least one value; it may return multiple values. If
|
|
the first return value is \ex{\#f} or if \ex{maker} raises the ``file already
|
|
exists'' syscall error exception, \ex{temp-file-iterate} will loop,
|
|
generating a new filename and calling \ex{maker} again.
|
|
If the first return value is true, the loop is terminated,
|
|
returning whatever \ex{maker} returned.
|
|
|
|
After a number of unsuccessful trials, \ex{temp-file-iterate} may give up
|
|
and signal an error.
|
|
|
|
To rename a file to a temporary name, we write:
|
|
\begin{code}
|
|
(temp-file-iterate (\l{backup-name}
|
|
(create-hard-link old-file
|
|
backup-name)
|
|
backup-name)
|
|
".#temp.~a") ; Keep link in cwd.
|
|
(delete-file old-file)\end{code}
|
|
Note the guarantee: if \ex{temp-file-iterate} returns successfully,
|
|
then the hard link was definitely created, so we can safely delete the
|
|
old link with the following \ex{delete-file}.
|
|
|
|
To create a unique temporary directory, we write:
|
|
%
|
|
\codex{(temp-file-iterate (\l{dir} (create-directory dir) dir))}
|
|
%
|
|
Similar operations can be used to generate unique symlinks and fifos,
|
|
or to return values other than the new filename (\eg, an open file
|
|
descriptor or port).
|
|
\end{document}
|
|
|
|
% LocalWords: Mips grep sed awk ls wc email flamefest philes SRC's dup int pid
|
|
% LocalWords: foobar fds perror waitpid execlp kb rc's epf pf fdes fv
|
|
% LocalWords: stdports dup'd subforms backquoted usr backquoting ref tmp
|
|
% LocalWords: buf stdin stderr stdout sync prog arg Readme xrdb xyzzy SunOS
|
|
% LocalWords: mbox txt cc preprocess Errlog exe outfile errfile PLAINTEXT des
|
|
% LocalWords: plaintext DIR perl cwd dir dsw ll conns xhost lpr ksh namespaces
|
|
% LocalWords: ms texinfo doc fd RS Wn Ansi esh zcat tex detex enscript madvise
|
|
% LocalWords: mmap stat fname eq fileinfo backquote readlink symlink fil nul
|
|
% LocalWords: bufsiz def bthp statbuf progname uid Tempfile IFS pre Ascii bp
|
|
% LocalWords: reparse setuid
|