167 lines
7.8 KiB
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167 lines
7.8 KiB
HTML
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>The Meta Switch (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Guile-Scripting.html" rel="up" title="Guile Scripting">
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<div class="subsection-level-extent" id="The-Meta-Switch">
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<p>
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Next: <a href="Command-Line-Handling.html" accesskey="n" rel="next">Command Line Handling</a>, Previous: <a href="The-Top-of-a-Script-File.html" accesskey="p" rel="prev">The Top of a Script File</a>, Up: <a href="Guile-Scripting.html" accesskey="u" rel="up">Guile Scripting</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
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<h4 class="subsection" id="The-Meta-Switch-1"><span>4.3.2 The Meta Switch<a class="copiable-link" href="#The-Meta-Switch-1"> ¶</a></span></h4>
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<p>Guile’s command-line switches allow the programmer to describe
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reasonably complicated actions in scripts. Unfortunately, the POSIX
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script invocation mechanism only allows one argument to appear on the
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‘<samp class="samp">#!</samp>’ line after the path to the Guile executable, and imposes
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arbitrary limits on that argument’s length. Suppose you wrote a script
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starting like this:
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</p><div class="example">
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<pre class="example-preformatted">#!/usr/local/bin/guile -e main -s
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!#
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(define (main args)
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(map (lambda (arg) (display arg) (display " "))
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(cdr args))
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(newline))
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</pre></div>
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<p>The intended meaning is clear: load the file, and then call <code class="code">main</code>
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on the command-line arguments. However, the system will treat
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everything after the Guile path as a single argument — the string
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<code class="code">"-e main -s"</code> — which is not what we want.
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</p>
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<p>As a workaround, the meta switch <code class="code">\</code> allows the Guile programmer to
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specify an arbitrary number of options without patching the kernel. If
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the first argument to Guile is <code class="code">\</code>, Guile will open the script file
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whose name follows the <code class="code">\</code>, parse arguments starting from the
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file’s second line (according to rules described below), and substitute
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them for the <code class="code">\</code> switch.
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</p>
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<p>Working in concert with the meta switch, Guile treats the characters
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‘<samp class="samp">#!</samp>’ as the beginning of a comment which extends through the next
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line containing only the characters ‘<samp class="samp">!#</samp>’. This sort of comment may
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appear anywhere in a Guile program, but it is most useful at the top of
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a file, meshing magically with the POSIX script invocation mechanism.
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</p>
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<p>Thus, consider a script named <samp class="file">/u/jimb/ekko</samp> which starts like this:
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</p><div class="example">
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<pre class="example-preformatted">#!/usr/local/bin/guile \
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-e main -s
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!#
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(define (main args)
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(map (lambda (arg) (display arg) (display " "))
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(cdr args))
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(newline))
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</pre></div>
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<p>Suppose a user invokes this script as follows:
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</p><div class="example">
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<pre class="example-preformatted">$ /u/jimb/ekko a b c
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</pre></div>
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<p>Here’s what happens:
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</p><ul class="itemize mark-bullet">
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<li>the operating system recognizes the ‘<samp class="samp">#!</samp>’ token at the top of the
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file, and rewrites the command line to:
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<div class="example">
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<pre class="example-preformatted">/usr/local/bin/guile \ /u/jimb/ekko a b c
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</pre></div>
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<p>This is the usual behavior, prescribed by POSIX.
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</p>
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</li><li>When Guile sees the first two arguments, <code class="code">\ /u/jimb/ekko</code>, it opens
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<samp class="file">/u/jimb/ekko</samp>, parses the three arguments <code class="code">-e</code>, <code class="code">main</code>,
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and <code class="code">-s</code> from it, and substitutes them for the <code class="code">\</code> switch.
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Thus, Guile’s command line now reads:
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<div class="example">
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<pre class="example-preformatted">/usr/local/bin/guile -e main -s /u/jimb/ekko a b c
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</pre></div>
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</li><li>Guile then processes these switches: it loads <samp class="file">/u/jimb/ekko</samp> as a
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file of Scheme code (treating the first three lines as a comment), and
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then performs the application <code class="code">(main "/u/jimb/ekko" "a" "b" "c")</code>.
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</li></ul>
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<p>When Guile sees the meta switch <code class="code">\</code>, it parses command-line
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argument from the script file according to the following rules:
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</p><ul class="itemize mark-bullet">
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<li>Each space character terminates an argument. This means that two
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spaces in a row introduce an argument <code class="code">""</code>.
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</li><li>The tab character is not permitted (unless you quote it with the
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backslash character, as described below), to avoid confusion.
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</li><li>The newline character terminates the sequence of arguments, and will
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also terminate a final non-empty argument. (However, a newline
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following a space will not introduce a final empty-string argument;
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it only terminates the argument list.)
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</li><li>The backslash character is the escape character. It escapes backslash,
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space, tab, and newline. The ANSI C escape sequences like <code class="code">\n</code> and
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<code class="code">\t</code> are also supported. These produce argument constituents; the
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two-character combination <code class="code">\n</code> doesn’t act like a terminating
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newline. The escape sequence <code class="code">\<var class="var">NNN</var></code> for exactly three octal
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digits reads as the character whose ASCII code is <var class="var">NNN</var>. As above,
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characters produced this way are argument constituents. Backslash
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followed by other characters is not allowed.
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</li></ul>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Command-Line-Handling.html">Command Line Handling</a>, Previous: <a href="The-Top-of-a-Script-File.html">The Top of a Script File</a>, Up: <a href="Guile-Scripting.html">Guile Scripting</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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