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<title>execline: language design and grammar</title>
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<p>
<a href="index.html">execline</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
<a href="http://skarnet.org/">skarnet.org</a>
</p>
<h1> The <tt>execline</tt> language design and grammar </h1>
<a name="principles" />
<h2> <tt>execline</tt> principles </h2>
<p>
Here are some basic Unix facts:
</p>
<ul>
<li> Unix programs are started with the <tt>execve()</tt>
system call, which takes 3 arguments: the command name (which
we won't discuss here because it's redundant in most cases),
the command line <em>argv</em>, which specifies the program name and its
arguments, and the environment <em>envp</em>. </li>
<li> The <em>argv</em> structure makes it easy to read some
arguments at the beginning of <em>argv</em>, perform some action,
then <tt>execve()</tt> into the rest of <em>argv</em>. For
instance, the <tt>nice</tt> command works that way:
<pre> nice -10 echo blah </pre> will read <tt>nice</tt> and <tt>-10</tt>
from the argv, change the process' <em>nice</em> value, then exec into
the command <tt>echo blah</tt>. This is called
<a href"http://en.wikipedia.org/wiki/Chain_loading">chain loading</a>
by some people, and <a href="http://www.faqs.org/docs/artu/ch07s02.html">
Bernstein chaining</a> by others. </li>
<li> The purpose of the environment is to preserve some state across
<tt>execve()</tt> calls. This state is usually small: most programs
keep their information in the filesystem. </li>
<li> A <em>script</em> is basically a text file whose meaning is a
sequence of actions, i.e. calls to Unix programs, with some control
over the execution flow. You need a program to interpret your script.
Traditionally, this program is <tt>/bin/sh</tt>: scripts are written
in the <em>shell</em> language. </li>
<li> The shell reads and interprets the script command after command.
That means it must preserve a state, and stay in memory while the
script is running. </li>
<li> Standard shells have lots of built-in features and commands, so
they are big. Spawning (i.e. <tt>fork()</tt>ing then <tt>exec()</tt>ing)
a shell script takes time, because the shell program itself must be
initialized. For simple programs like <tt>nice -10 echo blah</tt>,
a shell is overpowered - we only need a way to make an <em>argv</em>
from the "<tt>nice -10 echo blah</tt>" string, and <tt>execve()</tt>
into that <em>argv</em>. </li>
<li> Unix systems have a size limit for <em>argv</em>+<em>envp</em>,
but it is high. POSIX states that this limit must not be inferior to
4 KB - and most simple scripts are smaller than that. Modern systems
have a much higher limit: for instance, it is 64 KB on FreeBSD-4.6,
and 128 KB on Linux. </li>
</ul>
<p>
Knowing that, and wanting lightweight and efficient scripts, I
wondered: "Why should the interpreter stay in memory while the script
is executing ? Why not parse the script once and for all, put
it all into one <em>argv</em>, and just execute into that <em>argv</em>,
relying on external commands (which will be called from within the
script) to control the execution flow ?"
</p>
<p> <tt>execline</tt> was born. </p>
<ul>
<li> <tt>execline</tt> is the first script language to rely
<em>entirely</em> on chain loading. An execline script is a
single <em>argv</em>, made of a chain of programs designed to
perform their action then <tt>exec()</tt> into the next one. </li>
<li> The <a href="execlineb.html">execlineb</a> command is a
<em>launcher</em>: it reads and parses a text file, converting it
to an <em>argv</em>, then executes into that <em>argv</em>. It
does nothing more. </li>
<li> Straightforward scripts like <tt>nice -10 echo blah</tt>
will be run just as they are, without the shell overhead.
Here is what the script could look like:
<pre>
#!/command/execlineb -P
nice -10
echo blah
</pre>
</li>
<li> More complex scripts will include calls to other <tt>execline</tt>
commands, which are meant to provide some control over the process state
and execution flow from inside an <em>argv</em>. </li>
</ul>
<a name="grammar" />
<h2> Grammar of an execline script </h2>
<p>
An execline script can be parsed as follows:
</p>
<pre>
<instruction> = <> | external options <arglist> <instruction> | builtin options <arglist> <blocklist> <instruction>
<arglist> = <> | arg <arglist>
<blocklist> = <> | <block> <blocklist>
<block> = { <arglist> } | { <instrlist> }
<instrlist> = <> | <instruction> <instrlist>
</pre>
<p>
(This grammar is ambivalent, but much simpler to understand than the
non-ambivalent ones.)
</p>
<ul>
<li> An execline script is valid if it reduces to an
<em>instruction</em>. </li>
<li> The empty <em>instruction</em> is the same as the <tt>true</tt>
command: when an execline component must exec into the empty
instruction, it exits 0. </li>
<li> Basically, every non-empty <em>instruction</em>, be it
"<em>builtin</em>" - an execline command - or "<em>external</em>"
- a program such as <tt>echo</tt> or <tt>cp</tt> - takes a number of
arguments, the <em>arglist</em>, then executes into a (possibly empty)
<em>instruction</em>. </li>
<li> Some <em>builtin</em>s are special because they also take a
non-empty <em>blocklist</em> after their <em>arglist</em>. For instance,
the <a href="foreground.html">foreground</a> command takes an empty
<em>arglist</em> and one <em>block</em>: <pre>
#!/command/execlineb -P
foreground { sleep 1 } echo blah
</pre> is a valid <a href="execlineb.html">execlineb</a> script.
The <a href="foreground.html">foreground</a> command uses the
<tt>sleep 1</tt> <em>block</em> then execs into the
remaining <tt>echo blah</tt> <em>instruction</em>. </li>
</ul>
<a name="features"></a>
<h2> execline features </h2>
<p>
<tt>execline</tt> commands can perform some transformations on
their <em>argv</em>, to emulate some aspects of a shell. Here are
descriptions of these features:
</p>
<ul>
<li> <a href="el_semicolon.html">Block management</a> </li>
<li> <a href="el_substitute.html">Variable substitution</a> </li>
</ul>
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