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<title>execline: block management</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> Blocks </h1>
<p>
A command line (and thus an execline script) is one-dimensional. But a
Unix execution flow can be <em>two</em>-dimensional: when two
instructions are sequenced, for instance. In that case, we need a
way to extract <em>two</em> command lines from <em>one</em> argv.
That is precisely what <em>blocks</em> are made for.
</p>
<p>
execline commands that need more than one linear set of arguments
use blocks. For instance, the
<a href="foreground.html">foreground</a> command needs to spawn a
first process, then execute into a second one. It reads the command
line for the first process from a block, and the command line for the
second process from the rest of the argv. In the following script:
</p>
<pre>
#!/command/execlineb
foreground { echo 1 } echo 2
</pre>
<p>
<tt>echo 1</tt> is read from a block and spawned; then
<tt>echo 2</tt> is executed.
</p>
<h2> execlineb syntax </h2>
<p>
In <a href="execlineb.html">execlineb</a> scripts, blocks are
delimited by braces. They can be nested.
</p>
<h2> argv syntax </h2>
<p>
execlineb reads and parses the script, and converts it into an <em>argv</em>
(a simple Unix command line) with a different syntax for blocks.
In an argv, blocks are not delimited by braces;
they are made of <em>quoted arguments</em> and terminated by an
empty word (""). A quoted argument begins with a space.
Nested blocks are represented by arguments being
quoted several times, i.e. having several spaces in front of them;
an empty word inside a block
gets quoted too, i.e. it will be represented as a series of
spaces.
</p>
<p>
Actually, the block-reading commands know nothing about braces;
they only understand the "quoted arguments + empty word" syntax.
So if you want to use <a href="foreground.html">foreground</a>
from your shell to sequence <tt>echo 1</tt> and
<tt>echo 2</tt>, you will have to write
</p>
<pre>
$ foreground ' echo' ' 1' '' echo 2
</pre>
<p>
You do not really need to quote every argument inside a block in
that simple case. The following command works as well:
</p>
<pre>
$ foreground echo 1 '' echo 2
</pre>
<p>
However, this is bad practice, because it leads to a security hole:
commands that perform
<a href="el_substitute.html">substitution</a> inside a block may
produce empty words, which may modify your script's execution flow.
</p>
<pre>
$ define FOO '' foreground ' echo' ' ${FOO}' ' rm' ' -rf' ' /' '' echo blah
</pre>
<p>
is safe, whereas
</p>
<pre>
$ define FOO '' foreground echo '${FOO}' rm -rf / '' echo blah
</pre>
<p>
has very much unwanted results. (Don't try this at home.)
</p>
<p>
You can use the <tt>EXECLINE_STRICT</tt> environment variable to
check proper block quoting. If that variable contains <tt>1</tt>,
commands that read blocks will print a warning message everytime
they find an unquoted argument inside a block. If that variable
contains <tt>2</tt> or a bigger integer, commands will print an
error message and die on unquoted arguments.
<br /> You can use <a href="execlineb.html">execlineb</a>'s
<tt>-w</tt> or <tt>-W</tt>
switch to set <tt>EXECLINE_STRICT</tt> to <tt>1</tt> or <tt>2</tt>.
</p>
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