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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> Value transformation </h1>
<p>
You can apply 3 kinds of transformations to a value which is to be
<a href="el_substitute.html">substituted</a> for a variable:
crunching, chomping and splitting. They
always occur in that order.
</p>
<a name="delim">
<h2> Delimiters </h2>
</a>
<p>
The transformations work around <em>delimiters</em>. Delimiters are
the semantic bounds of the "words" in your value.
You can use any character (except the null character, which you cannot
use in execline scripts) as a delimiter, by giving a string consisting
of all the delimiters you want as the argument to the <tt>-d</tt> option
used by substitution commands. By default, the string "<tt> \n\r\t</tt>"
is used, which means that the default delimiters are spaces, newlines,
carriage returns and tabs.
</p>
<a name="crunch">
<h2> Crunching </h2>
</a>
<p>
You can tell the substitution command to merge sets of consecutive
delimiters into a single delimiter. For instance, to replace
three consecutive spaces, or a space and 4 tab characters, with a
single space. This is called <em>crunching</em>, and it is done
by giving the <tt>-C</tt> switch to the substitution command. The
remaining delimiter will always be the first in the sequence.
</p>
<p>
Crunching is mainly useful when also <a href="#split">splitting</a>.
</p>
<a name="chomp">
<h2> Chomping </h2>
</a>
<p>
Sometimes you don't want the last delimiter in a value.
<em>Chomping</em> deletes the last character of a value if it is a
delimiter. It can be requested by giving the <tt>-n</tt> switch to the
substitution command. Note that chomping always happens <em>after</em>
crunching, which means you can use crunching+chomping to ignore, for
instance, a set of trailing spaces.
</p>
<a name="split">
<h2> Splitting </h2>
</a>
<p>
In a shell, when you write
</p>
<pre>
$ A='foo bar' ; echo $A
</pre>
<p>
the <tt>echo</tt> command is given two arguments, <tt>foo</tt>
and <tt>bar</tt>. The <tt>$A</tt> value has been <em>split</em>,
and the space between <tt>foo</tt> and <tt>bar</tt> acted as a
<em>delimiter</em>.
</p>
<p>
If you want to avoid splitting, you must write something like
</p>
<pre>
$ A='foo bar' ; echo "$A"
</pre>
<p>
The doublequotes "protect" the spaces. Unfortunately, it's easy
to forget them and perform unwanted splits during script execution
- countless bugs happen because of the shell's splitting behaviour.
</p>
<p>
<tt>execline</tt> provides a <em>splitting</em> facility, with
several advantages over the shell's:
</p>
<ul>
<li> Splitting has to be explicitly requested, by specifying the
<tt>-s</tt> option to commands that perform
<a href="el_substitute.html">substitution</a>. By default,
substitutions are performed as is, without interpreting the
characters in the value. </li>
<li> Positional parameters are never split, so that execline
scripts can handle arguments the way the user intended to. To
split <tt>$1</tt>, for instance, you have to ask for it
specifically:
<pre>
#!/command/<a href="execlineb.html">execlineb</a> -S1
<a href="define.html">define</a> -sd" " ARG1S $1
blah $ARG1S
</pre>
and $ARG1S will be split using the space character as only delimiter.
</li>
<li> Any character can be a delimiter. </li>
</ul>
<h3> How it works </h3>
<ul>
<li> A substitution command can request that the substitution value
be split, via the <tt>-s</tt> switch. </li>
<li> The splitting function parses the value, looking for delimiters.
It fills up a structure, marking the split points, and the number
<em>n</em> of words the value is to be split into.
<ul>
<li> A word is a sequence of characters in the value <em>terminated
by a delimiter</em>. The delimiter is not included in the word. </li>
<li> If the value begins with <em>x</em> delimiters, the word list
will begin with <em>x</em> empty words. </li>
<li> The last sequence of characters in the value will be recognized
as a word even if it is not terminated by a delimiter, unless you have
requested <a href="#chomp">chomping</a> and there was no delimiter at
the end of the value <em>before</em> the chomp operation - in which case
that last sequence will not appear at all. </li>
</ul> </li>
<li> The substitution rewrites the argv. A non-split value will
be written as one word in the argv; a split value will be written
as <em>n</em> separate words. </li>
<li> Substitution of split values is
<a href="el_substitute.html#recursive">performed recursively</a>. </li>
</ul>
<a name="netstrings">
<h3> Decoding netstrings </h3>
</a>
<p>
<a href="http://cr.yp.to/proto/netstrings.txt">Netstrings</a> are
a way to reliably encode strings containing arbitrary characters.
<tt>execline</tt> takes advantage of this to offer a completely safe
splitting mechanism. If a substitution command is given an empty
delimiter string (by use of the <tt>-d ""</tt> option), the
splitting function will try to interpret the value as a sequence
of netstrings, every netstring representing a word. For instance,
in the following command line:
</p>
<pre>
$ define -s -d "" A '1:a,2:bb,0:,7:xyz 123,1: ,' echo '$A'
</pre>
<p>
the <tt>echo</tt> command will be given five arguments:
</p>
<ul>
<li> the "<tt>a</tt>" string </li>
<li> the "<tt>bb</tt>" string </li>
<li> the empty string </li>
<li> the "<tt>xyz 123</tt>" string </li>
<li> the "<tt> </tt>" string (a single space) </li>
</ul>
<p>
However, if the value is not a valid sequence of netstrings, the
substitution command will die with an error message.
</p>
<p>
The <a href="dollarat.html">dollarat</a> command, for instance,
can produce a sequence of netstrings (encoding all the arguments
given to an execline script), meant to be decoded by a substitution
command with the <tt>-d ""</tt> option.
</p>
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