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<title>execline: the multisubstitute command</title>
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<p>
<a href="index.html">execline</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
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<h1> The <tt>multisubstitute</tt> program </h1>
<p>
<tt>multisubstitute</tt> performs several substitutions at once in
its <em>argv</em>, then executes another program.
</p>
<h2> Interface </h2>
<p>
In an <a href="execlineb.html">execlineb</a> script:
</p>
<pre>
multisubstitute
{
[ <a href="define.html">define</a> [ -n ] [ -s ] [ -C | -c ] [ -d <em>delim</em> ] <em>variable</em> <em>value</em> ]
[ <a href="importas.html">importas</a> [ -i | -D <em>default</em> ] [ -n ] [ -s ] [ -C | -c ] [ -d <em>delim</em> ] <em>variable</em> <em>envvar</em> ]
[ <a href="elglob.html">elglob</a> [ -v ] [ -w ] [ -s ] [ -m ] [ -e ] [ -0 ] <em>variable</em> <em>pattern</em> ]
[ <a href="elgetpositionals.html">elgetpositionals</a> [ -P <em>sharp</em> ] ]
[ <a href="multidefine.html">multidefine</a> <em>value</em> { <em>variable...</em> } ]
<em>...</em>
}
<em>prog...</em>
</pre>
<ul>
<li> <tt>multisubstitute</tt> reads a <a href="el_semicolon.html">block</a>
containing a series of substitution commands. It performs all
those <a href="el_substitute.html">substitutions</a> on
<em>prog...</em> in parallel. Check the relevant documentation page
to learn about the syntax of each substitution command. </li>
<li> <tt>multisubstitute</tt> then execs into the modified <em>prog...</em>. </li>
</ul>
<h2> Options </h2>
<ul>
<li> If an <tt>importas</tt> directive was given with the
<tt>-i</tt> option, and the looked up variable is undefined,
<tt>multisubstitute</tt> will exit 100. </li>
</ul>
<h2> Rationale </h2>
<h3> Security </h3>
<p>
<tt>multisubstitute</tt> can be used to avoid unwanted
<em>serial substitutions</em>. Consider the following script:
</p>
<pre>
#!/command/execlineb
export A wrong
define B ${A}
importas A A
echo ${B}
</pre>
<p>
Running it will print <tt>wrong</tt>, because <tt>A</tt> is substituted
<em>after</em> B. On the contrary, the following script:
</p>
<pre>
#!/command/execlineb
export A wrong
multisubstitute
{
define B ${A}
importas A A
}
echo ${B}
</pre>
<p>
will print <tt>${A}</tt>, because A and B are substituted at the same
time. Serial substitution may be what you want - but when in doubt,
always perform parallel substitution.
</p>
<h3> Efficiency </h3>
<p>
<a href="el_substitute.html">Substitution</a> is a costly mechanism:
the whole <em>argv</em> is read three times and rewritten twice.
Serial substitution multiplies the cost by the number of
substitutions, whereas parallel substitution pays the price only once.
</p>
<h2> Credits </h2>
<p>
<a href="http://code.dogmap.org/">Paul Jarc</a> first originated the
idea of the <tt>multisubstitute</tt> command and a possible syntax.
</p>
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