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<title>execline: pushing and popping the environment</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>
</p>
<h1> Pushing and popping the environment </h1>
<p>
The <a href="execlineb.html">execlineb</a> launcher
can store <em>positional
parameters</em>, i.e. arguments given to your script, into the
environment. The <tt>#</tt> variable contains the number of arguments;
the <tt>0</tt> variable contains the name of your execline script;
the <tt>1</tt> variable contains the first argument; and so on.
</p>
<p>
Up to execline-1.04, this could create problems with nested scripts:
the inner script would overwrite the outer script's parameters, and
there was no way to get them back. In particular, writing execline
commands in the execline language via the
<a href="runblock.html">runblock</a> command was impossible.
</p>
<p>
<div id="push">To solve that issue</div>, execline now implements a kind of <em>environment
stack</em>. When execlineb reads the arguments, it does
not overwrite the positional parameters, but <em>pushes</em> them on a
stack:
</p>
<ul>
<li> <tt>#</tt> will be set to the current number of arguments </li>
<li> but if a variable named <tt>#</tt> existed before, it is renamed <tt>#:1</tt> </li>
<li> and if a variable named <tt>#:1</tt> also existed, it is renamed <tt>#:2</tt> </li>
<li> ... and so on until <tt>#:<em>n+1</em></tt> doesn't exist. </li>
</ul>
<p>
Same goes for the other <em>positional parameters</em>.
</p>
<p>
The script then runs; and commands such as
<a href="elgetpositionals.html">elgetpositionals</a> use the current
frame of positional parameters, without paying attention to the deeper
levels.
</p>
<p>
<div id="pop">When you are done with the arguments</div>, it is advisable to <em>drop</em>
the current frame, and <em>pop</em> the environment stack to get it back
to its previous state:
</p>
<ul>
<li> <tt>#</tt> will be unset </li>
<li> but if <tt>#:1</tt> exists, it will be renamed <tt>#</tt> </li>
<li> and if <tt>#:2</tt> exists, it will be renamed <tt>#:1</tt> </li>
<li> ... and so on until <tt>#:<em>n+1</em></tt> doesn't exist. </li>
</ul>
<p>
Again, same goes for the other <em>positional parameters</em>. <br />
The <a href="runblock.html">runblock</a> command will perform that
<em>pop</em> operation automatically; the standard "manual" way to
perform it is to use the <a href="emptyenv.html">emptyenv -P</a> command.
</p>
<h2> A pop example </h2>
<p>
Suppose you want to run the long-lived program <em>prog</em> after
printing the list of its arguments.
</p>
<pre>
#!/command/execlineb
elgetpositionals
foreground { echo $0 $@ }
prog $@
</pre>
<p>
will work, but will pollute <em>prog</em>'s environment with a set of
positional parameters that have no meaning to it. A better script is:
</p>
<pre>
#!/command/execlineb
elgetpositionals
foreground { echo $0 $@ }
emptyenv -P
prog $@
</pre>
<p>
which will run <em>prog</em> with the same environment as the script's
caller.
</p>
<div id="integrated">
<h2> Substituting positional parameters without touching the environment </h2>
</div>
<p>
Most of the time, you just need to substitute the positional parameters
in your execline script, and don't need to go through the whole
<a href="elgetpositionals.html">elgetpositionals</a> and
<a href="emptyenv.html">emptyenv</a> chain. execline comes with an
integrated substitution mechanism, that does not touch the environment
at all: the <tt>-S <em>n</em></tt> option.
</p>
<p>
Scripts beginning with:
</p>
<pre>
#!/command/execlineb -S<em>n</em>
<em>foobar...</em>
</pre>
<p>
are equivalent to:
</p>
<pre>
#!/command/execlineb
elgetpositionals -P<em>n</em>
emptyenv -P
<em>foobar...</em>
</pre>
<p>
So, to summarize, from most efficient (but less flexible) to least efficient
(but more flexible):
</p>
<ul>
<li> Use <tt>execlineb -P</tt> if you don't need positional parameters
at all; for instance, in
<a href="//skarnet.org/software/s6/">s6</a> or
<a href="http://smarden.org/runit/">runit</a> <em>run scripts</em>. </li>
<li> Use <tt>execlineb -S<em>n</em></tt> if you need only simple
positional parameter substitution in your script - no
<a href="shift.html">shift</a> or <a href="elgetopt.html">elgetopt</a>,
no <tt>importas 1 1</tt>. </li>
<li> Use <tt>execlineb -p</tt>, then <tt>elgetpositionals</tt> if
you don't mind overwriting the current stack of positional parameters. </li>
<li> Use <tt>execlineb</tt>, then <tt>elgetpositionals</tt>, then
<tt>emptyenv -P</tt> if you need the full power of positional parameter
handling. </li>
</ul>
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