<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE comref SYSTEM "comref.dtd">
<comref>
<header>
<copyright>
<year>2007</year><year>2010</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<title>escript</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>escript.xml</file>
</header>
<com>escript</com>
<comsummary>Erlang scripting support</comsummary>
<description>
<p><c><![CDATA[escript]]></c> provides support for running short Erlang programs
without having to compile them first and an easy way to retrieve the
command line arguments.</p>
</description>
<funcs>
<func>
<name>script-name script-arg1 script-arg2...</name>
<name>escript escript-flags script-name script-arg1 script-arg2...</name>
<fsummary>Run a script written in Erlang</fsummary>
<desc>
<p><c><![CDATA[escript]]></c> runs a script written in Erlang.</p>
<p>Here follows an example.</p>
<pre>
$ <input>cat factorial</input>
#!/usr/bin/env escript
%% -*- erlang -*-
%%! -smp enable -sname factorial -mnesia debug verbose
main([String]) ->
try
N = list_to_integer(String),
F = fac(N),
io:format("factorial ~w = ~w\n", [N,F])
catch
_:_ ->
usage()
end;
main(_) ->
usage().
usage() ->
io:format("usage: factorial integer\n"),
halt(1).
fac(0) -> 1;
fac(N) -> N * fac(N-1).
$ <input>factorial 5</input>
factorial 5 = 120
$ <input>factorial</input>
usage: factorial integer
$ <input>factorial five</input>
usage: factorial integer </pre>
<p>The header of the Erlang script in the example differs from
a normal Erlang module. The first line is intended to be the
interpreter line, which invokes
<c><![CDATA[escript]]></c>. However if you invoke the
<c><![CDATA[escript]]></c> like this</p>
<pre>
$ <input>escript factorial 5</input> </pre>
<p>the contents of the first line does not matter, but it
cannot contain Erlang code as it will be ignored.</p>
<p>The second line in the example, contains an optional
directive to the <c>Emacs</c> editor which causes it to
enter the major mode for editing Erlang source files. If the
directive is present it must be located on the second
line.</p>
<p>On the third line (or second line depending on the presence
of the Emacs directive), it is possible to give arguments to
the emulator, such as </p>
<pre>
%%! -smp enable -sname factorial -mnesia debug verbose</pre>
<p>Such an argument line must start with <c>%%!</c> and the
rest of the line will interpreted as arguments to the emulator.</p>
<p>If you know the location of the <c><![CDATA[escript]]></c> executable, the first
line can directly give the path to <c><![CDATA[escript]]></c>. For instance:</p>
<pre>
#!/usr/local/bin/escript </pre>
<p>As any other kind of scripts, Erlang scripts will not work on
Unix platforms if the execution bit for the script file is not set.
(Use <c><![CDATA[chmod +x script-name]]></c> to turn on the execution bit.)
</p>
<p>The rest of the Erlang script file may either contain
Erlang <c>source code</c>, an <c>inlined beam file</c> or an
<c>inlined archive file</c>.</p>
<p>An Erlang script file must always contain the function
<em>main/1</em>. When the script is run, the
<c><![CDATA[main/1]]></c> function will be called with a list
of strings representing the arguments given to the script (not
changed or interpreted in any way).</p>
<p>If the <c><![CDATA[main/1]]></c> function in the script returns successfully,
the exit status for the script will be 0. If an exception is generated
during execution, a short message will be printed and the script terminated
with exit status 127.</p>
<p>To return your own non-zero exit code, call <c><![CDATA[halt(ExitCode)]]></c>;
for instance:</p>
<pre>
halt(1).</pre>
<p>Call <c><![CDATA[escript:script_name/0]]></c> from your to
script to retrieve the pathname of the script (the pathname
is usually, but not always, absolute).</p>
<p>If the file contains source code (as in the example above),
it will be processed by the preprocessor <c>epp</c>. This
means that you for example may use pre-defined macros (such as
<c><![CDATA[?MODULE]]></c>) as well as include directives like
the <c><![CDATA[-include_lib]]></c> directive. For instance, use</p>
<pre>
-include_lib("kernel/include/file.hrl"). </pre>
<p>to include the record definitions for the records used by the
<c><![CDATA[file:read_link_info/1]]></c> function.</p>
<p>The script will be checked for syntactic and semantic
correctness before being run. If there are warnings (such as
unused variables), they will be printed and the script will
still be run. If there are errors, they will be printed and
the script will not be run and its exit status will be
127.</p>
<p>Both the module declaration and the export declaration of
the <c><![CDATA[main/1]]></c> function are optional.</p>
<p>By default, the script will be interpreted. You can force
it to be compiled by including the following line somewhere
in the script file:</p><pre>
-mode(compile).</pre>
<p>Execution of interpreted code is slower than compiled code.
If much of the execution takes place in interpreted code it
may be worthwhile to compile it, even though the compilation
itself will take a little while.</p>
<p>As mentioned earlier, it is possible to have a script which
contains precompiled <c>beam</c> code. In a precompiled
script, the interpretation of the script header is exactly
the same as in a script containing source code. That means
that you can make a <c>beam</c> file executable by
prepending the file with the lines starting with <c>#!</c>
and <c>%%!</c> mentioned above. In a precompiled script, the
function
<c>main/1</c> must be exported.</p>
<p>As yet another option it is possible to have an entire
Erlang archive in the script. In a archive script, the
interpretation of the script header is exactly the same as
in a script containing source code. That means that you can
make an archive file executable by prepending the file with
the lines starting with <c>#!</c> and <c>%%!</c> mentioned
above. In an archive script, the function <c>main/1</c> must
be exported. By default the <c>main/1</c> function in the
module with the same name as the basename of the
<c>escript</c> file will be invoked. This behavior can be
overridden by setting the flag <c>-escript main Module</c>
as one of the emulator flags. The <c>Module</c> must be the
name of a module which has an exported <c>main/1</c>
function. See <seealso marker="kernel:code">code(3)</seealso>
for more information about archives and code loading.</p>
<p>In many cases it is very convenient to have a header in
the escript, especially on Unix platforms. But the header is
in fact optional. This means that you directly can "execute"
an Erlang module, beam file or archive file without adding
any header to them. But then you have to invoke the script
like this:</p>
<pre>
$ <input>escript factorial.erl 5</input>
factorial 5 = 120
$ <input>escript factorial.beam 5</input>
factorial 5 = 120
$ <input>escript factorial.zip 5</input>
factorial 5 = 120
</pre>
</desc>
</func>
</funcs>
<section>
<title>Options accepted by escript</title>
<taglist>
<tag>-c</tag>
<item>Compile the escript regardless of the value of the mode attribute.
</item>
<tag>-d</tag>
<item>Debug the escript. Starts the debugger, loads the module
containing the <c>main/1</c> function into the debugger, sets a
breakpoint in <c>main/1</c> and invokes <c>main/1</c>. If the
module is precompiled, it must be explicitly compiled with the
<c>debug_info</c> option.
</item>
<tag>-i</tag>
<item>Interpret the escript regardless of the value of the mode attribute.
</item>
<tag>-s</tag>
<item>Only perform a syntactic and semantic check of the script file.
Warnings and errors (if any) are written to the standard output, but
the script will not be run. The exit status will be 0 if there were
no errors, and 127 otherwise.</item>
</taglist>
</section>
</comref>
