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<chapter>
<header>
<copyright>
<year>1996</year><year>2018</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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See the License for the specific language governing permissions and
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</legalnotice>
<title>System Principles</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>system_principles.xml</file>
</header>
<marker id="system principles"></marker>
<section>
<title>Starting the System</title>
<p>An Erlang runtime system is started with command <c>erl</c>:</p>
<pre>
% <input>erl</input>
Erlang/OTP 17 [erts-6.0] [hipe] [smp:8:8]
Eshell V6.0 (abort with ^G)
1> </pre>
<p><c>erl</c> understands a number of command-line arguments, see
the <seealso marker="erts:erl">erl(1)</seealso> manual page in
ERTS. Some of them are also described in this chapter.</p>
<p>Application programs can access the values of the command-line
arguments by calling the function <c>init:get_argument(Key)</c>
or <c>init:get_arguments()</c>. See the
<seealso marker="erts:init">init(3)</seealso> manual page in
ERTS.</p>
</section>
<section>
<title>Restarting and Stopping the System</title>
<p>The runtime system is halted by calling <c>halt/0,1</c>. For
details, see the <seealso marker="erts:erlang">erlang(3)</seealso>
manual page in ERTS.</p>
<p>The module <c>init</c> contains functions for restarting,
rebooting, and stopping the runtime system:</p>
<pre>
init:restart()
init:reboot()
init:stop()</pre>
<p>For details, see the <seealso marker="erts:init">init(3)</seealso>
manual page in ERTS.</p>
<p>The runtime system terminates if the Erlang shell is
terminated.</p>
</section>
<section>
<marker id="BOOTSCRIPT"></marker>
<title>Boot Scripts</title>
<p>The runtime system is started using a <em>boot script</em>.
The boot script contains instructions on which code to load and
which processes and applications to start.</p>
<p>A boot script file has the extension <c>.script</c>.
The runtime system uses a binary version of the script. This
<em>binary boot script</em> file has the extension <c>.boot</c>.</p>
<p>Which boot script to use is specified by the command-line flag
<c>-boot</c>. The extension <c>.boot</c> is to be omitted.
For example, using the boot script <c>start_all.boot</c>:</p>
<pre>
% <input>erl -boot start_all</input></pre>
<p>If no boot script is specified, it defaults to
<c>ROOT/bin/start</c>, see <seealso marker="#default_boot_scripts">
Default Boot Scripts</seealso>.</p>
<p>The command-line flag <c>-init_debug</c> makes the <c>init</c>
process write some debug information while interpreting the boot
script:</p>
<pre>
% <input>erl -init_debug</input>
{progress,preloaded}
{progress,kernel_load_completed}
{progress,modules_loaded}
{start,heart}
{start,logger}
...</pre>
<p>For a detailed description of the syntax and contents of the
boot script, see the <c>script(4)</c> manual page in SASL.</p>
<section>
<marker id="default_boot_scripts"></marker>
<title>Default Boot Scripts</title>
<p>Erlang/OTP comes with these boot scripts:</p>
<list type="bulleted">
<item><c>start_clean.boot</c> - Loads the code for and starts
the applications Kernel and STDLIB.</item>
<item><c>start_sasl.boot</c> - Loads the code for and starts
the applications Kernel, STDLIB, and
SASL).</item>
<item><c>no_dot_erlang.boot</c> - Loads the code for and
starts the applications Kernel and STDLIB.
Skips loading the file <c>.erlang</c>. Useful for scripts and
other tools that are to behave the same irrespective of user
preferences.</item>
</list>
<p>Which of <c>start_clean</c> and <c>start_sasl</c> to use as
default is decided by the user when installing Erlang/OTP using
<c>Install</c>. The user is asked "Do you want to use a minimal
system startup instead of the SASL startup". If the answer is
yes, then <c>start_clean</c> is used, otherwise
<c>start_sasl</c> is used. A copy of the selected boot script is
made, named <c>start.boot</c> and placed in directory
<c>ROOT/bin</c>.</p>
</section>
<section>
<title>User-Defined Boot Scripts</title>
<p>It is sometimes useful or necessary to create a user-defined
boot script. This is true especially when running Erlang in
embedded mode, see <seealso marker="#code_loading">
Code Loading Strategy</seealso>.</p>
<p>A boot script can be written manually. However, it is
recommended to create a boot script by generating it from a
release resource file <c>Name.rel</c>, using the function
<c>systools:make_script/1,2</c>. This requires that the source
code is structured as applications according to the OTP design
principles. (The program does not have to be started in terms of
OTP applications, but can be plain Erlang).</p>
<p>For more information about <c>.rel</c> files, see
<seealso marker="doc/design_principles:release_handling">
OTP Design Principles</seealso> and the
<seealso marker="sasl:rel">rel(4)</seealso> manual page in
SASL.</p>
<p>The binary boot script file <c>Name.boot</c> is generated from
the boot script file <c>Name.script</c>, using the function
<c>systools:script2boot(File)</c>.</p>
</section>
</section>
<section>
<marker id="code_loading"></marker>
<title>Code Loading Strategy</title>
<p>The runtime system can be started in either <em>embedded</em> or
<em>interactive</em> mode. Which one is decided by the
command-line flag <c>-mode</c>.</p>
<pre>
% <input>erl -mode embedded</input></pre>
<p>Default mode is <c>interactive</c> and extra <c>-mode</c> flags are
ignored.</p>
<p>The mode properties are as follows:</p>
<list type="bulleted">
<item>In embedded mode, all code is loaded during system startup
according to the boot script. (Code can also be loaded later
by explicitly ordering the code server to do so.)</item>
<item>In interactive mode, the code is dynamically loaded when first
referenced. When a call to a function in a module is made, and
the module is not loaded, the code server searches the code path
and loads the module into the system.</item>
</list>
<p>Initially, the code path consists of the current
working directory and all object code directories under
<c>ROOT/lib</c>, where <c>ROOT</c> is the installation directory
of Erlang/OTP. Directories can be named <c>Name[-Vsn]</c>. The
code server, by default, chooses the directory with
the highest version number among those which have the same
<c>Name</c>. The <c>-Vsn</c> suffix is optional. If an
<c>ebin</c> directory exists under the <c>Name[-Vsn]</c>
directory, this directory is added to the code path.</p>
<p>The code path can be extended by using the command-line flags
<c>-pa Directories</c> and <c>-pz Directories</c>. These add
<c>Directories</c> to the head or the end of the code path,
respectively. Example:</p>
<pre>
% <input>erl -pa /home/arne/mycode</input></pre>
<p>The code server module <c>code</c> contains a number of
functions for modifying and checking the search path, see the
<c>code(3)</c> manual page in Kernel.</p>
</section>
<section>
<title>File Types</title>
<p>The following file types are defined in Erlang/OTP:</p>
<table>
<row>
<cell align="left" valign="middle"><em>File Type</em></cell>
<cell align="left" valign="middle"><em>File Name/Extension</em></cell>
<cell align="left" valign="middle"><em>Documented in</em></cell>
</row>
<row>
<cell align="left" valign="middle">Module</cell>
<cell align="left" valign="middle"><c>.erl</c></cell>
<cell align="left" valign="middle">
<seealso marker="doc/reference_manual:modules">
Erlang Reference Manual</seealso></cell>
</row>
<row>
<cell align="left" valign="middle">Include file</cell>
<cell align="left" valign="middle"><c>.hrl</c></cell>
<cell align="left" valign="middle">
<seealso marker="doc/reference_manual:modules">
Erlang Reference Manual</seealso></cell>
</row>
<row>
<cell align="left" valign="middle">Release resource file</cell>
<cell align="left" valign="middle"><c>.rel</c></cell>
<cell align="left" valign="middle">
<seealso marker="sasl:rel">rel(4)</seealso>
manual page in SASL</cell>
</row>
<row>
<cell align="left" valign="middle">Application resource file</cell>
<cell align="left" valign="middle"><c>.app</c></cell>
<cell align="left" valign="middle">
<seealso marker="kernel:app">app(4)</seealso>
manual page in Kernel</cell>
</row>
<row>
<cell align="left" valign="middle">Boot script</cell>
<cell align="left" valign="middle"><c>.script</c></cell>
<cell align="left" valign="middle">
<seealso marker="sasl:script">script(4)</seealso>
manual page in SASL</cell>
</row>
<row>
<cell align="left" valign="middle">Binary boot script</cell>
<cell align="left" valign="middle"><c>.boot</c></cell>
<cell align="left" valign="middle">-</cell>
</row>
<row>
<cell align="left" valign="middle">Configuration file</cell>
<cell align="left" valign="middle"><c>.config</c></cell>
<cell align="left" valign="middle">
<seealso marker="kernel:config">config(4)</seealso>
manual page in Kernel</cell>
</row>
<row>
<cell align="left" valign="middle">Application upgrade file</cell>
<cell align="left" valign="middle"><c>.appup</c></cell>
<cell align="left" valign="middle">
<seealso marker="sasl:appup">appup(4)</seealso>
manual page in SASL</cell>
</row>
<row>
<cell align="left" valign="middle">Release upgrade file</cell>
<cell align="left" valign="middle"><c>relup</c></cell>
<cell align="left" valign="middle">
<seealso marker="sasl:relup">relup(4)</seealso>
manual page in SASL</cell>
</row>
<tcaption>File Types</tcaption>
</table>
</section>
</chapter>
