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<header>
<copyright>
<year>1997</year><year>2016</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
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Licensed under the Apache License, Version 2.0 (the "License");
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<title>config</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
</header>
<file>config</file>
<filesummary>Configuration file.</filesummary>
<description>
<p>A <em>configuration file</em> contains values for configuration
parameters for the applications in the system. The <c>erl</c>
command-line argument <c>-config Name</c> tells the system to use
data in the system configuration file <c>Name.config</c>.</p>
<p>Configuration parameter values in the configuration file
override the values in the application resource files (see
<seealso marker="app"><c>app(4)</c></seealso>.
The values in the configuration file can be
overridden by command-line flags (see
<seealso marker="erts:erl"><c>erts:erl(1)</c></seealso>.</p>
<p>The value of a configuration parameter is retrieved by calling
<c>application:get_env/1,2</c>.</p>
</description>
<section>
<title>File Syntax</title>
<p>The configuration file is to be called <c>Name.config</c>, where
<c>Name</c> is any name.</p>
<p>File <c>.config</c> contains a single Erlang term and
has the following syntax:</p>
<code type="none">
[{Application1, [{Par11, Val11}, ...]},
...
{ApplicationN, [{ParN1, ValN1}, ...]}].</code>
<taglist>
<tag><c>Application = atom()</c></tag>
<item><p>Application name.</p></item>
<tag><c>Par = atom()</c></tag>
<item><p>Name of a configuration parameter.</p></item>
<tag><c>Val = term()</c></tag>
<item><p>Value of a configuration parameter.</p></item>
</taglist>
</section>
<section>
<title>sys.config</title>
<p>When starting Erlang in embedded mode, it is assumed that
exactly one system configuration file is used, named
<c>sys.config</c>. This file is to be located in
<c>$ROOT/releases/Vsn</c>, where <c>$ROOT</c> is the Erlang/OTP
root installation directory and <c>Vsn</c> is the release version.</p>
<p>Release handling relies on this assumption. When installing a
new release version, the new <c>sys.config</c> is read and used
to update the application configurations.</p>
<p>This means that specifying another <c>.config</c> file, or more
<c>.config</c> files, leads to inconsistent update of application
configurations. Therefore, in Erlang 5.4/OTP R10B, the syntax of
<c>sys.config</c> was extended to allow pointing out other
<c>.config</c> files:</p>
<code type="none">
[{Application, [{Par, Val}]} | File].</code>
<taglist>
<tag><c>File = string()</c></tag>
<item>Name of another <c>.config</c> file.
Extension <c>.config</c> can be omitted. It is
recommended to use absolute paths. A relative path is
relative the current working directory of the emulator.</item>
</taglist>
<p>When traversing the contents of <c>sys.config</c> and a filename
is encountered, its contents are read and merged with the result
so far. When an application configuration tuple
<c>{Application, Env}</c> is found, it is merged with the result
so far. Merging means that new parameters are added and existing
parameter values overwritten.</p>
<p><em>Example:</em></p>
<code type="none">
sys.config:
[{myapp,[{par1,val1},{par2,val2}]},
"/home/user/myconfig"].
myconfig.config:
[{myapp,[{par2,val3},{par3,val4}]}].</code>
<p>This yields the following environment for <c>myapp</c>:</p>
<code type="none">
[{par1,val1},{par2,val3},{par3,val4}]</code>
<p>The behavior if a file specified in <c>sys.config</c> does not
exist, or is erroneous, is backwards compatible.
Starting the runtime system will fail. Installing a new release
version will not fail, but an error message is returned and
the erroneous file is ignored.</p>
</section>
<section>
<title>See Also</title>
<p><seealso marker="app"><c>app(4)</c></seealso>,
<seealso marker="erts:erl"><c>erts:erl(1)</c></seealso>,
<seealso marker="doc/design_principles:des_princ">OTP Design Principles</seealso></p>
</section>
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