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  <header>
    <copyright>
      <year>1997</year><year>2009</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>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 will
      override the values in the application resource files (see
      <c>app(4)</c>). The values in the configuration file can be
      overridden by command line flags (see <c>erl(1)</c>).</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 should be called <c>Name.config</c> where
      <c>Name</c> is an arbitrary name.</p>
    <p>The <c>.config</c> file contains one single Erlang term.
      The file has the following syntax:</p>
    <code type="none">
[{Application1, [{Par11, Val11}, ..]},
 ..
 {ApplicationN, [{ParN1, ValN1}, ..]}].</code>
    <list type="bulleted">
      <item>
        <p><c>Application = atom()</c> is the name of the application.</p>
      </item>
      <item>
        <p><c>Par = atom()</c> is the name of a configuration parameter.</p>
      </item>
      <item>
        <p><c>Val = term()</c> is the value of a configuration
          parameter.</p>
      </item>
    </list>
  </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 should 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, or additional, <c>.config</c>
      files would lead 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>
    <list type="bulleted">
      <item>
        <p><c>File = string()</c> is the name of another <c>.config</c>
          file. The extension <c>.config</c> may be omitted. It is
          recommended to use absolute paths. A relative path is
          relative the current working directory of the emulator.</p>
      </item>
    </list>
    <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. Example:</p>
    <code type="none">
sys.config:

[{myapp,[{par1,val1},{par2,val2}]},
 "/home/user/myconfig"].


myconfig.config:

[{myapp,[{par2,val3},{par3,val4}]}].</code>
    <p>This will yield the following environment for <c>myapp</c>:</p>
    <code type="none">
[{par1,val1},{par2,val3},{par3,val4}]</code>
    <p>The behaviour if a file specified in <c>sys.config</c> does not
      exist or is erroneous in some other way, is backwards compatible.
      Starting the runtime system will fail. Installing a new release
      version will not fail, but an error message is given and
      the erroneous file is ignored.</p>
  </section>

  <section>
    <title>SEE ALSO</title>
    <p><c>app(4)</c>, <c>erl(1)</c>, <em>OTP Design Principles</em></p>
  </section>
</fileref>

