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  <header>
    <copyright>
      <year>1997</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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    <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
      <seefile marker="app"><c>app(4)</c></seefile>).
      The values in the configuration file can be
      overridden by command-line flags (see
      <seecom marker="erts:erl"><c>erts:erl(1)</c></seecom>).</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. There is, however, a syntax for
      <c>sys.config</c> that allows 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. If a relative path is used,
        <c>File</c> is searched, first, relative from <c>sys.config</c> directory, then relative
        to the current working directory of the emulator, for backward compatibility.
        This allow to use a <c>sys.config</c> pointing out other <c>.config</c> files in a release
        or in a node started manually using <c>-config ...</c> with same result whatever
        the current working directory.
        </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><seefile marker="app"><c>app(4)</c></seefile>,
      <seecom marker="erts:erl"><c>erts:erl(1)</c></seecom>,
      <seeguide marker="system/design_principles:des_princ">OTP Design Principles</seeguide></p>
  </section>
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