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  <header>
    <copyright>
      <year>1997</year><year>2021</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      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 <seecom marker="erts:erl#config"><c>-config
    Name</c></seecom> tells the system to use data in the system configuration
    file <c>Name.config</c>.</p>
    <p>The erl command-line argument <seecom
    marker="erts:erl#configfd"><c>-configfd</c></seecom> works the
    same way as the <c>-config</c> option but specifies a file
    descriptor to read configuration data from instead of a file.</p>
    <p>The configuration data from configuration files and file
    descriptors are read in the same order as they are given on the
    command line. For example, <c>erl -config a -configfd 3 -config b
    -configfd 4</c> would cause the system to read configuration data
    in the following order <c>a.config</c>, file descriptor <c>3</c>,
    <c>b.config</c>, and file descriptor <c>4</c>. If a configuration
    parameter is specified more than once in the given files and file
    descriptors, the last one overrides the previous ones.</p>
    <p>Configuration parameter values in a configuration file or file
    descriptor override the values in the application resource files
    (see <seefile marker="app"><c>app(4)</c></seefile>). The values in
    the configuration file are always 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's configurations.</p>
    <p>This means that specifying another <c>.config</c> file, or more
      <c>.config</c> files, leads to an inconsistent update of application
      configurations. There is, however, a way to point out other config
      files from a <c>sys.config</c>. How to do this is described in
      the next section.</p>
  </section>

  <section>
    <title>Including Files from sys.config and -configfd Configurations</title>

    <p>There is a way to include other configuration files from a
    <c>sys.config</c> file and from a configuration that comes
    from a file descriptor that has been pointed out with the <seecom
    marker="erts:erl#configfd"><c>-configfd</c></seecom> command-line
    arguemnt.</p>

    <p>The syntax for including files can be described by the
    <seeguide marker="system/reference_manual:typespec">Erlang type
    language</seeguide> like this:</p>
    <code type="none">
[{Application, [{Par, Val}]} | IncludeFile].</code>
    <taglist>
      <tag><c>IncludeFile = string()</c></tag>
      <item>Name of a <c>.config</c> file.  The extension
      <c>.config</c> can be omitted. It is recommended to use absolute
      paths. If a relative path is used in a <c>sys.config</c>,
      <c>IncludeFile</c> is searched, first, relative to the
      <c>sys.config</c> directory, then relative to the current
      working directory of the emulator.  If a relative path is used
      in a <c>-configfd</c> configuration, <c>IncludeFile</c> is
      searched, first, relative to the dictionary containing the
      <seefile marker="sasl:script">boot script</seefile> (see also
      the <seecom marker="erts:erl#boot"><c>-boot</c></seecom>
      command-line argument) for the emulator, then relative to the
      current working directory of the emulator.  This makes it
      possible to use <c>sys.config</c> for pointing out other
      <c>.config</c> files in a release or in a node started manually
      using <c>-config</c> or <c>-configfd</c> with the same result
      whatever the current working directory is.
        </item>
    </taglist>
    <p>When traversing the contents of a <c>sys.config</c> or a
    <c>-configfd</c> configuration 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
    are overwritten.</p>
    <p><em>Example:</em></p>
    <code type="none">
sys.config:

["/home/user/myconfig1"
 {myapp,[{par1,val1},{par2,val2}]},
 "/home/user/myconfig2"].

myconfig1.config:

[{myapp,[{par0,val0},{par1,val0},{par2,val0}]}].

myconfig2.config:

[{myapp,[{par2,val3},{par3,val4}]}].</code>
    <p>This yields the following environment for <c>myapp</c>:</p>
    <code type="none">
[{par0,val0},{par1,val1},{par2,val3},{par3,val4}]</code>
    <p>The run-time system will abort before staring up if an include file
    specified in <c>sys.config</c> or a <c>-configfd</c> configuration
    does not exist, or is erroneous. However, installing a new release
    version will not fail if there is an error while loading an
    include file, 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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