path: root/lib/kernel/doc/src/config.xml

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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 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>
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