<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE fileref SYSTEM "fileref.dtd"> <fileref> <header> <copyright> <year>1997</year><year>2013</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>