config(5)

NAME

   config - Configuration file.

DESCRIPTION

   A  configuration  file contains values for configuration parameters for
   the applications in the system. The erl command-line  argument  -config
   Name  tells  the  system  to  use data in the system configuration file
   Name.config.

   Configuration parameter values in the configuration file  override  the
   values in the application resource files (see app(5). The values in the
   configuration  file  can  be  overridden  by  command-line  flags  (see
   erts:erl(1).

   The  value  of  a  configuration  parameter  is  retrieved  by  calling
   application:get_env/1,2.

FILE SYNTAX

   The configuration file is to be called Name.config, where Name  is  any
   name.

   File  .config  contains  a  single  Erlang  term  and has the following
   syntax:

   [{Application1, [{Par11, Val11}, ...]},
    ...
    {ApplicationN, [{ParN1, ValN1}, ...]}].

     Application = atom():
       Application name.

     Par = atom():
       Name of a configuration parameter.

     Val = term():
       Value of a configuration parameter.

SYS.CONFIG

   When starting Erlang in embedded mode, it is assumed that  exactly  one
   system configuration file is used, named sys.config. This file is to be
   located in $ROOT/releases/Vsn,  where  $ROOT  is  the  Erlang/OTP  root
   installation directory and Vsn is the release version.

   Release  handling  relies  on  this  assumption.  When installing a new
   release version, the new sys.config is read  and  used  to  update  the
   application configurations.

   This means that specifying another .config file, or more .config files,
   leads to inconsistent update of application configurations.  Therefore,
   in  Erlang 5.4/OTP R10B, the syntax of sys.config was extended to allow
   pointing out other .config files:

   [{Application, [{Par, Val}]} | File].

     File = string():
       Name of another .config file. Extension .config can be omitted.  It
       is  recommended  to use absolute paths. A relative path is relative
       the current working directory of the emulator.

   When  traversing  the  contents  of  sys.config  and  a   filename   is
   encountered,  its  contents are read and merged with the result so far.
   When an application configuration tuple {Application, Env} is found, it
   is merged with the result so far. Merging means that new parameters are
   added and existing parameter values overwritten.

   Example:

   sys.config:

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

   myconfig.config:

   [{myapp,[{par2,val3},{par3,val4}]}].

   This yields the following environment for myapp:

   [{par1,val1},{par2,val3},{par3,val4}]

   The behavior if a file specified in sys.config 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.

SEE ALSO

   app(5), erts:erl(1), OTP Design Principles

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