Go to main content

man pages section 5: File Formats

Exit Print View

Updated: Wednesday, July 27, 2022

config (5erl)


config - Configuration file.


Please see following description for synopsis


config(5)                            Files                           config(5)

       config - Configuration file.

       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

       The erl command-line argument -configfd works the same way as the -con-
       fig  option  but specifies a file descriptor to read configuration data
       from instead of a file.

       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, erl -config a -configfd 3 -config b -configfd  4  would  cause
       the  system to read configuration data in the following order a.config,
       file descriptor 3, b.config, and file descriptor 4. If a  configuration
       parameter  is  specified  more  than  once  in the given files and file
       descriptors, the last one overrides the previous ones.

       Configuration parameter values in a configuration file or file descrip-
       tor override the values in the application resource files (see app(4)).
       The values in the configuration file are always overridden by  command-
       line flags (see erts:erl(1)).

       The value of a configuration parameter is retrieved by calling applica-

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

       File  .config  contains a single Erlang term and has the following syn-

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

         Application = atom():
           Application name.

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

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

       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's configurations.

       This means that specifying another .config file, or more .config files,
       leads to an inconsistent update of  application  configurations.  There
       is,  however,  a way to point out other config files from a sys.config.
       How to do this is described in the next section.

       There is a way to include other configuration files from  a  sys.config
       file  and  from  a configuration that comes from a file descriptor that
       has been pointed out with the -configfd command-line arguemnt.

       The syntax for including files can be described by the Erlang type lan-
       guage like this:

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

         IncludeFile = string():
           Name of a .config file. The extension .config can be omitted. It is
           recommended to use absolute paths. If a relative path is used in  a
           sys.config,   IncludeFile  is  searched,  first,  relative  to  the
           sys.config directory, then relative to the current  working  direc-
           tory  of  the  emulator.  If a relative path is used in a -configfd
           configuration, IncludeFile is searched, first, relative to the dic-
           tionary containing the boot script (see also the -boot command-line
           argument) for the emulator, then relative to  the  current  working
           directory of the emulator. This makes it possible to use sys.config
           for pointing out other .config files in a  release  or  in  a  node
           started  manually  using  -config or -configfd with the same result
           whatever the current working directory is.

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








       This yields the following environment for myapp:


       The  run-time  system  will  abort before staring up if an include file
       specified in sys.config or a -configfd 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  mes-
       sage is returned and the erroneous file is ignored.

       app(4), erts:erl(1), OTP Design Principles

Ericsson AB                       kernel 8.2                         config(5)