config

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 will override the values in the application resource files (see app(4)). The values in the configuration file can be overridden by command line flags (see erl(1)).

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

The configuration file should be called Name.config where Name is an arbitrary name.

The .config file contains one single Erlang term. The file has the following syntax:

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

• Application = atom() is the name of the application.

• Par = atom() is the name of a configuration parameter.

• Val = term() is the 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 should 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, or additional, .config files would lead 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() is the name of another .config file. The extension .config may 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 will yield the following environment for myapp:

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

The behaviour if a file specified in sys.config 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.

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