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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 Name.config.

Configuration parameter values in the configuration file 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 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. There is, however, a syntax for sys.config that allows 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. If a relative path is used, File is searched, first, relative from sys.config directory, then relative to the current working directory of the emulator, for backward compatibility. This allow to use a sys.config pointing out other .config files in a release or in a node started manually using -config ... with same result whatever the current working directory.

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