config(5) Files 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 applica-
tion: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 syn-
tax:
[{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 in-
stallation directory and Vsn is the release version.
Release handling relies on this assumption. When installing a new re-
lease version, the new sys.config is read and used to update the appli-
cation 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 us-
ing -config ... with same result whatever the current working di-
rectory.
When traversing the contents of sys.config and a filename is encoun-
tered, 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 mes-
sage is returned and the erroneous file is ignored.
SEE ALSO
app(5), erts:erl(1), OTP Design Principles
Ericsson AB kernel 7.0 config(5)