config - Configuration file.
Please see following description for synopsis
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.
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-
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
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.
INCLUDING FILES FROM SYS.CONFIG AND -CONFIGFD CONFIGURATIONS
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
overwritten.
Example:
sys.config:
["/home/user/myconfig1"
{myapp,[{par1,val1},{par2,val2}]},
"/home/user/myconfig2"].
myconfig1.config:
[{myapp,[{par0,val0},{par1,val0},{par2,val0}]}].
myconfig2.config:
[{myapp,[{par2,val3},{par3,val4}]}].
This yields the following environment for myapp:
[{par0,val0},{par1,val1},{par2,val3},{par3,val4}]
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.
SEE ALSO
app(4), erts:erl(1), OTP Design Principles
Ericsson AB kernel 8.2 config(5)