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Common Test
User's Guide
Version 1.11

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8 External Configuration Data

8.1  General

To avoid hard coding data values related to the test and/or SUT (System Under Test) in the test suites, the data may instead be specified by means of configuration files or strings that Common Test reads before the start of a test run. External configuration data makes it possible to change test properties without having to modify the actual test suites using the data. Examples of configuration data:

  • Addresses to the test plant or other instruments
  • User login information
  • Names of files needed by the test
  • Names of programs that should be executed during the test
  • Any other variable needed by the test

8.2  Syntax

A configuration file can contain any number of elements of the type:



      CfgVarName = atom()
      Value = term() | [{CfgVarName,Value}]

8.3  Requiring and reading configuration data

In a test suite, one must require that a configuration variable (CfgVarName in the definition above) exists before attempting to read the associated value in a test case or config function.

require is an assert statement that can be part of the test suite info function or test case info function. If the required variable is not available, the test is skipped (unless a default value has been specified, see the test case info function chapter for details). There is also a function ct:require/1/2 which can be called from a test case in order to check if a specific variable is available. The return value from this function must be checked explicitly and appropriate action be taken depending on the result (e.g. to skip the test case if the variable in question doesn't exist).

A require statement in the test suite info- or test case info-list should look like this: {require,CfgVarName} or {require,AliasName,CfgVarName}. The arguments AliasName and CfgVarName are the same as the arguments to ct:require/1/2 which are described in the reference manual for ct. AliasName becomes an alias for the configuration variable, and can be used as reference to the configuration data value. The configuration variable may be associated with an arbitrary number of alias names, but each name must be unique within the same test suite. There are two main uses for alias names:

  • They may be introduced to identify connections (see below).
  • They may used to help adapt configuration data to a test suite (or test case) and improve readability.

To read the value of a config variable, use the function get_config/1/2/3 which is also described in the reference manual for ct.


      suite() -> 
          [{require, domain, 'CONN_SPEC_DNS_SUFFIX'}].

      testcase(Config) ->
          Domain = ct:get_config(domain),

8.4  Using configuration variables defined in multiple files

If a configuration variable is defined in multiple files and you want to access all possible values, you may use the ct:get_config/3 function and specify all in the options list. The values will then be returned in a list and the order of the elements corresponds to the order that the config files were specified at startup. Please see the ct reference manual for details.

8.5  Encrypted configuration files

It is possible to encrypt configuration files containing sensitive data if these files must be stored in open and shared directories.

Call ct:encrypt_config_file/2/3 to have Common Test encrypt a specified file using the DES3 function in the OTP crypto application. The encrypted file can then be used as a regular configuration file, in combination with other encrypted files or normal text files. The key for decrypting the configuration file must be provided when running the test, however. This can be done by means of the decrypt_key or decrypt_file flag/option, or a key file in a predefined location.

Common Test also provides decryption functions, ct:decrypt_config_file/2/3, for recreating the original text files.

Please see the ct reference manual for more information.

8.6  Opening connections by using configuration data

There are two different methods for opening a connection by means of the support functions in e.g. ct_ssh, ct_ftp, and ct_telnet:

  • Using a configuration target name (an alias) as reference.
  • Using the configuration variable as reference.

When a target name is used for referencing the configuration data (that specifies the connection to be opened), the same name may be used as connection identity in all subsequent calls related to the connection (also for closing it). It's only possible to have one open connection per target name. If attempting to open a new connection using a name already associated with an open connection, Common Test will return the already existing handle so that the previously opened connection will be used. This is a practical feature since it makes it possible to call the function for opening a particular connection whenever useful. An action like this will not necessarily open any new connections unless it's required (which could be the case if e.g. the previous connection has been closed unexpectedly by the server). Another benefit of using named connections is that it's not necessary to pass handle references around in the suite for these connections.

When a configuration variable name is used as reference to the data specifying the connection, the handle returned as a result of opening the connection must be used in all subsequent calls (also for closing the connection). Repeated calls to the open function with the same variable name as reference will result in multiple connections being opened. This can be useful e.g. if a test case needs to open multiple connections to the same server on the target node (using the same configuration data for each connection).

8.7  User specific configuration data formats

It is possible for the user to specify configuration data on a different format than key-value tuples in a text file, as described so far. The data can e.g. be read from arbitrary files, fetched from the web over http, or requested from a user specific process. To support this, Common Test provides a callback module plugin mechanism to handle configuration data.

Default callback modules for handling configuration data

The Common Test application includes default callback modules for handling configuration data specified in standard config files (see above) and in xml files:

  • ct_config_plain - for reading configuration files with key-value tuples (standard format). This handler will be used to parse configuration files if no user callback is specified.
  • ct_config_xml - for reading configuration data from XML files.

Using XML configuration files

This is an example of an XML configuration file:


This configuration file, once read, will produce the same configuration variables as the following text file:

{ftp_host, [{ftp,"targethost"},

{lm_directory, "/test/loadmodules"}.

How to implement a user specific handler

The user specific handler can be written to handle special configuration file formats. The parameter can be either file name(s) or configuration string(s) (the empty list is valid).

The callback module implementing the handler is responsible for checking correctness of configuration strings.

To perform validation of the configuration strings, the callback module should have the following function exported:


The input argument will be passed from Common Test, as defined in the test specification or given as an option to ct_run or ct:run_test.

The return value should be any of the following values indicating if given configuration parameter is valid:

  • {ok, {file, FileName}} - parameter is a file name and the file exists,
  • {ok, {config, ConfigString}} - parameter is a config string and it is correct,
  • {error, {nofile, FileName}} - there is no file with the given name in the current directory,
  • {error, {wrong_config, ConfigString}} - the configuration string is wrong.

To perform reading of configuration data - initially before the tests start, or as a result of data being reloaded during test execution - the following function should be exported from the callback module:


The input argument is the same as for the check_parameter/1 function.

The return value should be either:

  • {ok, Config} - if the configuration variables are read successfully,
  • {error, {Error, ErrorDetails}} - if the callback module fails to proceed with the given configuration parameters.

Config is the proper Erlang key-value list, with possible key-value sublists as values, like for the configuration file example above:

        [{ftp_host, [{ftp, "targethost"}, {username, "tester"}, {password, "letmein"}]},
         {lm_directory, "/test/loadmodules"}]

8.8  Examples of configuration data handling

A config file for using the FTP client to access files on a remote host could look like this:

    {ftp_host, [{ftp,"targethost"},

    {lm_directory, "/test/loadmodules"}.

The XML version shown in the chapter above can also be used, but it should be explicitly specified that the ct_config_xml callback module is to be used by Common Test.

Example of how to assert that the configuration data is available and use it for an FTP session:

    init_per_testcase(ftptest, Config) ->
        {ok,_} = ct_ftp:open(ftp),

    end_per_testcase(ftptest, _Config) ->

    ftptest() ->

    ftptest(Config) ->
        Remote = filename:join(ct:get_config(lm_directory), "loadmodX"),
        Local = filename:join(?config(priv_dir,Config), "loadmodule"),
        ok = ct_ftp:recv(ftp, Remote, Local),

An example of how the above functions could be rewritten if necessary to open multiple connections to the FTP server:

    init_per_testcase(ftptest, Config) ->
        {ok,Handle1} = ct_ftp:open(ftp_host),
        {ok,Handle2} = ct_ftp:open(ftp_host),
        [{ftp_handles,[Handle1,Handle2]} | Config].

    end_per_testcase(ftptest, Config) ->
        lists:foreach(fun(Handle) -> ct_ftp:close(Handle) end, 

    ftptest() ->

    ftptest(Config) ->
        Remote = filename:join(ct:get_config(lm_directory), "loadmodX"),
        Local = filename:join(?config(priv_dir,Config), "loadmodule"),
        [Handle | MoreHandles] = ?config(ftp_handles,Config),
        ok = ct_ftp:recv(Handle, Remote, Local),

8.9  Example of user specific configuration handler

A simple configuration handling driver which will ask an external server for configuration data can be implemented this way:

-export([read_config/1, check_parameter/1]).

    ServerModule = list_to_atom(ServerName),

    ServerModule = list_to_atom(ServerName),
    case code:is_loaded(ServerModule) of
        {file, _}->
            {ok, {config, ServerName}};
            case code:load_file(ServerModule) of
                {module, ServerModule}->
                    {ok, {config, ServerName}};
                {error, nofile}->
                    {error, {wrong_config, "File not found: " ++ ServerName ++ ".beam"}}

The configuration string for this driver may be "config_server", if the config_server.erl module below is compiled and exists in the code path during test execution:

-export([start/0, stop/0, init/1, get_config/0, loop/0]).

-define(REGISTERED_NAME, ct_test_config_server).

    case whereis(?REGISTERED_NAME) of
            spawn(?MODULE, init, [?REGISTERED_NAME]),

    register(Name, self()),

    call(self(), get_config).

    call(self(), stop).

call(Client, Request)->
    case whereis(?REGISTERED_NAME) of
            {error, {not_started, Request}};
            Pid ! {Client, Request},
                    {ok, Reply}
            after 4000->
                {error, {timeout, Request}}

        {Pid, stop}->
            Pid ! ok;
        {Pid, get_config}->
            {D,T} = erlang:localtime(),
            Pid !
                [{localtime, [{date, D}, {time, T}]},
                 {node, erlang:node()},
                 {now, erlang:now()},
                 {config_server_pid, self()},
                 {config_server_vsn, ?vsn}],

    case whereis(?REGISTERED_NAME) of

In this example, the handler also provides the ability to dynamically reload configuration variables. If ct:reload_config(localtime) is called from the test case function, all variables loaded with config_driver:read_config/1 will be updated with their latest values, and the new value for variable localtime will be returned.