The configuration files for each program have the standard program executable name with a ‘.conf’ suffix. When you download the source code, you can find them in the same directory as the source code of each program, see Program source.
Any line in the configuration file whose first non-white character is a # is considered to be a comment and is ignored. An empty line is also similarly ignored. The long name of the option should be used as an identifier. The parameter name and parameter value have to be separated by any number of ‘white-space’ characters: space, tab or vertical tab. By default several space characters are used. If the value of an option has space characters (most commonly for the hdu option), then the full value can be enclosed in double quotation signs (", similar to the example in Arguments and options). If it is an option without a value in the --help output (on/off option, see Options), then the value should be 1 if it is to be ‘on’ and 0 otherwise.
In each non-commented and non-blank line, any text after the first two words (option identifier and value) is ignored. If an option identifier is not recognized in the configuration file, the name of the file, the line number of the unrecognized option, and the unrecognized identifier name will be reported and the program will abort. If a parameter is repeated more more than once in the configuration files, accepts only one value, and is not set on the command-line, then only the first value will be used, the rest will be ignored.
You can build or edit any of the directories and the configuration files yourself using any text editor. However, it is recommended to use the --setdirconf and --setusrconf options to set default values for the current directory or this user, see Operating mode options. With these options, the values you give will be checked before writing in the configuration file. They will also print a set of commented lines guiding the reader and will also classify the options based on their context and write them in their logical order to be more understandable.