GNU Astronomy Utilities manual

4.2 Configuration files

Each program needs a certain number of parameters to run. Supplying all the necessary parameters each time you run the program is very frustrating and prone to errors. Therefore all the programs read the values for the necessary options you have not given in the command line from one of several plain text files (which you can view and edit with any text editor). These files are known as configuration files and are usually kept in a directory named etc/ according to the file system hierarchy standard³⁶.

The thing to have in mind is that none of the programs in Gnuastro keep any internal default value. All the values must either be stored in one of the configuration files or explicitly called in the command line. In case the necessary parameters are not given through any of these methods, the program will list the missing necessary parameters and abort. The only exception to this is --numthreads, whose default value is set at $ ./configure time internally, see Threads in GNU Astronomy Utilities. Of course, you can still provide a default value for the number of threads at any of the levels below, but if you don’t, the program will not abort. Also note that through automatic output name genertion, the value to the --output option is also not mandatory on the command line or in the configuration files for all programs which don’t rely on that value as an input³⁷, see Automatic output.

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GNU Astronomy Utilities manual, November 2015.