How Should a Project Document Configuration Precedence?
From the earlier reference collection. Read the current software blog.
State how command-line options, environment variables, and files interact when the same setting appears more than once.
Write down the actual order
When a setting can come from several places, readers need a defined precedence order. A common design gives command-line options priority over environment variables, and environment variables priority over a configuration file, but a project must document its actual rule rather than assume the convention. Say whether built-in defaults sit below all user-provided values and whether a missing value differs from an explicitly empty one.
A compact statement might be: command line overrides environment, which overrides the selected file, which overrides the default. Include that sentence in both the overview and the detailed configuration reference if users encounter settings in multiple interfaces. Be clear about exceptions, such as a command-line flag that appends to a list instead of replacing it.
Show a conflict example
Give a concrete example with the same option set in two places. Suppose a file sets timeout to 20 seconds and the environment contains APP_TIMEOUT=45; show which value is used. Then add a command-line --timeout 10 and explain whether the final value is 10. Mark sample values as illustrative and ensure names correspond to the product's real interface.
Also explain what happens when a value is malformed. Does the application reject startup, ignore that source and fall back, or emit a warning? A clear example might say that an invalid command-line value exits with an error instead of silently using a lower-priority file value. This behavior affects deployment scripts and should not be left to guesswork.
Clarify loading and merging
Precedence is only part of the story. Document whether the program reads one configuration file or several, how it chooses a file path, and whether included files merge or replace earlier content. If a list is merged while a scalar is replaced, describe both behaviors. Explain when values are read: at startup, on each request, or after a reload signal. Users need this to know whether editing a file takes effect immediately.
For example, a service might load /etc/example.conf at startup and require a restart after changes. If it also supports an environment variable for the file path, show how that selection happens before the file is parsed. Avoid promising live reload unless it has been implemented and documented.
Make configuration inspectable
Where possible, provide a way to inspect effective settings without exposing secrets. A diagnostic command might print resolved non-sensitive values and identify their source, such as “timeout: 10 (command line).” If no such feature exists, a documentation precedence table can still help readers reason about the result. Warn if values are redacted or if defaults differ by platform.
End with one checkable outcome: readers should be able to predict the value the program will use and know when a restart or correction is needed. Keep the rule near every setting entry and update it when parsing behavior changes.