The preinit.d folder contains script files that are concatenated into a single file which is then executed, as such, every file must have an empty line at the end or concatenation will fail and will result in failure to boot.
There are two types of files in this folder, both are included at different times
Naming convention[edit source]
b files[edit source]
These files should only contain function or variable definitions and should not contain code that executes.