-printf and -fprintf support the following format
directives to print information about the file being processed. The C
printf function, field width and precision specifiers are
supported, as applied to string (%s) types. That is, you can specify
"minimum field width"."maximum field width" for each directive.
Format flags (like # for example) may not work as you expect
because many of the fields, even numeric ones, are printed with %s.
The format flag - does work; it forces left-alignment of the
field.
%% is a literal percent sign. A % character followed by an unrecognised character (i.e. not a known directive or printf field width and precision specifier), is discarded (but the unrecognised character is printed), and a warning message is printed to the standard error output (because it was probably a typo).