fprintf — Formatted output to a file
The fprintf function is intended for writing formatted output
to a file. It is similar to the C language function of the same name,
though some features are limited or not available, since doesn't have all the data types of C.
The first argument is a string which specifies which file the
formatted string is to be written to. If this argument starts with
|, then the rest of the argument is taken as a process which
should be invoked, and the formatted output string is sent to that
process.
The second string argument is the format string. It consists of the
text to be written out, and possibly some conversion specifications.
A conversion specification is a sequence of commands that determine
how the remainder of the arguments are to be displayed. The left most
conversion specifier is matched to the third argument, the next
specifier to the fourth argument, and so on. Each argument has to be
matched to a conversion specification. When the format string is
being scanned, the argument that matches the the specifier that the
scanner is up to is called the current argument.
Conversion specifications always begin with a +