@FILEWRITE

@FILEWRITE[n,text]:  Writes a line to the file whose handle is n.  Returns 
the number of bytes written, or "-1" if an error occurred.  n must be a 
handle returned by @FILEOPEN; or 1 (for standard output) or 2 (for 
standard error). 

Be sure to read the cautionary note about file functions under Variable 
Functions.