Puts each sequence on one (its own) line, after the header line.
awk '/^>/ {printf("\n%s\n",$0);next; } { printf("%s",$0);} END {printf("\n");}' < INPUT.fa > OUTPUT_oneline.fa
WARNING - if the header lines are longer than the number of characters to wrap, these will be wrapped too, which you do not want.
fold -w 50 INPUT.fa > OUTPUT_wrapped.fa