mfoll

BEGIN {
  OFS = "\t"; # tab-delimited output
}
# Use substr instead of regex to match a starting ">"
substr($0, 1, 1) == ">" {
  if (seqlen) {
    # Only print info for this sequence if no target was given
    # or its id matches the target.
    if (! target || id == target) {
      print id, seqlen;