split a file into pieces but keep a common header of first n lines

split-with-header.sh

#!/bin/bash

export SPLIT_header_n=$1
target_size=$2
export SPLIT_input_file=$3

target_wo_header=$(( "$target_size" - "$SPLIT_header_n" ))

[[ -n $SPLIT_input_file ]] || {
  echo "usage: $(basename "$0") SPLIT_header_n target_n SPLIT_input_file" >&2
  exit 1
}

function my_header {
  head -n "$SPLIT_header_n" "$SPLIT_input_file"
  cat
}

export -f my_header

sed "1,${SPLIT_header_n}d" "$SPLIT_input_file" |
split \
  -d \
  -l "$target_wo_header" \
  --filter='my_header > $FILE' \
  - "$(basename "$SPLIT_input_file")."