prepend.sh

#!/usr/bin/env bash

for file in $(ls *.csv)
do
  prefix=$(echo ${file} | cut -d'_' -f1)
  echo $prefix
  tmpfile=$(tempfile)

  while read line
    do echo "${prefix},${line}" >> ${tmpfile}
  done < ${file}

  mv ${tmpfile} ${file}
  rm ${tmpfile}
done