Shell Programming and Scripting

cd /DATA/CSV_FILES                      # change dir
for i in *.csv                          # loop for all files with .csv extension
  do if [ -f "$i" ]                     # if regular file
       then sed "s/^/$i,/" "$i" > temp  # prefix every line with respective file name
            sed 's/.*/&,/' temp >temp2  # suffix every line with comma
     fi
  cat temp2                             # list the file to stdout
  done > output.txt                     # redirect all above (= every .csv prefixed) to file 
rm -f temp*                             # remove tmp files

cd /DATA/CSV_FILES

cd /DATA/CSV_FILES # Get in correct directory
rm -f output.txt   # Clear output file
for i in *.csv; do       # For each input .csv file
  if [ -f "$i" ]; then   # If the file is a "regular" file (test not really needed)
    sed "s/^/$i,/" "$i" > temp          # Add file name as first column for all lines and write to output.txt
    sed 's/.*/&,/' temp >> output.txt    # Do you really want another comma at the end of the line?
  fi
done
rm -f temp

sed  "s/^/$i,/ ; s/.*/&,/" /DATA/CSV_FILES/*.csv > /DATA/CSV_FILES/output.txt