cp ./file.txt ./file_copy.txt;
FILE_SUM=`md5sum ./file.txt | head -c 5`;
FILE_COPYSUM=`md5sum ./file_copy.txt | head -c 5`;
 if [[ "$FILE_SUM" == "$FILE_COPYSUM" ]]
  then
echo "Буфер есть. Пишем..." && echo  "New string" > file.txt && cat file_copy.txt >> file.txt && cat file.txt
  else
echo "Буфера нет...";
 fi