SRC="/data/files_to_transfer"
TGT="/data/final_files"
OWN=myappuser                                                                                      # Define file owner
GRP=myappgroup                                                                                     # Define file group
PRM=440                                                                                            # Define permissions
SUMS="${TGT}/sha1sums"

exec 50> "${SUMS}.new"                                                                             # Define the output file for checksums

cd "${SRC}"                                                                                        # Change to the source directory
for FN in *.txt
  do
     CKS=$(sha1sum "${FN}")                                                                        # Generate a checksum
     echo "${CKS}"  >&55                                                                           # Write the sha1sum record to intermediate file descriptor 55
     > "${TGT}/${FN%.*}.pdy"                                                                       # Create en empty file
     echo "${FN}", $(date +%Y/%m/%d), $(wc -l < ${FN}), ${CKS}, "sha1sum" > "${TGT}/${FN%.*}.ldy"  # Still creating this as before if you need it
     cp "${FN}" "${TGT}/${FN}"
     chown "${OWN}:${GRP}" "${TGT}/${FN}"                                                          # Set desired ownership
     chmod "${PRM}" "${TGT}/${FN}"                                                                 # Set desired permissions
  done  55>&50                                                                                     # Catch all the sha1sum output into the new file in one IO operation

grep -vf - "${SUMS}" < <(cut -f3 -d" "  "${SUMS}.new") > "${SUMS}.tmp"                             # Ignore previous checksums of the same filename
sort "${SUMS}.tmp" "${SUMS}.new" > "${TGT}/sha1sums"                                               # Put the older & new file checksums together neatly

sha1sum -c /data/final_files/sha1sums