Is it possible to execute a command that only adds the sha256 of a file if it has been modified?

Question

Is it possible to execute a command that adds the sha256 of a file in the previously created list and only if it has been modified according to the last date registered in that list?

I do not know if they understand me. What I want to do is create a sha256 list of all the files along with the modification dates.

So, to keep the list updated, instead of going through all the files (a process that would take a lot of time and disk usage), I would need a command that compares the dates of changes registered in the list created with the current modification dates of each one of the files on the disk. In case of detecting a modification, then, the program generates the sha256 of the file and re-registers it in the list, overwriting the previous value (only of that file).

Is there a program that does this or some idea?

Update:

What I need is a program that in addition to creating the sha, register in turn the modification date belonging to each file.

E.g:

Contents of the file database_of_SHA256.txt:

0adca15c96d77a38aa0447fa87af9c297c *document1.txt  2018-12-03 04:12:23
dca15c96d77a9d30d2a7defad30d2a47fa *document2.txt  2018-09-09 10:19:11
77a915c9defad30d2c96d77aa0447fa87a *document3.txt  2017-01-20 17:34:04

The code what you should do is, I do not know if it is the most appropriate but you should create a list of dates and names of all the files on the computer and compare them with this data. If the dates are different, then you should generate and verify the checksum.

If the checksum does not match, then it means that the file has actually been modified and that list must be updated with the new information (it would be good to ask for a confirmation for this), if the user confirms that he wants to continue, then the list is recorded. new date and checksum of the file.

Answer 1

You can easily achieve that with a shell script:

• Compare the modification time of the files with that of the list (or the registered point in time)
• If a file is newer than the list, run sha256sum on it and replace in the list

Something along the lines of

HASHCHANGED=$(stat -c '%Y' "/path/to/listfile") # Or extract the timestamp from the list
for FILE in /path/to/files/* ; do
  FILECHANGED=$(stat -c '%Y' "$FILE")
  if test $FILECHANGED -ge $HASHCHANGED; then
    NEWHASH=$(sha256sum "$FILE")
    # put the hash into your list, this depends on the list format
  fi
done

EDIT

From the comments I understand, that there is no predefined list format, as I had assumed from the OQ. this makes it natural to use the standard sha256sum format. We can now expand the script from above to a full solution:

# Configuration
HASHFILE="/path/to/hashfile"
FILEMASK="/path/to/files/*.suffix"

HASHCHANGED=$(stat -c '%Y' "$HASHFILE")
while read FILE ; do
  test -f "$FILE" || continue
  FILECHANGED=$(stat -c '%Y' "$FILE")
  if test $FILECHANGED -ge $HASHCHANGED; then
    NEWHASH=$(sha256sum "$FILE")
    cat "$HASHFILE" | grep -ve "\\s$FILE\$" > "$HASHFILE.tmp"
    echo "$NEWHASH" >> "$HASHFILE.tmp"
    cat "$HASHFILE.tmp" | sort -u > "$HASHFILE"
  fi
done < <(eval "ls -1 $FILEMASK") #Need an eval here to allow * in mask

You should seed the initial list with sha256sum /path/to/files/*mask