How to check data with SHA1?

Question

In this answer's last paragraph, Michael K. says:

If you do go the multiple storage media route, too, remember to keep checking each for signs of degredation. It's fairly quick and easy to do a SHA1 hash run over all the files on a drive and compare the results, as well as storing the list of hashes itself in multiple locations. That way, even if you get read errors at some point, you can determine which copy is "good".

On Windows, can this be done on the command line without downloading a checksum utility program? How then should we store the list of hashes? Will there be any difference between NTFS and FAT32 systems?

Answer 1

SHA1 is simply a mathematical calculation on the contents of files, and SHOULD be unique all the time. As a file changes, the SHA1 hash changes (hash=result). What you would do (I don't know the specific programs):

1. Run the SHA1 program and get a result back
2. Copy the result to a file on a separate media (i.e. flash drive)
3. Run the copy command of your choice (i.e. robocopy)
4. Run the SHA1 program and get the result back from the files freshly copied
5. Open both result files and make sure they're the same

It should not be different for NTFS, FAT32, HFS+, EXT2/3/4, etc. It checks the file specifically IIRC, and therefore should not be different for the different OSes either

Answer 2

After some googling I found a program for Windows that does this whole process pretty elegantly: ExactFile. It does checksums on both files and folders (with subfolders).

1. After downloading and installing the program you first create a "digest" of the repository that you want to be able to check. You can choose MD5, SHA1, and 16 other methods/formats.

2. Under the Create Digest tab, you choose the folder, the output file name, and the format. You then click on the "Go" button and it will recursively compute the checksums of all the files in the folder, and store the results in the output file.

3. After moving the folder (for example from Dropbox to Google Drive, or from your hard disk to a flash drive) you can run a check: on the Test Digest tab, put the file (output from the creation step) and the folder to test and click on the "Go" button.

   The program then goes through and details which files are exactly the same and which files had any errors.

The only conceivable downside to this program is that you have to re-create a digest anytime the fileset has changed, but this will probably be the case regardless of the program / implementation.