How to compare two files. Solved

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  1.    16 Jan 2016 #1

    How to compare two files.


    What is the best way to do a byte by bye comparison of two large files - Windows 10 64 bit.

    There is an FC command but when run in the Power Shell I think it wants to do a 'Format Custom'!
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  2. Posts : 134
    Windows 10 Pro X64 Version 1803 Build 17134.165
       16 Jan 2016 #2

    Try Winmerge.

    WinMerge
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  3.    16 Jan 2016 #3

    Thanks, I have just tried that. Unfortunately it has a 2GB limit. The files I want to compare are Windows image back up files and are more like 25GB.
      My ComputerSystem Spec

  4.    16 Jan 2016 #4

    Use a tool that calculates a check sum like MD5 or SHA. I use a tool called Hash Tab which conveniently integrates itself into the file properties dialogue. It supports various check sums. If the check sums match the files must be the same, otherwise you can be sure that they are not the same.
      My ComputerSystem Spec

  5.    17 Jan 2016 #5

    Thank you
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  6.    21 Oct 2018 #6

    altae said: View Post
    Use a tool that calculates a check sum like MD5 or SHA. I use a tool called Hash Tab which conveniently integrates itself into the file properties dialogue. It supports various check sums. If the check sums match the files must be the same, otherwise you can be sure that they are not the same.
    This is true.

    However, this is COMPLETELY wrong. A cursory analysis proves that it is impossible for a check sum to be unique when applied to files larger than the check sum. So long as there are more possible files than there are possible check sums (which is true), many different files must have the same check sum.
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  7. Bree's Avatar
    Posts : 9,910
    10 Home x64 (1809) (10 Pro on 2nd pc)
       21 Oct 2018 #7

    machare said: View Post
    There is an FC command but when run in the Power Shell I think it wants to do a 'Format Custom'!
    Then just run it in a Command Prompt. You can get one in PowerShell by typing the command CMD

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  8. jimbo45's Avatar
    Posts : 6,352
    Windows / Linux : Centos, Ubuntu, OpenSuse
       21 Oct 2018 #8

    Hi there

    if you have a NAS or Linux server -- quite easy

    use rsync or the GUI version -- grsync.

    Not sure about Windows but I know a lot here have things like NAS etc.

    I suppose another way is to use something like Macrium / other backup software in differential / incremental mode which captures the changes.

    Cheers
    jimbo
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  9.    21 Oct 2018 #9

    technovelist said: View Post
    A cursory analysis proves that it is impossible for a check sum to be unique when applied to files larger than the check sum. So long as there are more possible files than there are possible check sums (which is true), many different files must have the same check sum.
    But, there are about 100 million times as many SHA-256 hashes than there are atoms in the milky way.

    You are right - you could theoretically get 2 different files with the same hash but it's rather unlikely. According to Why havent any SHA-256 collisions been found yet? - Cryptography Stack Exchange you could reasonably expect to wait in the order of 39 times the age of the universe
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  10.    22 Oct 2018 #10

    Bree said: View Post
    Then just run it in a Command Prompt. You can get one in PowerShell by typing the command CMD

    Click image for larger version. 

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    The fc command runs just fine in PowerShell.
    The only thing that needs to be done, is to run it as fc.exe and not plain fc.
    e.g. fc.exe file1 file2
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