How to diff two folders from a Windows command prompt

In most cases where I need to compare two folders recursively on a Windows system I use my go-to tool Beyond Compare. It is an excellent utility, and one that I think should be among the first utilities any developer should install on a new machine.

However, today I was doing a reconciliation as part of a very large file migration project that required comparing two folders that each contained hundreds of millions of files spread across thousands of sub-folders. BC was having a lot of trouble and choked on many of my comparisons. It just wasn’t the tool for today’s job. I needed another solution.

Necessity mothered some invention and I found an inventive way to use a combination of command switches on  RoboCopy to perform the comparison. If you are not familiar with RoboCopy, and you do a lot of mass copying of files, you need to stop what you are doing and learn about it pronto. It is a supercharged version of XCopy that has been included with Windows since Vista. It has a ton of great features such as multi-threaded file copying, selectively copying changed files, and resumable copies that make it a must especially for big file copy jobs over flaky network connections.

Diff Command Using RoboCopy

So here’s the command to perform a basic comparison of two folders and write a log file listing the differences.

ROBOCOPY “\\FileShare\SourceFolder” “\\FileShare\ComparisonFolder” /e /l /ns /njs /njh /ndl /fp /log:reconcile.txt

Explanation of the command switches used above:

/e  Recurse through sub-directories (including empty ones)
/l  Don’t modify or copy files, log differences only
/fp  Include the full path of files in log (only necessary if you omit /ndl)
/ns  Don’t include file sizes in log
/ndl  Don’t include folders in log
/njs   Don’t include Job Summary
/njh   Don’t include Job Header
/log:reconcile.txt   Write log to reconcile.txt (Recreate if exists)
/log+: reconcile.txt   (Optional variant) Write log to reconcile.txt (Append if exists)

Usage Notes and Warnings Regarding the /NDL Option

The /NDL option is a handy way to suppress the inclusion of every folder checked (regardless of whether it contains differences) in the log, but there because of the way it works it is not a good idea in all circumstances. Consider the following before you use /NDL.

  • Folders that exist only on source or destination are not logged unless at least one mismatched file is present or a source file is missing on destination.
  • Folders that exist only on the destination are not logged at all regardless of contents.

If you omit the /NDL option, it is necessary to include the /FP option if you want full paths listed for each file.

Example Output

(with /NDL option)

*EXTRA File         c:\dest\log.txt
New File              c:\source\newfolder\Blah.txt
Newer                 c:\source\Files\CONCORD.DAT
New File              c:\source\Files\COWCO.DAT

(without NDL Option)

 c:\work\test\source\    (extraneous folder listing)
*EXTRA Dir      c:\dest\newfolderdest\
*EXTRA Dir      c:\dest\newfolderrestempty\
*EXTRA File     c:\dest\log.txt
New Dir           c:\source\newfolder\
New File          c:\source\newfolder\Blah.txt
New Dir           c:\source\newfolderempty\
c:\source\Files\   (extraneous folder listing)
Newer             c:\source\Files\CONCORD.DAT
New File          c:\source\Files\COWCO.DAT
c:\test\source\FilesSame\   (Included despite no diffs)

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