Having migrated to Linux from Windows, I would like to find an alternative software to Winmerge or rather learn command line tools to compare and sync two folders on Linux. I would be grateful if you could tell me how to do the following tasks on the command line... (I have studied diff and rsync, but I still need some help.)

We have two folders: "/home/user/A" and "/home/user/B"

Folder A is the place where regular files and folders are saved and folder B is a backup folder that serves as a complete mirror of folder A. (Nothing is directly saved or modified by the user in folder B.)

My questions are:

• How to list files that exist only in folder B? (E.g. the ones deleted from folder A since the last synchronization.)




• How to copy files that exist in only folder B back into folder A?




• How to list files that exist in both folders but have different timestamps or sizes? (The ones that have been modified in folder A since last synronization. I would like to avoid using checksums, because there are tens of thousands of files and it'd make the process too slow.)




• How to make an exact copy of folder A into folder B? I mean, copy everything from folder A into folder B that exists only in folder A and delete everything from folder B that exists only in folder B, but without touching the files that are the same in both folders.

This puts folder A into folder B:

rsync -avu --delete "/home/user/A" "/home/user/B"  

If you want the contents of folders A and B to be the same, put /home/user/A/ (with the slash) as the source. This takes not the folder A but all of it's content and puts it into folder B. Like this:

rsync -avu --delete "/home/user/A/" "/home/user/B"  

Explanation by SonicARG

-a Do the sync preserving all filesystem attributes

-v run verbosely

-u skip files that are newer on the receiver

--delete delete the files in target folder that do not exist in the source, /home/user/A: source folder, /home/user/B: target folder

You could unison tool developed by Benjamin Pierce at U Penn.

Let us assume you have two directories,

/home/user/Documents/dirA/ and /home/user/Documents/dirB/

To synchronize these two, you may use:

~$unison -ui text /home/user/Documents/dirA/ /home/user/Documents/dirB/

In output, unison will display each and every directory and file that is different in the two directories you have asked to sync. It will recommend to additively synchronize (replicate missing file in both locations) on the initial run, then create and maintain a synchronization tree on your machine, and on subsequent runs it will implement true synchronization (i.e., if you delete a file from .../dirA, it will get deleted from .../dirB as well. You can also compare each and every change and optionally choose to forward or reverse synchronize between the two directories.

Optionally, to launch graphical interface, simply remove the -ui text option from your command, although I find the cli simpler and faster to use.

More on this: Unison tutorial at Unison user documentation.

This is not completely the same as what you ask for, but you could considered using a version-control tool. Tools like Git do everything you ask for, and more, especially if you do not work in folder B directly it could be interesting to take a look at it. you can find some more information on git here

You can use it this way:

rsync -avu --delete /home/user/A/* /home/user/B/

This way you will copy the content of folder A into folder B, not the content of folder A itself.

The answer from TuxForLife is pretty good, but I strongly suggest you use -c when syncing locally. You can argue that it's not worth the time/network penalty to do it for remote syncs, but it is totally worth it for local files because the speed is so great.

-c, --checksum

       This forces the sender to checksum every regular file using a 128-bit  MD4

       checksum.   It  does this during the initial file-system scan as it builds

       the list of all available files. The receiver then checksums  its  version

       of  each  file  (if  it exists and it has the same size as its sender-side

       counterpart) in order to decide which files need to be updated: files with

       either  a  changed  size  or a changed checksum are selected for transfer.

       Since this whole-file checksumming of all files on both sides of the  con-

       nection  occurs  in  addition to the automatic checksum verifications that

       occur during a file's transfer, this option can be quite slow.



       Note that rsync always verifies that each transferred file  was  correctly

       reconstructed  on  the receiving side by checking its whole-file checksum,

       but that automatic after-the-transfer verification has nothing to do  with

       this  option's  before-the-transfer  "Does  this file need to be updated?"

       check.

This shows how having the same size and time stamps can fail you.

The setup

$ cd /tmp



$ mkdir -p {A,b}/1/2/{3,4}



$ echo "___________from A" | 

      tee A/1/2/x  | tee A/1/2/3/y  | tee A/1/2/4/z  | 

  tr A b | 

      tee b/1/2/x  | tee b/1/2/3/y  | tee b/1/2/4/z  | 

      tee b/1/2/x0 | tee b/1/2/3/y0 >     b/1/2/4/z0



$ find A b -type f | xargs -I% sh -c "echo %; cat %;"

A/1/2/3/y

___________from A

A/1/2/4/z

___________from A

A/1/2/x

___________from A

b/1/2/3/y

___________from b

b/1/2/3/y0

___________from b

b/1/2/4/z

___________from b

b/1/2/4/z0

___________from b

b/1/2/x

___________from b

b/1/2/x0

___________from b

The rsync that copies nothing because the files all have the same size and timestamp

$ rsync -avu A/ b

building file list ... done



sent 138 bytes  received 20 bytes  316.00 bytes/sec

total size is 57  speedup is 0.36



$ find A b -type f | xargs -I% sh -c "echo %; cat %;"

A/1/2/3/y

___________from A

A/1/2/4/z

___________from A

A/1/2/x

___________from A

b/1/2/3/y

___________from b

b/1/2/3/y0

___________from b

b/1/2/4/z

___________from b

b/1/2/4/z0

___________from b

b/1/2/x

___________from b

b/1/2/x0

___________from b    

The rsync that works correctly because it compares checksums

$ rsync -cavu A/ b

building file list ... done

1/2/x

1/2/3/y

1/2/4/z



sent 381 bytes  received 86 bytes  934.00 bytes/sec

total size is 57  speedup is 0.12



$ find A b -type f | xargs -I% sh -c "echo %; cat %;"

A/1/2/3/y

___________from A

A/1/2/4/z

___________from A

A/1/2/x

___________from A

b/1/2/3/y

___________from A

b/1/2/3/y0

___________from b

b/1/2/4/z

___________from A

b/1/2/4/z0

___________from b

b/1/2/x

___________from A

b/1/2/x0

___________from b