How can I intelligently sync directories on Linux?

Question

I have two directories on my Linux system that I want to sync. Most of the contents of these directories are the same, however some files differ:

Directory A contains:

File 1
File 2
File 4
File 5

Directory B contains:

File 1
File 2
File 3
File 5

After synchronization, I want both directory A and directory B to contain:

File 1
File 2
File 3
File 4
File 5

This is an example, the real directories contain thousands of files and their size is several GBs.

How can I do this on Linux?

Update: What if File 1, 2, 4, 5 are not in directory A but in a subdirectory AA that is in directory A? The files in directory B remain the same. After syncing, I don't want to end up with multiple copies of File 1, 2, 5.

Answer 1

What you want is rsync:

$ cd -- "$(mktemp --directory)
$ mkdir A B
$ touch A/File\ {1,2,4,5}
$ touch B/File\ {1,2,3,5}
$ rsync -a A/ B
$ rsync -a B/ A
$ ls A B
A:
'File 1'  'File 2'  'File 3'  'File 4'  'File 5'

B:
'File 1'  'File 2'  'File 3'  'File 4'  'File 5'

rsync will transport only the files necessary to sync up the directories. Unlike other *nix tools the slash after the first directory name is significant - it indicates that the content of the source directory rather than the directory itself should be copied.

Answer 2

Assuming both directories A and B are in the same level

$ rsync -a A/ B/ & rsync -a B/ A/