Comparing each file in two directories and copying it to another if it differs from its counterpart

Question

How can I compare all files in two directories and copy the ones that are different to another directory? For example, say we have dir1 and dir2:

dir1:
     build.gradle
     gradle.properties
     somejar.jar
     javacode.java
     anotherjar.jar

dir2:
     build.gradle      <-- different from build.gradle in dir1
     gradle.properties
     somejar.jar
     javacode.java     <-- different from javacode.java in dir1
     yetanotherjar.jar

How may I create a new directory dir3 that contains the different files from dir2, the common files in dir1 and dir2 and all uncommon files in both dir1 and dir2? dir3 should contain:

dir3:
     build.gradle      <-- from dir2
     gradle.properties <-- these are common files both in dir1 and dir2
     somejar.jar       <--
     javacode.java     <-- from dir2
     anotherjar.jar    <-- from dir1
     yetanotherjar.jar <-- from dir2

Answer 1

All you need is

cp -n dir2/* dir1/* dir3/

Answer 2

Do I understand you correctly in that dir3 should contain all files from the two base directories, and if a file appears in both directories, the one from dir2 should be taken? Then a simple

cp dir1/* dir3/
cp dir2/* dir3/

should do the trick: Files that differ in both source directories will be overwritten with the version from dir2. If you do not want to copy identical files twice, you might also rsync them instead of cping.

Answer 3

One way to do it:

cp -a /path/to/dir2/* /path/to/dir3
cd /path/to/dir1
comm -23 <(ls -1 | sort) <(cd /path/to/dir2; ls -1 | sort) | cpio -pam /path/to/dir3

This assumes you only have files (i.e. no subdirectories) in dir1 and dir2, and your filenames don't contain newlines. It also assumes your shell is smart enough to handle <(...) constructs.

Answer 4

cp -pu dir2/* dir1/* dir3/

from man:

-p    same as --preserve=mode,ownership,timestamps
-u    copy only when the SOURCE file is  newer  than  the  destination
      file or when the destination file is missing

Note:

1. Always put the updated directory first. (From example above: cp -pu dir1/* dir2/* dir3/ will not copy the updated files)
2. dir3 must exist.