How to copy hidden (starting with a dot) files and subdirectories in linux?

As long as you're only looking for hidden files and folders at the level of A and don't want, for example

A/b/.hidden

to be copied, you should be able to use this:

cp -r A/.[^.]* B

It basically means copy anything that starts with a . and then any character other than a . That filters out . and ..

Edit: Removed the -p from the cp command since Asker hasn't indicated he wants to preserve any ownerships, dates, etc.

The problem with A/.* is that there is the directory . in A which also matches the pattern.

You can turn on extended glob patterns and use the following:

shopt -s extglob
cp -r A/.!(?(.)) B    

It matches files whose name starts with a dot and whose second character is neither a dot nor nothing ( ?(.) matches nothing or a dot, !(...) negates it, i.e. !(?(.)) matches everything else than nothing or a dot).

For cases like this would recommend using find instead of cp like this:

find A/ -type f -maxdepth 1 -name '.*' -exec cp -p {} B/ \;

The basic syntax breaks down like this:

• find A/ -type f: find items in the directory A/ whose type is a file (instead of a directory)…
• -maxdepth 1 -name '.*': To this for a maxdepth of 1 directories and whose name begins with ..
• -exec cp -p {} B/ \;: And once these files are found, exec the cp command with a -p flag to preserve dates/times from the source ({}) to the destination of B/.

I like using maxdepth to add a layer of control so I am not accidentally copying a whole filesystem. But feel free to remove that.