How to force 'cp' to overwrite directory instead of creating another one inside?

You can do this using -T option in cp.
See Man page for cp.

-T, --no-target-directory    treat DEST as a normal file

So as per your example, following is the file structure.

$ tree testtest|-- bar|   |-- a|   `-- b`-- foo    |-- a    `-- b2 directories, 4 files

You can see the clear difference when you use -v for Verbose.
When you use just -R option.

$ cp -Rv foo/ bar/`foo/' -> `bar/foo'`foo/b' -> `bar/foo/b'`foo/a' -> `bar/foo/a' $ tree |-- bar |   |-- a |   |-- b |   `-- foo |       |-- a |       `-- b `-- foo     |-- a     `-- b3 directories, 6 files

When you use the option -T it overwrites the contents, treating the destination like a normal file and not directory.

$ cp -TRv foo/ bar/`foo/b' -> `bar/b'`foo/a' -> `bar/a'$ tree|-- bar|   |-- a|   `-- b`-- foo    |-- a    `-- b2 directories, 4 files

This should solve your problem.

If you want to ensure bar/ ends up identical to foo/, use rsync instead:

rsync -a --delete foo/ bar/

If just a few things have changed, this will execute much faster than removing and re-copying the whole directory.

• -a is 'archive mode', which copies faithfully files in foo/ to bar/
• --delete removes extra files not in foo/ from bar/ as well, ensuring bar/ ends up identical
• If you want to see what it's doing, add -vh for verbose and human-readable
• Note: the slash after foo is required, otherwise rsync will copy foo/ to bar/foo/ rather than overwriting bar/ itself.
  • (Slashes after directories in rsync are confusing; if you're interested, here's the scoop. They tell rsync to refer to the contents of the directory, rather than the directory itself. So to overwrite from the contents of foo/ onto the contents of bar/, we use a slash on both. It's confusing because it won't work as expected with a slash on neither, though; rsync sneakily always interprets the destination path as though it has a slash, even though it honors an absence of a slash on the source path. So we need a slash on the source path to make it match the auto-added slash on the destination path, if we want to copy the contents of foo/ into bar/, rather than the directory foo/ itself landing into bar/ as bar/foo.)

rsync is very powerful and useful, if you're curious look around for what else it can do (such as copying over ssh).

Do it in two steps.

rm -r bar/cp -r foo/ bar/