copy and replace to folders to another fastest way - linux bash

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http://serverfault.com – I have a three folders: core, project and merge with many files inside. I want to delete all files and folders (including dot files) inside the merge folder, and then copy all files (including dot files) from the core folder to the merge folder, as well as merge all files in the project folder into the merge files. Situations How can i remove all files and folders (including dot files) from special folders, without removing them? (rm -rf /path/to/folder/* did not delete files and folders.) What is fastest way to copy (with and without merge and replace) all files recursively from special fo (HowTos)