Rsync
rsync copies files and directories. It is often used for backups, or duplicating directory contents on a remote server. rsync can copy only changed files, and can use SSH to securely copy the file.
Common Options
-a - Combo flag, same as -rlptgoD (recursive; copies symlinks; preserve: permissions, modification times, group, and owner; device/special)
dir vs dir/:
dir- Copy the directory
dir/- Copy the contents of the directory
Sync Directories on the same computer
rsync -a dir1/ dir2 - Copies the contents of dir1 into dir2