Which command is primarily used for transferring and synchronizing files across directories in Linux?

The command that is primarily used for transferring and synchronizing files across directories in Linux is RSYNC. This utility is highly regarded for its efficiency and versatility in file transfer operations. RSYNC works by only transferring the differences between the source and the destination files, rather than copying all data each time, which saves bandwidth and time, especially when dealing with large files or directories.

Additionally, RSYNC offers a range of features that make it suitable for file synchronization tasks, such as support for remote transfers over SSH, the ability to preserve file permissions and timestamps, and options for compression, which enhances performance during transfers. This makes it an ideal choice for system administrators and users looking to keep files and directories in sync across different locations.

In contrast, while the other commands might be useful in specific scenarios, they do not serve the primary purpose of syncing files as effectively as RSYNC does. CPIO is mainly used for copying files to and from archives, DD is primarily used for low-level copying of files and disk images, and TAR is used for archiving multiple files into a single file, rather than for transferring or synchronizing files across directories.