rsync

Basics

Recursively synchronise the contents of directory source to directory target on the same system:

rsync -a source/ target

-a: stands for "archive" and its effect is to synchronise recursively and preserve symbolic links, special and device files, modification times, group, owner, and permissions. It is more commonly used than -r and is usually what we want to use.
The trailing slash (/) after source indicates that we want to synchronise the contents of source. Without the trailing slash, we would instead place the directory source itself inside target.

Other commonly used flags:

-n: equivalent to --dry-run
-v: verbose
-L: equivalent to --copy-links, which copies the actual content of symbolic links (see https://lists.samba.org/archive/rsync/2006-November/016724.html

We can also use rsync to synchronise content with a remote system. To synchronise content from a local to a remote directory (push):

rsync -a source/ username@remote_host:target_directory

To synchronise remote content to a local directory (pull):

rsync -a username@remote_host:/home/username/source local_directory

Some useful flags are the following:

-z: the source data is compressed before synchronisation (useful when transferring through a network)
--partial: keeps partially transferred files in the target directory even if thhe the connection gets interrupted. This is useful when synchronising big files.
--progress: shows the progress during transfer
-P: equivalent to --partial --progress

To exclude files with a pattern:

rsync -a --exclude '*.pyc' source/ target

To exclude a directory:

rsync -a --exclude 'edir' source/ target