Rsync, standing for Remote SYNChorize, is an open source tool which provide fast incremental file transfer. As same as scp command, rsync can simply transfer data via ssh/rsh. More over, rsync itself can also be a daemon if needing for lower CPU overhead and faster transfer rate.

Two different way for use rsync to contact a remote system

1. Using a remote-shell program as the transport (ssh/rsh):

Remote-shell transport format would be familiar to you. A single colon (:) separate a host specification and source/destination path.

To sync files from remote to local is just like cp/scp:

$ rsync user@hostname:path/to/file /path/to/syncfile

Or backup to remote

$ rsync /path/to/file user@hostname:path/to/syncfile

If you use non-default port for your remote-shell program, or want to add some different feature while transfer via ssh/rsh, just add specified remote-shell command after -e option:

$ rsync -e 'ssh -p 10022' user@hostname:path/to/syncfile

2. Contacting an rsync daemon directly via TCP

A rsync daemon is mostly used when your want to constantly maintain a synchronization. It can save some CPU usage and speedup transfer rate a bit.

To use rsync daemon for sync, a double colon (::) is required to separate the host specification and the module, or using rsync:// schema. Using double colon almost the same as single colon. The only difference is the module parameter. We will cover it later.

The default rsync daemon TCP port typically using 873.

$ rsync user@hostname::module /path/to/syncfile

However, rsync daemon would not encrypt data while transfering by default, you should manually encrypt by ssh command as your needs. (Encryption may lower some speed, but is necessary when connecting to internet.)

$ rsync -e 'ssh' user@hostname::path/to/file /path/to/syncfile

The rsync schema is another way to do the same thing:

# Example for rsync:// schema
# rsync [OPTION]... rsync://[USER@]HOST[:PORT]/MODULE[/SRC] [DEST]
rsync rsync:///module /path/to/your/copyfile

ATTENTAION!! : rsync has a trick that if the source path without an trailling slash, it would create a directory with the same name inside the destination directory. Otherwise, it would only sync files inside source directory.

# This will sync files with the directory.
rsync /directory-no-slash /path/to/store/a/new/directory

# This will sync files under the directory.
rsync /directory-no-slash/ /path/to/store/some/new/files

Useful rsync options

Here are some common options for rsync

  • -a : archive mode, the same as using -rlptgoD.
    • -r : rescursive
    • -l : copies symlinks as symlinks
    • -p : preserves permissions
    • -t : preserves modification times
    • -g : preserves group
    • -o : preserves owner
    • -D : preserves device files
  • -A : preserves ACLs (A more specific access rights table. See wiki.)
  • -H : preserves hard links
  • -S : handle sparse files efficiently
  • -X : preserves extended attributes
  • -v : increase verbosity (can add up to tree -vvv for the most verbosity)
  • -x : don't cross file-system boundaries (A filesystem boundary is a mount point. See more!)
  • -z : compress data during the transfer
  • --delete : differential clean-up during sync, like a mirror backup
  • --numeric-ids : do not map uid/gid values by user/group name
  • --progress : show the transfer progress during
  • --password-file=**FILE** : read rsync-daemon-access password from this FILE

Following rsync combined with options is the most usefule combination I've used.

rsync -aHAXzxv --numeric-ids --delete --progress -e "ssh -T -o Compression=no -x" [USER@]HOST:[SOURCE] [DEST]

Rsync daemon

As mentioned above, using rsync daemon have a bit advantage when sync constantly. To setup a rsync daemon is as simple as setting up a ssh server. For convenience, We will explian how to setup a daemon with a Network-attached Storage in the next chapter.

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