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Original author(s) Andrew Tridgell, Paul Mackerras
Developer(s) Wayne Davison
Initial release June 19, 1996; 20 years ago (1996-06-19)[1]
Stable release 3.1.2 (December 21, 2015; 12 months ago (2015-12-21)) [±][2]
Development status active
Written in C
Platform Unix-like, Windows
Type Data transfer, Differential backup
License GNU GPLv3

rsync is a utility that keeps copies of a file on two computer systems.[3] It is commonly found on Unix-like systems and functions as both a file synchronization and file transfer program. The rsync algorithm is a type of delta encoding, and is used to minimize network usage. Zlib may be used for additional compression,[4] and SSH or stunnel can be used for data security.

Rsync is typically used to synchronize files and directories between two different systems. For example, if the command rsync local-file user@remote-host:remote-file is run, rsync will use SSH to connect as user to remote-host.[5] Once connected, it will invoke the remote host's rsync and then the two programs will determine what parts of the file need to be transferred over the connection.

Rsync can also operate in a daemon mode, serving files in the native rsync protocol (using the "rsync://" syntax).

It is released under version 3 of the GNU General Public License.[3][6][7][8]


Andrew Tridgell and Paul Mackerras wrote the original rsync, which was first announced on 19 June 1996.[1] Tridgell discusses the design, implementation, and performance of rsync in chapters 3 through 5 of his Ph.D. thesis in 1999.[9] It is currently maintained by Wayne Davison.[10]

Because of the flexibility, speed, and scriptability of rsync, it has become a standard Linux utility, included in all popular Linux distributions. It has been ported to Windows (via Cygwin, Grsync, or SFU[11]), FreeBSD,[12] NetBSD,[13] OpenBSD,[14] and macOS.


Similar to rcp and scp, rsync requires the specification of a source and of a destination; either of them may be remote, but not both.[15]

Generic syntax:


where SRC is the file or directory (or a list of multiple files and directories) to copy from, DEST is the file or directory to copy to, and square brackets indicate optional parameters.

rsync can synchronize Unix clients to a central Unix server using rsync/ssh and standard Unix accounts. It can be used in desktop environments, for example to efficiently synchronize files with a backup copy on an external hard drive. A scheduling utility such as cron can carry out tasks such as automated encrypted rsync-based mirroring between multiple hosts and a central server.

By default, rsync uses the remote-shell program SSH for its communication. It can be configured to use a different remote-shell program, or to contact an rsync daemon directly via TCP, which per default then is via TCP port 873.


A command line to mirror FreeBSD might look like:

$ rsync -avz—delete /pub/FreeBSD/[16]

The Apache HTTP Server supports only rsync for updating mirrors.

$ rsync -avz—delete—safe-links /path/to/mirror[17]

The preferred (and simplest) way to mirror the PuTTY website to the current directory is to use rsync.

$ rsync -auH rsync:// .[18]

A way to mimic the capabilities of Time Machine (macOS) - see also tym.[19]

$ date=$(date "+%FT%H-%M-%S") # rsync interprets ":" as separator between host and port (i. e. host:port), so we cannot use %T or %H:%M:%S here, so we use %H-%M-%S
$ rsync -aP --link-dest=$HOME/Backups/current /path/to/important_files $HOME/Backups/back-$date
$ ln -nfs $HOME/Backups/back-$date $HOME/Backups/current

Make a full backup of system root directory:

 $ rsync -aczvAXHS --progress --exclude={"/dev/*","/proc/*","/sys/*","/tmp/*","/run/*","/mnt/*","/media/*","/lost+found"} /* /path/to/backup/folder



Determining which files to send[edit]

By default rsync determines which files differ between the sending and receiving systems by checking the modification time and size of each file. If time and/or size is different between the systems, it transfers the file from the sending to the receiving system. As this only requires reading file directory information, it is quick, but it will miss unusual modifications which change neither.

Rsync performs a slower but comprehensive check if invoked with --checksum. This forces a full checksum comparison on every file present on both systems. Barring rare checksum collisions, this avoids the risk of missing changed files at the cost of reading every file present on both systems.

Determining which parts of a file have changed[edit]

The rsync utility uses an algorithm invented by Australian computer programmer Andrew Tridgell for efficiently transmitting a structure (such as a file) across a communications link when the receiving computer already has a similar, but not identical, version of the same structure.

The recipient splits its copy of the file into chunks and computes two checksums for each chunk: the MD5 hash, and a weaker but easier to compute 'rolling checksum'.[21] It sends these checksums to the sender.

The sender quickly computes the rolling checksum for each chunk in its version of the file; if they differ, it must be sent. If they're the same, the sender uses the more computationally expensive MD5 hash to verify the chunks are the same.

The sender then sends the recipient those parts of its file that did not match, along with information on where to merge these blocks into the recipient's version. This makes the copies identical. There is a small probability that differences between chunks in the sender and recipient are not detected, and thus remain uncorrected. With 128 bits from MD5 plus 32 bits from the rolling checksum, the probability is on the order of 2−(128+32) = 2−160.

The rolling checksum used in rsync is based on Mark Adler's adler-32 checksum, which is used in zlib, and is itself based on Fletcher's checksum.

If the sender's and recipient's versions of the file have many sections in common, the utility needs to transfer relatively little data to synchronize the files. If typical data compression algorithms are used, files that are similar when uncompressed may be very different when compressed, and thus the entire file will need to be transferred. Some compression programs, such as gzip, provide a special "rsyncable" mode which allows these files to be efficiently rsynced, by ensuring that local changes in the uncompressed file yield only local changes in the compressed file.

Rsync supports other key features that aid significantly in data transfers or backup. They include compression and decompression of data block by block using zlib, and support for protocols such as ssh and stunnel.


The rdiff utility uses the rsync algorithm to generate delta files with the difference from file A to file B (like the utility diff, but in a different delta format). The delta file can then be applied to file A, turning it into file B (similar to the patch utility). rdiff works well with binary files.

rdiff-backup maintains a backup mirror of a file or directory either locally or remotely over the network, on another server. rdiff-backup stores incremental rdiff deltas with the backup, with which it is possible to recreate any backup point.[22]

The librsync library used by rdiff is an independent implementation of the rsync algorithm. It does not use the rsync network protocol and does not share any code with the rsync application.[23] It is used by Dropbox, rdiff-backup, duplicity, and other utilities.[23]

The acrosync library is an independent, cross-platform implementation of the rsync network protocol.[24] Unlike librsync, it is wire-compatible with rsync (protocol version 29 or 30). It is released under the Reciprocal Public License and used by the commercial rsync software Acrosync.[25]

Duplicity is a variation on rdiff-backup that allows for backups without cooperation from the storage server, as with simple storage services like Amazon S3. It works by generating the hashes for each block in advance, encrypting them, and storing them on the server. It then retrieves them when doing an incremental backup. The rest of the data is also stored encrypted for security purposes.

The BackupPC suite performs automatic scheduled backups and supports the rsync protocol.

As of macOS 10.5 and later, there is a special -E or --extended-attributes switch which allows retaining much of the HFS file metadata when syncing between two machines supporting this feature. This is achieved by transmitting the Resource Fork along with the Data Fork.[26]

zsync is a rsync-like tool optimized for many downloads per file version. zsync is used by Linux distributions such as Ubuntu[27] for distributing fast changing beta ISO image files. zsync uses the HTTP protocol and .zsync files with pre-calculated rolling hash to minimize server load yet permit diff transfer for network optimization.

rsync applications[edit]

Program Operating system Free software Description
Linux macOS Windows
Back In Time Yes No No Yes
BackupAssist No No Yes No Direct mirror or with history, VSS.
Cwrsync No No Yes No Based on Cygwin.
Grsync Yes Yes Yes[28] Yes Graphical Interface for rsync on Linux Systems
GS RichCopy 360 No No Yes [29] No Designed only for MS Windows workstations and servers with VSS support.
Jotasync Yes Yes Yes Yes
LuckyBackup Yes Yes Yes Yes
QtdSync Yes No Yes Yes
Unison Yes Yes Yes Yes Two-way file synchronizer using Rsync algorithm

See also[edit]


  1. ^ a b Tridgell, Andrew (19 June 1996). "First release of rsync - rcp replacement". Newsgroupcomp.os.linux.announce. Usenet: Retrieved 2007-07-19. 
  2. ^ "NEWS for rsync 3.1.2 (21 Dec 2015)". rsync. 2015-12-21. Retrieved 2015-12-24. 
  3. ^ a b Sayood, Khalid (2002-12-18). Lossless compression handbook. Retrieved 2014-08-18. 
  4. ^ "rsync(1) - Linux man page". Retrieved 2014-08-18. 
  5. ^ "Using Rsync and SSH". Retrieved 2014-08-18. 
  6. ^ Web content caching and distribution: proceedings of the 8th International Workshop. Retrieved 2014-08-18. 
  7. ^ In-Place Rsync: File Synchronization for Mobile and Wireless Devices, David Rasch and Randal Burns, Department of Computer Science, Johns Hopkins University
  8. ^ Dempsey, Bert J.; Weiss, Debra (April 30, 1999). "Towards an Efficient, Scalable Replication Mechanism for the I2-DSI Project". Technical Report TR-1999-01. CiteSeerX accessible. 
  9. ^ Andrew Tridgell: Efficient Algorithms for Sorting and Synchronization, February 1999. Retrieved 29 Sept. 2009.
  10. ^ "rsync". Retrieved 28 Nov 2014. 
  11. ^ "Tool Warehouse". SUA Community. Archived from the original on 6 April 2013. 
  12. ^ "FreeBSD Ports". Retrieved 24 Oct 2016. 
  13. ^ "NetBSD Ports". Retrieved 24 Oct 2016. 
  14. ^ "OpenBSD Ports". Retrieved 24 Oct 2016. 
  15. ^ See the README file
  16. ^ "How to Mirror FreeBSD (With rsync)". Retrieved 2014-08-18. 
  17. ^ "How to become a mirror for the Apache Software Foundation". Retrieved 2014-08-18. 
  18. ^ "PuTTY Web Site Mirrors: Mirroring guidelines". 2007-12-20. Retrieved 2014-08-18. 
  19. ^ "Rsync set up to run like Time Machine". Retrieved 2014-08-18. 
  20. ^ "Full system backup with rsync". Retrieved 2014-12-15. 
  21. ^ NEWS for rsync 3.0.0 (1 Mar 2008)
  22. ^ rdiff-backup
  23. ^ a b Martin Pool. "librsync".
  24. ^ Chen, Gilbert. "acrosync-library". 
  25. ^ "". 
  26. ^ "Mac Developer Library". Retrieved 2014-08-18. 
  27. ^ "Zsync Cd Image". Retrieved 2015-01-06. 
  28. ^ Grsync for Windows
  29. ^ GS RichCopy 360 Enterprise for Windows

External links[edit]