Rsync

Another great Digital Ocean tutorial. Thanks a lot @jellingwood!The only thing I have missed is the mysterious rsyncd or rsync --daemon which initially seemed to me something the keeps running all the time watching for live file changes and it would be great to link to a folder. Could you please help me on clarifying what is that rsyncd really does?

rsync is a utility for efficiently transferring and synchronizing files between a computer and a storage drive and across networked computers by comparing the modification times and sizes of files.[8] It is commonly found on Unix-like operating systems and is under the GPL-3.0-or-later license.[4][5][9][10][11][12]

Rsync

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rsync is written in C as a single threaded application.[13] The rsync algorithm is a type of delta encoding, and is used for minimizing network usage. Zstandard, LZ4, or Zlib may be used for additional data compression,[8] and SSH or stunnel can be used for security.

rsync is typically used for synchronizing 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.[14] Once connected, it will invoke the remote host's rsync and then the two programs will determine what parts of the local file need to be transferred so that the remote file matches the local one. One application of rsync is the synchronization of software repositories on mirror sites used by package management systems.[15][16]

Andrew Tridgell and Paul Mackerras wrote the original rsync, which was first announced on 19 June 1996.[1] It is similar in function and invocation to rdist (rdist -c), created by Ralph Campbell in 1983 and released as part of 4.3BSD.[17] Tridgell discusses the design, implementation, and performance of rsync in chapters 3 through 5 of his 1999 Ph.D. thesis.[18] As of 2023[ref], it is maintained by Wayne Davison.[2]

Because of the flexibility, speed, and scriptability of rsync, it has become a standard Linux utility, included in all popular Linux distributions.[citation needed] It has been ported to Windows (via Cygwin, Grsync, or SFU[19]), FreeBSD,[20] NetBSD,[21] OpenBSD,[22] and macOS.

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.

An rsync process operates by communicating with another rsync process, a sender and a receiver. At startup, an rsync client connects to a peer process. If the transfer is local (that is, between file systems mounted on the same host) the peer can be created with fork, after setting up suitable pipes for the connection. If a remote host is involved, rsync starts a process to handle the connection, typically Secure Shell. Upon connection, a command is issued to start an rsync process on the remote host, which uses the connection thus established. As an alternative, if the remote host runs an rsync daemon, rsync clients can connect by opening a socket on TCP port 873, possibly using a proxy.[30]

Rsync has numerous command line options and configuration files to specify alternative shells, options, commands, possibly with full path, and port numbers. Besides using remote shells, tunnelling can be used to have remote ports appear as local on the server where an rsync daemon runs. Those possibilities allow adjusting security levels to the state of the art, while a naive rsync daemon can be enough for a local network.

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 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.[8]

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.[31]

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.

The .mw-parser-output .vanchor>:target~.vanchor-text{background-color:#b1d2ff}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.

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.[35] It is used by Dropbox, rdiff-backup, duplicity, and other utilities.[35]

The acrosync library is an independent, cross-platform implementation of the rsync network protocol.[36] 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.[37]

The duplicity backup software written in python allows for incremental backups with simple storage backend services like local file system, sftp, Amazon S3 and many others. It utilizes librsync to generate delta data against signatures of the previous file versions, encrypting them using gpg, and storing them on the backend. For performance reasons a local archive-dir is used to cache backup chain signatures, but can be re-downloaded from the backend if needed.

zsync is an rsync-like tool optimized for many downloads per file version. zsync is used by Linux distributions such as Ubuntu[39] 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.[40]

Rclone is an open-source tool inspired by rsync that focuses on cloud and other high latency storage. It supports more than 50 different providers and provides an rsync-like interface for cloud storage.[41] However, Rclone does not support rolling checksums for partial file syncing (binary diffs) because cloud storage providers do not usually offer the feature and Rclone avoids storing additional metadata.[42]

Whether transferring files locally or remotely, rsync first creates a file-list containing information (by default, it is the file size and last modification timestamp) which will then be used to determine if a file needs to be constructed. For each file to be constructed, a weak and strong checksum is found for all blocks such that each block is of length S bytes, non-overlapping, and has an offset which is divisible by S. Using this information a large file can be constructed using rsync without having to transfer the entire file. For a more detailed practical and mathematical explanation refer to how rsync works and the rsync algorithm, respectively.

This behavior is different from that of GNU cp, which treats "source" and "source/" identically (but not "source/."). Also, some shells automatically append the trailing slash when tab-completing directory names. Because of these factors, there can be a tendency among new or occasional rsync users to forget about rsync's different behavior, and inadvertently create a mess or even overwrite important files by leaving the trailing slash on the command line.

The rsync protocol can easily be used for backups, only transferring files that have changed since the last backup. This section describes a very simple scheduled backup script using rsync, typically used for copying to removable media.

Instead of running time interval backups with time based schedules, such as those implemented in cron, it is possible to run a backup every time one of the files you are backing up changes. systemd.path units use inotify to monitor the filesystem, and can be used in conjunction with systemd.service files to start any process (in this case your rsync backup) based on a filesystem event.

There must be a symlink to a full backup already in existence as a target for --link-dest. If the most recent snapshot is deleted, the symlink will need to be recreated to point to the most recent snapshot. If --link-dest does not find a working symlink, rsync will proceed to copy all source files instead of only the changes.

This section is about using rsync to transfer a copy of the entire / tree, excluding a few selected directories. This approach is considered to be better than disk cloning with dd since it allows for a different size, partition table and filesystem to be used, and better than copying with cp -a as well, because it allows greater control over file permissions, attributes, Access Control Lists and extended attributes.

rsync will work even while the system is running, but files changed during the transfer may or may not be transferred, which can cause undefined behavior of some programs using the transferred files.

rsync provides a way to do a copy of all data in a file system while preserving as much information as possible, including the file system metadata. It is a procedure of data cloning on a file system level where source and destination file systems do not need to be of the same type. It can be used for backing up, file system migration or data recovery. 2351a5e196