<s>
rsync	O
is	O
a	O
utility	O
for	O
efficiently	O
transferring	B-Protocol
and	O
synchronizing	B-Application
files	B-Operating_System
between	O
a	O
computer	O
and	O
a	O
storage	O
drive	O
and	O
across	O
networked	B-Architecture
computers	O
by	O
comparing	O
the	O
modification	O
times	O
and	O
sizes	O
of	O
files	B-Operating_System
.	O
</s>
<s>
It	O
is	O
commonly	O
found	O
on	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
and	O
is	O
under	O
the	O
GPL-3.0-or-later	B-License
license	O
.	O
</s>
<s>
Rsync	O
is	O
written	O
in	O
C	B-Language
as	O
a	O
single	B-Operating_System
threaded	I-Operating_System
application	O
.	O
</s>
<s>
The	O
rsync	O
algorithm	O
is	O
a	O
type	O
of	O
delta	B-Algorithm
encoding	I-Algorithm
,	O
and	O
is	O
used	O
for	O
minimizing	O
network	O
usage	O
.	O
</s>
<s>
Zlib	B-Language
may	O
be	O
used	O
for	O
additional	O
data	B-General_Concept
compression	I-General_Concept
,	O
and	O
SSH	B-Protocol
or	O
stunnel	B-Protocol
can	O
be	O
used	O
for	O
security	O
.	O
</s>
<s>
Rsync	O
is	O
typically	O
used	O
for	O
synchronizing	B-Application
files	B-Operating_System
and	O
directories	O
between	O
two	O
different	O
systems	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
command	O
rsync	O
local-file	O
user	O
@remote	O
-host:remote-file	O
is	O
run	O
,	O
rsync	O
will	O
use	O
SSH	B-Protocol
to	O
connect	O
as	O
user	O
to	O
remote-host	O
.	O
</s>
<s>
One	O
application	O
of	O
rsync	O
is	O
the	O
synchronization	O
of	O
software	B-Application
repositories	I-Application
on	O
mirror	O
sites	O
used	O
by	O
package	B-Application
management	I-Application
systems	I-Application
.	O
</s>
<s>
Rsync	O
can	O
also	O
operate	O
in	O
a	O
daemon	B-Operating_System
mode	O
(	O
rsyncd	O
)	O
,	O
serving	O
and	O
receiving	O
files	B-Operating_System
in	O
the	O
native	O
rsync	O
protocol	O
(	O
using	O
the	O
"	O
rsync://	O
"	O
syntax	O
)	O
.	O
</s>
<s>
It	O
is	O
similar	O
in	O
function	O
and	O
invocation	O
to	O
rdist	O
(	O
rdist	O
-c	O
)	O
,	O
created	O
by	O
Ralph	O
Campbell	O
in	O
1983	O
and	O
released	O
under	O
the	O
Berkeley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
.	O
</s>
<s>
It	O
has	O
been	O
ported	O
to	O
Windows	O
(	O
via	O
Cygwin	B-Language
,	O
Grsync	B-Language
,	O
or	O
SFU	O
)	O
,	O
FreeBSD	B-Operating_System
,	O
NetBSD	B-Device
,	O
OpenBSD	B-Operating_System
,	O
and	O
macOS	B-Application
.	O
</s>
<s>
Similar	O
to	O
cp	B-Application
,	O
rcp	B-Application
and	O
scp	O
,	O
rsync	O
requires	O
the	O
specification	O
of	O
a	O
source	O
and	O
of	O
a	O
destination	O
,	O
of	O
which	O
at	O
least	O
one	O
must	O
be	O
local	O
.	O
</s>
<s>
where	O
SRC	O
is	O
the	O
file	O
or	O
directory	O
(	O
or	O
a	O
list	O
of	O
multiple	O
files	B-Operating_System
and	O
directories	O
)	O
to	O
copy	O
from	O
,	O
DEST	O
is	O
the	O
file	O
or	O
directory	O
to	O
copy	O
to	O
,	O
and	O
square	O
brackets	O
indicate	O
optional	O
parameters	O
.	O
</s>
<s>
rsync	O
can	O
synchronize	O
Unix	O
clients	O
to	O
a	O
central	O
Unix	O
server	O
using	O
rsync/ssh	O
and	O
standard	O
Unix	O
accounts	O
.	O
</s>
<s>
It	O
can	O
be	O
used	O
in	O
desktop	O
environments	O
,	O
for	O
example	O
to	O
efficiently	O
synchronize	O
files	B-Operating_System
with	O
a	O
backup	B-Protocol
copy	I-Protocol
on	O
an	O
external	O
hard	O
drive	O
.	O
</s>
<s>
A	O
scheduling	O
utility	O
such	O
as	O
cron	B-General_Concept
can	O
carry	O
out	O
tasks	O
such	O
as	O
automated	O
encrypted	O
rsync-based	O
mirroring	O
between	O
multiple	O
hosts	O
and	O
a	O
central	O
server	O
.	O
</s>
<s>
A	O
command	O
line	O
to	O
mirror	O
FreeBSD	B-Operating_System
might	O
look	O
like	O
:	O
</s>
<s>
The	O
Apache	B-Application
HTTP	I-Application
Server	I-Application
supports	O
rsync	O
only	O
for	O
updating	O
mirrors	O
.	O
</s>
<s>
The	O
preferred	O
(	O
and	O
simplest	O
)	O
way	O
to	O
mirror	O
a	O
PuTTY	B-Operating_System
website	O
to	O
the	O
current	O
directory	O
is	O
to	O
use	O
rsync	O
.	O
</s>
<s>
$	O
rsync	O
-auH	O
rsync://rsync.chiark.greenend.org.uk/ftp/users/sgtatham/putty-website-mirror/	O
.	O
</s>
<s>
A	O
way	O
to	O
mimic	O
the	O
capabilities	O
of	O
Time	B-Operating_System
Machine	I-Operating_System
(	O
macOS	B-Application
)	O
;	O
</s>
<s>
Make	O
a	O
full	B-Protocol
backup	I-Protocol
of	O
system	O
root	O
directory	O
:	O
</s>
<s>
Delete	O
all	O
files	B-Operating_System
and	O
directories	O
,	O
within	O
a	O
directory	O
,	O
extremely	O
fast	O
:	O
</s>
<s>
If	O
a	O
remote	O
host	O
is	O
involved	O
,	O
rsync	O
starts	O
a	O
process	O
to	O
handle	O
the	O
connection	O
,	O
typically	O
Secure	B-Protocol
Shell	I-Protocol
.	O
</s>
<s>
As	O
an	O
alternative	O
,	O
if	O
the	O
remote	O
host	O
runs	O
an	O
rsync	O
daemon	B-Operating_System
,	O
rsync	O
clients	O
can	O
connect	O
by	O
opening	O
a	O
socket	O
on	O
TCP	O
port	O
873	O
,	O
possibly	O
using	O
a	O
proxy	O
.	O
</s>
<s>
Rsync	O
has	O
numerous	O
command	O
line	O
options	O
and	O
configuration	O
files	B-Operating_System
to	O
specify	O
alternative	O
shells	O
,	O
options	O
,	O
commands	O
,	O
possibly	O
with	O
full	O
path	O
,	O
and	O
port	O
numbers	O
.	O
</s>
<s>
Besides	O
using	O
remote	O
shells	O
,	O
tunnelling	O
can	O
be	O
used	O
to	O
have	O
remote	O
ports	O
appear	O
as	O
local	O
on	O
the	O
server	O
where	O
an	O
rsync	O
daemon	B-Operating_System
runs	O
.	O
</s>
<s>
Those	O
possibilities	O
allow	O
adjusting	O
security	O
levels	O
to	O
the	O
state	O
of	O
the	O
art	O
,	O
while	O
a	O
naive	O
rsync	O
daemon	B-Operating_System
can	O
be	O
enough	O
for	O
a	O
local	O
network	O
.	O
</s>
<s>
There	O
is	O
"	O
–	O
dry-run	O
"	O
option	O
allows	O
the	O
rsync	O
command	O
to	O
run	O
a	O
trial	O
without	O
making	O
any	O
changes	O
to	O
check	O
syntax	O
and	O
test	O
communication	O
without	O
data	O
transferring	B-Protocol
.	O
</s>
<s>
By	O
default	O
,	O
rsync	O
determines	O
which	O
files	B-Operating_System
differ	O
between	O
the	O
sending	O
and	O
receiving	O
systems	O
by	O
checking	O
the	O
modification	O
time	O
and	O
size	O
of	O
each	O
file	O
.	O
</s>
<s>
Rsync	O
performs	O
a	O
slower	O
but	O
comprehensive	O
check	O
if	O
invoked	O
with	O
--checksum	O
.	O
</s>
<s>
This	O
forces	O
a	O
full	O
checksum	B-Algorithm
comparison	O
on	O
every	O
file	O
present	O
on	O
both	O
systems	O
.	O
</s>
<s>
Barring	O
rare	O
checksum	B-Algorithm
collisions	I-Algorithm
,	O
this	O
avoids	O
the	O
risk	O
of	O
missing	O
changed	O
files	B-Operating_System
at	O
the	O
cost	O
of	O
reading	O
every	O
file	O
present	O
on	O
both	O
systems	O
.	O
</s>
<s>
The	O
recipient	O
splits	O
its	O
copy	O
of	O
the	O
file	O
into	O
chunks	O
and	O
computes	O
two	O
checksums	B-Algorithm
for	O
each	O
chunk	O
:	O
the	O
MD5	B-Algorithm
hash	I-Algorithm
,	O
and	O
a	O
weaker	O
but	O
easier	O
to	O
compute	O
'	O
rolling	B-Error_Name
checksum	I-Error_Name
 '	O
.	O
</s>
<s>
It	O
sends	O
these	O
checksums	B-Algorithm
to	O
the	O
sender	O
.	O
</s>
<s>
The	O
sender	O
computes	O
the	O
checksum	B-Algorithm
for	O
each	O
rolling	O
section	O
in	O
its	O
version	O
of	O
the	O
file	O
having	O
the	O
same	O
size	O
as	O
the	O
chunks	O
used	O
by	O
the	O
recipient	O
's	O
.	O
</s>
<s>
While	O
the	O
recipient	O
calculates	O
the	O
checksum	B-Algorithm
only	O
for	O
chunks	O
starting	O
at	O
full	O
multiples	O
of	O
the	O
chunk	O
size	O
,	O
the	O
sender	O
calculates	O
the	O
checksum	B-Algorithm
for	O
all	O
sections	O
starting	O
at	O
any	O
address	O
.	O
</s>
<s>
If	O
any	O
such	O
rolling	B-Error_Name
checksum	I-Error_Name
calculated	O
by	O
the	O
sender	O
matches	O
a	O
checksum	B-Algorithm
calculated	O
by	O
the	O
recipient	O
,	O
then	O
this	O
section	O
is	O
a	O
candidate	O
for	O
not	O
transmitting	O
the	O
content	O
of	O
the	O
section	O
,	O
but	O
only	O
the	O
location	O
in	O
the	O
recipient	O
's	O
file	O
instead	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
sender	O
uses	O
the	O
more	O
computationally	O
expensive	O
MD5	B-Algorithm
hash	I-Algorithm
to	O
verify	O
that	O
the	O
sender	O
's	O
section	O
and	O
recipient	O
's	O
chunk	O
are	O
equal	O
.	O
</s>
<s>
This	O
allows	O
efficient	O
transmission	O
of	O
files	B-Operating_System
which	O
differ	O
by	O
insertions	O
and	O
deletions	O
.	O
</s>
<s>
The	O
rolling	B-Error_Name
checksum	I-Error_Name
used	O
in	O
rsync	O
is	O
based	O
on	O
Mark	O
Adler	O
's	O
adler-32	B-Algorithm
checksum	B-Algorithm
,	O
which	O
is	O
used	O
in	O
zlib	B-Language
,	O
and	O
is	O
itself	O
based	O
on	O
Fletcher	B-Algorithm
's	I-Algorithm
checksum	I-Algorithm
.	O
</s>
<s>
If	O
the	O
sender	O
's	O
and	O
recipient	O
's	O
versions	O
of	O
the	O
file	O
have	O
many	O
sections	O
in	O
common	O
,	O
the	O
utility	O
needs	O
to	O
transfer	O
relatively	O
little	O
data	O
to	O
synchronize	O
the	O
files	B-Operating_System
.	O
</s>
<s>
If	O
typical	O
data	B-General_Concept
compression	I-General_Concept
algorithms	I-General_Concept
are	O
used	O
,	O
files	B-Operating_System
that	O
are	O
similar	O
when	O
uncompressed	B-General_Concept
may	O
be	O
very	O
different	O
when	O
compressed	O
,	O
and	O
thus	O
the	O
entire	O
file	O
will	O
need	O
to	O
be	O
transferred	O
.	O
</s>
<s>
Some	O
compression	B-General_Concept
programs	I-General_Concept
,	O
such	O
as	O
gzip	B-Application
,	O
provide	O
a	O
special	O
"	O
rsyncable	O
"	O
mode	O
which	O
allows	O
these	O
files	B-Operating_System
to	O
be	O
efficiently	O
rsynced	O
,	O
by	O
ensuring	O
that	O
local	O
changes	O
in	O
the	O
uncompressed	B-General_Concept
file	O
yield	O
only	O
local	O
changes	O
in	O
the	O
compressed	O
file	O
.	O
</s>
<s>
Rsync	O
supports	O
other	O
key	O
features	O
that	O
aid	O
significantly	O
in	O
data	O
transfers	O
or	O
backup	B-Protocol
.	O
</s>
<s>
They	O
include	O
compression	O
and	O
decompression	O
of	O
data	O
block	O
by	O
block	O
using	O
zlib	B-Language
,	O
and	O
support	O
for	O
protocols	O
such	O
as	O
ssh	B-Protocol
and	O
stunnel	B-Protocol
.	O
</s>
<s>
The	O
utility	O
uses	O
the	O
rsync	O
algorithm	O
to	O
generate	O
delta	O
files	B-Operating_System
with	O
the	O
difference	O
from	O
file	O
A	O
to	O
file	O
B	O
(	O
like	O
the	O
utility	O
diff	B-Application
,	O
but	O
in	O
a	O
different	O
delta	O
format	O
)	O
.	O
</s>
<s>
The	O
delta	B-Algorithm
file	I-Algorithm
can	O
then	O
be	O
applied	O
to	O
file	O
A	O
,	O
turning	O
it	O
into	O
file	O
B	O
(	O
similar	O
to	O
the	O
patch	B-Application
utility	O
)	O
.	O
</s>
<s>
rdiff	B-Application
works	O
well	O
with	O
binary	O
files	B-Operating_System
.	O
</s>
<s>
The	O
rdiff-backup	B-Application
script	O
maintains	O
a	O
backup	B-Protocol
mirror	O
of	O
a	O
file	O
or	O
directory	O
either	O
locally	O
or	O
remotely	O
over	O
the	O
network	O
on	O
another	O
server	O
.	O
</s>
<s>
rdiff-backup	B-Application
stores	O
incremental	O
rdiff	B-Application
deltas	O
with	O
the	O
backup	B-Protocol
,	O
with	O
which	O
it	O
is	O
possible	O
to	O
recreate	O
any	O
backup	B-Protocol
point	O
.	O
</s>
<s>
The	O
librsync	O
library	O
used	O
by	O
rdiff	B-Application
is	O
an	O
independent	O
implementation	O
of	O
the	O
rsync	O
algorithm	O
.	O
</s>
<s>
It	O
is	O
used	O
by	O
Dropbox	B-Application
,	O
rdiff-backup	B-Application
,	O
duplicity	B-Application
,	O
and	O
other	O
utilities	O
.	O
</s>
<s>
The	O
acrosync	O
library	O
is	O
an	O
independent	O
,	O
cross-platform	B-Operating_System
implementation	O
of	O
the	O
rsync	O
network	O
protocol	O
.	O
</s>
<s>
It	O
is	O
released	O
under	O
the	O
Reciprocal	B-License
Public	I-License
License	I-License
and	O
used	O
by	O
the	O
commercial	O
rsync	O
software	O
Acrosync	O
.	O
</s>
<s>
The	O
duplicity	B-Application
backup	B-Protocol
software	O
written	O
in	O
python	O
allows	O
for	O
incremental	O
backups	B-Protocol
with	O
simple	O
storage	O
backend	O
services	O
like	O
local	O
file	O
system	O
,	O
sftp	B-Protocol
,	O
Amazon	B-General_Concept
S3	I-General_Concept
and	O
many	O
others	O
.	O
</s>
<s>
It	O
utilizes	O
librsync	O
to	O
generate	O
delta	O
data	O
against	O
signatures	O
of	O
the	O
previous	O
file	O
versions	O
,	O
encrypting	O
them	O
using	O
gpg	B-Application
,	O
and	O
storing	O
them	O
on	O
the	O
backend	O
.	O
</s>
<s>
For	O
performance	O
reasons	O
a	O
local	O
archive-dir	O
is	O
used	O
to	O
cache	O
backup	B-Protocol
chain	O
signatures	O
,	O
but	O
can	O
be	O
re-downloaded	O
from	O
the	O
backend	O
if	O
needed	O
.	O
</s>
<s>
As	O
of	O
macOS	B-Application
10.5	O
and	O
later	O
,	O
there	O
is	O
a	O
special	O
-E	O
or	O
--extended-attributes	O
switch	O
which	O
allows	O
retaining	O
much	O
of	O
the	O
HFS+	B-Application
file	O
metadata	O
when	O
syncing	O
between	O
two	O
machines	O
supporting	O
this	O
feature	O
.	O
</s>
<s>
This	O
is	O
achieved	O
by	O
transmitting	O
the	O
Resource	B-Operating_System
Fork	I-Operating_System
along	O
with	O
the	O
Data	B-Operating_System
Fork	I-Operating_System
.	O
</s>
<s>
zsync	O
is	O
used	O
by	O
Linux	O
distributions	O
such	O
as	O
Ubuntu	B-Operating_System
for	O
distributing	O
fast	O
changing	O
beta	O
ISO	B-General_Concept
image	I-General_Concept
files	B-Operating_System
.	O
</s>
<s>
zsync	O
uses	O
the	O
HTTP	O
protocol	O
and	O
.zsync	O
files	B-Operating_System
with	O
pre-calculated	O
rolling	B-Error_Name
hash	I-Error_Name
to	O
minimize	O
server	O
load	O
yet	O
permit	O
diff	B-Application
transfer	O
for	O
network	O
optimization	O
.	O
</s>
<s>
Rclone	B-General_Concept
is	O
an	O
open-source	O
tool	O
inspired	O
by	O
rsync	O
that	O
focuses	O
on	O
cloud	O
and	O
other	O
high	O
latency	O
storage	O
.	O
</s>
<s>
However	O
,	O
Rclone	B-General_Concept
does	O
not	O
support	O
rolling	B-Error_Name
checksums	I-Error_Name
for	O
partial	O
file	O
syncing	O
(	O
binary	O
diffs	B-Application
)	O
because	O
cloud	O
storage	O
providers	O
do	O
not	O
usually	O
offer	O
the	O
feature	O
and	O
Rclone	B-General_Concept
avoids	O
storing	O
additional	O
metadata	O
.	O
</s>
<s>
Program	O
Operating	B-General_Concept
system	I-General_Concept
Description	O
Linux	O
macOS	B-Application
Windows	O
Back	B-Language
In	I-Language
Time	I-Language
BackupAssist	B-Application
Direct	O
mirror	O
or	O
with	O
history	O
,	O
VSS	O
.	O
</s>
<s>
cwRsync	B-Protocol
Based	O
on	O
Cygwin	B-Language
.	O
</s>
<s>
Grsync	B-Language
Graphical	O
Interface	O
for	O
rsync	O
.	O
</s>
<s>
LuckyBackup	B-Language
rclone	B-General_Concept
Inspired	O
by	O
rsync	O
and	O
supports	O
more	O
than	O
50	O
cloud	O
storage	O
providers	O
and	O
other	O
high	O
latency	O
storage	O
services	O
.	O
</s>
<s>
Does	O
not	O
actually	O
use	O
rsync	O
or	O
support	O
rolling	B-Error_Name
checksums	I-Error_Name
and	O
partial	O
file	B-Application
synchronization	I-Application
.	O
</s>
<s>
tym	O
Time	O
rsYnc	O
Machine	O
–	O
backup	B-Protocol
à	O
la	O
Time	B-Operating_System
Machine	I-Operating_System
–	O
Bash	O
script	O
YARsync	O
Yet	O
Another	O
Rsync	O
synchronizes	O
files	B-Operating_System
between	O
UNIX-like	B-Operating_System
systems	I-Operating_System
or	O
backup	B-Protocol
drives	O
with	O
an	O
interface	O
similar	O
to	O
Git	B-Application
.	O
</s>
