<s>
A	O
file	B-Application
archiver	I-Application
is	O
a	O
computer	O
program	O
that	O
combines	O
a	O
number	O
of	O
files	B-Operating_System
together	O
into	O
one	O
archive	B-Application
file	I-Application
,	O
or	O
a	O
series	O
of	O
archive	B-Application
files	I-Application
,	O
for	O
easier	O
transportation	O
or	O
storage	O
.	O
</s>
<s>
File	B-Application
archivers	I-Application
may	O
employ	O
lossless	B-Algorithm
data	I-Algorithm
compression	I-Algorithm
in	O
their	O
archive	B-General_Concept
formats	I-General_Concept
to	O
reduce	O
the	O
size	O
of	O
the	O
archive	B-Application
.	O
</s>
<s>
Basic	O
archivers	O
just	O
take	O
a	O
list	O
of	O
files	B-Operating_System
and	O
concatenate	O
their	O
contents	O
sequentially	O
into	O
archives	O
.	O
</s>
<s>
The	O
archive	B-Application
files	I-Application
need	O
to	O
store	O
metadata	O
,	O
at	O
least	O
the	O
names	O
and	O
lengths	O
of	O
the	O
original	O
files	B-Operating_System
,	O
if	O
proper	O
reconstruction	O
is	O
possible	O
.	O
</s>
<s>
More	O
advanced	O
archivers	O
store	O
additional	O
metadata	O
,	O
such	O
as	O
the	O
original	O
timestamps	O
,	O
file	B-Application
attributes	I-Application
or	O
access	O
control	O
lists	O
.	O
</s>
<s>
The	O
process	O
of	O
making	O
an	O
archive	B-Application
file	I-Application
is	O
called	O
archiving	O
or	O
packing	O
.	O
</s>
<s>
Reconstructing	O
the	O
original	O
files	B-Operating_System
from	O
the	O
archive	B-Application
is	O
termed	O
unarchiving	O
,	O
unpacking	O
or	O
extracting	O
.	O
</s>
<s>
An	O
early	O
archiver	O
was	O
the	O
Multics	B-Application
command	O
archive	B-Application
,	O
descended	O
from	O
the	O
CTSS	B-Operating_System
command	O
of	O
the	O
same	O
name	O
,	O
which	O
was	O
a	O
basic	O
archiver	O
and	O
performed	O
no	O
compression	O
.	O
</s>
<s>
Multics	B-Application
also	O
had	O
a	O
"	O
tape_archiver	O
"	O
command	O
,	O
abbreviated	O
ta	O
,	O
which	O
was	O
perhaps	O
the	O
forerunner	O
of	O
the	O
Unix	B-Application
command	O
tar	B-Application
.	O
</s>
<s>
The	O
Unix	B-Application
tools	O
ar	B-Application
,	O
tar	B-Application
,	O
and	O
cpio	B-Application
act	O
as	O
archivers	O
but	O
not	O
compressors	O
.	O
</s>
<s>
Users	O
of	O
the	O
Unix	B-Application
tools	O
use	O
additional	O
compression	O
tools	O
,	O
such	O
as	O
gzip	B-Application
,	O
bzip2	B-Application
,	O
or	O
xz	B-Language
,	O
to	O
compress	O
the	O
archive	B-Application
file	I-Application
after	O
packing	O
or	O
remove	O
compression	O
before	O
unpacking	O
the	O
archive	B-Application
file	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
archiving	O
a	O
collection	O
of	O
files	B-Operating_System
with	O
tar	B-Application
and	O
then	O
compressing	O
the	O
resulting	O
archive	B-Application
file	I-Application
with	O
gzip	B-Application
results	O
a	O
file	O
with	O
.tar.gz	O
extension	O
.	O
</s>
<s>
It	O
follows	O
the	B-Operating_System
Unix	I-Operating_System
philosophy	I-Operating_System
that	O
each	O
program	O
should	O
accomplish	O
a	O
single	O
task	O
to	O
perfection	O
,	O
as	O
opposed	O
to	O
attempting	O
to	O
accomplish	O
everything	O
with	O
one	O
tool	O
.	O
</s>
<s>
The	O
archives	O
use	O
solid	B-General_Concept
compression	I-General_Concept
.	O
</s>
<s>
When	O
the	O
files	B-Operating_System
are	O
combined	O
,	O
the	O
compressor	O
can	O
exploit	O
redundancy	O
across	O
several	O
archived	O
files	B-Operating_System
and	O
achieve	O
better	O
compression	O
than	O
a	O
compressor	O
that	O
compresses	O
each	O
files	B-Operating_System
individually	O
.	O
</s>
<s>
Extracting	O
one	O
file	O
requires	O
decompressing	O
an	O
entire	O
archive	B-Application
,	O
which	O
can	O
be	O
time	O
-	O
and	O
space-consuming	O
.	O
</s>
<s>
Modifying	O
one	O
means	O
the	O
file	O
needs	O
to	O
be	O
put	O
back	O
into	O
archive	B-Application
and	O
the	O
archive	B-Application
recompressed	O
again	O
.	O
</s>
<s>
The	O
archive	B-Application
becomes	O
damage-prone	O
.	O
</s>
<s>
If	O
the	O
area	O
holding	O
shared	O
data	O
for	O
several	O
files	B-Operating_System
is	O
damaged	O
,	O
all	O
those	O
files	B-Operating_System
are	O
lost	O
.	O
</s>
<s>
It	O
is	O
impossible	O
to	O
take	O
advantage	O
of	O
redundancy	O
between	O
files	B-Operating_System
unless	O
the	O
compression	O
window	O
is	O
larger	O
than	O
the	O
size	O
of	O
an	O
individual	O
file	O
.	O
</s>
<s>
For	O
example	O
,	O
gzip	B-Application
uses	O
DEFLATE	B-Algorithm
,	O
which	O
typically	O
operates	O
with	O
a	O
32768-byte	O
window	O
,	O
whereas	O
bzip2	B-Application
uses	O
a	O
Burrows	B-Algorithm
–	I-Algorithm
Wheeler	I-Algorithm
transform	I-Algorithm
roughly	O
27	O
times	O
bigger	O
.	O
</s>
<s>
xz	B-Language
defaults	O
to	O
8MiB	O
but	O
supports	O
significantly	O
larger	O
windows	O
.	O
</s>
<s>
The	O
built-in	O
archiver	O
of	O
Microsoft	B-Application
Windows	I-Application
as	O
well	O
as	O
third-party	O
archiving	B-Application
software	I-Application
,	O
such	O
as	O
WinRAR	B-Application
and	O
7-zip	B-Language
,	O
often	O
use	O
a	O
graphical	B-Application
user	I-Application
interface	I-Application
.	O
</s>
<s>
They	O
also	O
offer	O
an	O
optional	O
command-line	B-Application
interface	I-Application
,	O
while	O
Windows	O
itself	O
does	O
not	O
.	O
</s>
<s>
Solid	B-General_Concept
compression	I-General_Concept
may	O
or	O
may	O
not	O
be	O
offered	O
,	O
depending	O
on	O
the	O
product	O
:	O
Windows	O
itself	O
does	O
not	O
support	O
it	O
;	O
WinRAR	B-Application
and	O
7-zip	B-Language
offer	O
it	O
as	O
an	O
option	O
that	O
can	O
be	O
turned	O
on	O
or	O
off	O
.	O
</s>
