<s>
Advanced	B-Application
package	I-Application
tool	I-Application
,	O
or	O
APT	B-Application
,	O
is	O
a	O
free-software	B-Application
user	B-Architecture
interface	I-Architecture
that	O
works	O
with	O
core	B-Library
libraries	I-Library
to	O
handle	O
the	O
installation	O
and	O
removal	O
of	O
software	O
on	O
Debian	O
,	O
and	O
Debian-based	O
Linux	B-Application
distributions	I-Application
.	O
</s>
<s>
APT	B-Application
simplifies	O
the	O
process	O
of	O
managing	O
software	O
on	O
Unix-like	B-Operating_System
computer	O
systems	O
by	O
automating	O
the	O
retrieval	O
,	O
configuration	O
and	O
installation	O
of	O
software	B-Application
packages	I-Application
,	O
either	O
from	O
precompiled	O
files	O
or	O
by	O
compiling	B-Language
source	O
code	O
.	O
</s>
<s>
APT	B-Application
is	O
a	O
collection	O
of	O
tools	O
distributed	O
in	O
a	O
package	O
named	O
apt	B-Application
.	O
</s>
<s>
A	O
significant	O
part	O
of	O
APT	B-Application
is	O
defined	O
in	O
a	O
C++	B-Language
library	O
of	O
functions	O
;	O
APT	B-Application
also	O
includes	O
command-line	B-Application
programs	I-Application
for	O
dealing	O
with	O
packages	O
,	O
which	O
use	O
the	O
library	O
.	O
</s>
<s>
Three	O
such	O
programs	O
are	O
apt	B-Application
,	O
apt-get	B-Application
and	O
apt-cache	B-Application
.	O
</s>
<s>
The	O
apt	B-Application
package	O
is	O
of	O
"	O
important	O
"	O
priority	O
in	O
all	O
current	O
Debian	O
releases	O
,	O
and	O
is	O
therefore	O
included	O
in	O
a	O
default	O
Debian	O
installation	O
.	O
</s>
<s>
APT	B-Application
can	O
be	O
considered	O
a	O
front-end	B-Architecture
to	O
dpkg	B-Language
,	O
friendlier	O
than	O
the	O
older	O
dselect	O
front-end	B-Architecture
.	O
</s>
<s>
While	O
dpkg	B-Language
performs	O
actions	O
on	O
individual	O
packages	O
,	O
APT	B-Application
manages	O
relations	O
(	O
especially	O
dependencies	O
)	O
between	O
them	O
,	O
as	O
well	O
as	O
sourcing	O
and	O
management	O
of	O
higher-level	O
versioning	O
decisions	O
(	O
release	O
tracking	O
and	O
version	O
pinning	O
)	O
.	O
</s>
<s>
APT	B-Application
is	O
often	O
hailed	O
as	O
one	O
of	O
Debian	O
's	O
best	O
features	O
,	O
which	O
Debian	O
developers	O
attribute	O
to	O
the	O
strict	O
quality	O
controls	O
in	O
Debian	O
's	O
policy	O
.	O
</s>
<s>
A	O
major	O
feature	O
of	O
APT	B-Application
is	O
the	O
way	O
it	O
calls	O
dpkg	B-Language
—	O
it	O
does	O
topological	B-Algorithm
sorting	I-Algorithm
of	O
the	O
list	O
of	O
packages	O
to	O
be	O
installed	O
or	O
removed	O
and	O
calls	O
dpkg	B-Language
in	O
the	O
best	O
possible	O
sequence	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
it	O
utilizes	O
the	O
--force	O
options	O
of	O
dpkg	B-Language
.	O
</s>
<s>
However	O
,	O
it	O
only	O
does	O
this	O
when	O
it	O
is	O
unable	O
to	O
calculate	O
how	O
to	O
avoid	O
the	O
reason	O
dpkg	B-Language
requires	O
the	O
action	O
to	O
be	O
forced	O
.	O
</s>
<s>
Notably	O
,	O
APT	B-Application
automatically	O
gets	O
and	O
installs	O
packages	O
upon	O
which	O
the	O
indicated	O
package	O
depends	O
(	O
if	O
necessary	O
)	O
.	O
</s>
<s>
This	O
was	O
an	O
original	O
distinguishing	O
characteristic	O
of	O
APT-based	O
package	B-Application
management	I-Application
systems	I-Application
,	O
as	O
it	O
avoided	O
installation	O
failure	O
due	O
to	O
missing	O
dependencies	O
,	O
a	O
type	O
of	O
dependency	B-Protocol
hell	I-Protocol
.	O
</s>
<s>
Another	O
distinction	O
is	O
the	O
retrieval	O
of	O
packages	O
from	O
remote	O
repositories	B-Application
.	O
</s>
<s>
APT	B-Application
uses	O
a	O
location	O
configuration	O
file	O
(	O
/etc/apt/sources.list	O
)	O
to	O
locate	O
the	O
desired	O
packages	O
,	O
which	O
might	O
be	O
available	O
on	O
the	O
network	O
or	O
a	O
removable	B-Device
storage	I-Device
medium	O
,	O
for	O
example	O
,	O
and	O
retrieve	O
them	O
,	O
and	O
also	O
obtain	O
information	O
about	O
available	O
(	O
but	O
not	O
installed	O
)	O
packages	O
.	O
</s>
<s>
APT	B-Application
provides	O
other	O
command	O
options	O
to	O
override	O
decisions	O
made	O
by	O
apt-get	B-Application
'	O
s	O
conflict	O
resolution	O
system	O
.	O
</s>
<s>
The	O
user	O
can	O
specify	O
packages	O
using	O
a	O
POSIX	O
regular	B-Language
expression	I-Language
.	O
</s>
<s>
APT	B-Application
searches	O
its	O
cached	O
list	O
of	O
packages	O
and	O
lists	O
the	O
dependencies	O
that	O
must	O
be	O
installed	O
or	O
updated	O
.	O
</s>
<s>
APT	B-Application
retrieves	O
,	O
configures	O
and	O
installs	O
the	O
dependencies	O
automatically	O
.	O
</s>
<s>
Triggers	B-Language
are	O
the	O
treatment	O
of	O
deferred	O
actions	O
.	O
</s>
<s>
Usage	O
modes	O
of	O
apt	B-Application
and	O
apt-get	B-Application
that	O
facilitate	O
updating	O
installed	O
packages	O
include	O
:	O
</s>
<s>
The	O
lists	O
of	O
available	O
packages	O
are	O
fetched	O
from	O
the	O
location(s )	O
specified	O
in	O
/etc/apt/sources.list	O
.	O
</s>
<s>
upgrade	O
is	O
used	O
to	O
install	O
the	O
newest	O
versions	O
of	O
all	O
packages	O
currently	O
installed	O
on	O
the	O
system	O
from	O
the	O
sources	O
enumerated	O
in	O
/etc/apt/sources.list	O
.	O
</s>
<s>
full-upgrade	O
(	O
apt	B-Application
)	O
and	O
dist-upgrade	O
(	O
apt-get	B-Application
)	O
,	O
in	O
addition	O
to	O
performing	O
the	O
function	O
of	O
upgrade	O
,	O
also	O
intelligently	O
handles	O
changing	O
dependencies	O
with	O
new	O
versions	O
of	O
packages	O
;	O
apt	B-Application
and	O
apt-get	B-Application
have	O
a	O
"	O
smart	O
"	O
conflict	O
resolution	O
system	O
,	O
and	O
will	O
attempt	O
to	O
upgrade	O
the	O
most	O
important	O
packages	O
at	O
the	O
expense	O
of	O
less	O
important	O
ones	O
if	O
necessary	O
.	O
</s>
<s>
The	O
/etc/apt/sources.list	O
file	O
contains	O
a	O
list	O
of	O
locations	O
from	O
which	O
to	O
retrieve	O
desired	O
package	O
files	O
.	O
</s>
<s>
/etc/apt	O
contains	O
the	O
APT	B-Application
configuration	O
folders	O
and	O
files	O
.	O
</s>
<s>
apt-config	O
is	O
the	O
APT	B-Application
Configuration	O
Query	O
program	O
.	O
</s>
<s>
apt-config	O
dump	O
shows	O
the	O
configuration	O
.	O
</s>
<s>
/etc/apt/sources.list	O
:	O
Locations	O
to	O
fetch	O
packages	O
from	O
.	O
</s>
<s>
/etc/apt/sources.list.d/	O
:	O
Additional	O
source	O
list	O
fragments	O
.	O
</s>
<s>
/etc/apt/apt.conf	O
:	O
APT	B-Application
configuration	O
file	O
.	O
</s>
<s>
/etc/apt/apt.conf.d/	O
:	O
APT	B-Application
configuration	O
file	O
fragments	O
.	O
</s>
<s>
/etc/apt/preferences.d/	O
:	O
Directory	O
with	O
version	O
preferences	O
files	O
.	O
</s>
<s>
/var/cache/apt/archives/	O
:	O
Storage	O
area	O
for	O
retrieved	O
package	O
files	O
.	O
</s>
<s>
/var/cache/apt/archives/partial/	O
:	O
Storage	O
area	O
for	O
package	O
files	O
in	O
transit	O
.	O
</s>
<s>
/var/lib/apt/lists/partial/	O
:	O
Storage	O
area	O
for	O
state	O
information	O
in	O
transit	O
.	O
</s>
