Process that runs in the background
A
background process
is a
computer process
that runs
behind the scenes
(i.e., in the background) and without user intervention.
[1]
Typical tasks for these processes include logging, system monitoring, scheduling,
[2]
and user notification.
[3]
On a
Windows
system, a background process is either a
computer program
that does not create a
user interface
, or a
Windows service
. The former are started just as any other program is started, e.g., via
Start menu
. Windows services, on the other hand, are started by
Service Control Manager
. In
Windows Vista
and later, they
are run in a separate session
.
[
citation needed
]
On a
Unix
or
Unix-like
system, a background process or job can be further identified as one whose
process group
ID differs from its terminal group ID (TGID). (The TGID of a process is the process ID of the process group leader that opened the terminal, which is typically the login shell. The TGID identifies the control terminal of the process group.) This type of process is unable to receive keyboard signals from its parent terminal, and typically will not send output to that terminal.
[4]
This more technical definition does not distinguish between whether or not the process can receive user intervention. Although background processes are typically used for purposes needing few resources, any process can be run in the background, and such a process will behave like any other process, with the exceptions given above.
[1]
Windows services
[
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]
In
Windows NT
family of
operating systems
, a Windows service is a dedicated background process.
[5]
A Windows service must conform to the interface rules and protocols of the
Service Control Manager
, the component responsible for managing Windows services.
[6]
Windows services can be configured to start when the operating system starts, and to run in the background as long as Windows runs. Alternatively, they can be started manually or by an event. Windows NT operating systems
include numerous services
which run in context of three
user accounts
:
System
,
Network Service
and
Local Service
. These Windows components are often associated with Host Process for Windows Services:
svchost.exe
. Since Windows services operate in the context of their own dedicated user accounts, they can operate when a user is not logged on.
Before
Windows Vista
, services installed as "interactive services" could interact with Windows
desktop
and show a
graphical user interface
. With Windows Vista, however, interactive services became
deprecated
and ceased operating properly, as a result of
Windows Service Hardening
.
[7]
[8]
The three principal means of managing Windows services are:
- Services snap-in for
Microsoft Management Console
sc.exe
- Windows PowerShell
Daemon
[
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]
A daemon is a type of background process designed to run continually in the background, waiting for event(s) to occur or condition(s) to be met.
[9]
When launched with the
daemon
function, daemons are disassociated from their parent terminal.
[10]
Background jobs in Unix
[
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]
From a Unix command line, a background process can be launched using the "&" operator. The
bg
command can resume a suspended job (sending
SIGCONT
), running it in the background. Using the
fg
command will also reconnect standard input its parent terminal, bringing it into the foreground. The
jobs
command will list all processes associated with the current terminal and can be used to bring background processes into the foreground.
[4]
[11]
When a
login session
ends, via explicit logout or network disconnection, all processes, including background processes, will by default be terminated, to prevent them from becoming
orphan processes
. Concretely, when the user exits the launching shell process, as part of shutdown it sends a
hangup
signal (
SIGHUP
) to all its
jobs
, to terminate all the processes in the corresponding
process group
. To have processes continue to run, one can either not end the session, or end the session without terminating the processes. A
terminal multiplexer
can be used to leave a session running but detach a virtual terminal from it, leaving processes running as child processes of the session; the user can then reattach session later. Or, termination can be prevented by either starting the process via the
nohup
command (telling the process to ignore SIGHUP), or by subsequently running
disown
with the job id, which either removes the job from the job list entirely, or simply prevents SIGHUP from being sent. In the latter case when the session ends, the child processes are not terminated, either because they are not sent SIGHUP or because they ignore it, and thus become orphan processes, which are then adopted by the
init
process (the kernel sets the init process as their parent), and they continue running without a session, now called
daemons
.
Example
[
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]
In this example running on
Unix
, the
sleep
utility was launched into the background. Afterward, the
ps
tool was run in the foreground, where it output the below text. Both were launched from the shell.
[12]
PID
TT
STAT
TIME
COMMAND
54659
10
S
0
:00.06
su
(
zsh
)
54703
10
IN
0
:00.00
-
sleep
1000
54852
10
R+
0
:00.00
-
ps
-U
botty
-axd
Smartphones
[
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]
Many newer versions of
smartphone
and PDA
operating systems
now include the ability to start background processes. Due to hardware limits, background processes on mobile operating systems are often restricted to certain tasks or consumption levels. On
Android
, CPU use for background processes may be bounded at 5 - 10%.
[13]
Applications on Apple's
iOS
are limited to a subset of functions while running in the background.
[3]
On both iOS and Android, background processes can be killed by the system if they are using too much memory.
[3]
[13]
See also
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]
References
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]