Daemon (computing) - Wikipedia

Daemon (computing) - Wikipedia
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Daemon (computer software)
Computer program that runs as a background process
For spirit in Classical Greek mythology, see
Daimon
. For broader coverage of the topic, see
Server (computing)
Components of some
Linux
desktop environments
that are daemons include
D-Bus
NetworkManager
(here called
unetwork
),
PulseAudio
usound
), and
Avahi
In
computing
, a
daemon
is a
program
that runs as a
background process
, rather than being under the direct control of an interactive user. Customary convention is to name a daemon process with the letter
as a suffix to indicate that it's a daemon. For example,
syslogd
is a daemon that implements system logging facility, and
sshd
is a daemon that serves incoming
SSH
connections.
Even though the concept can apply to many computing systems, the term
daemon
is used almost exclusively in the context of
Unix
-based systems. In other contexts, different terms are used for the same concept.
Systems often start daemons at
boot
time that will respond to network requests, hardware activity, or other programs by performing some task. Daemons such as
cron
may also perform defined tasks at scheduled times.
Terminology
edit
In the context of computing, the word is generally pronounced either as
iː
ən
DEE
-mən
or
eɪ
ən
DAY
-mən
The term was coined by the programmers at
MIT's Project MAC
. According to
Fernando J. Corbató
, who worked on
Project MAC
around 1963, his team was the first to use the term daemon, inspired by
Maxwell's demon
, an imaginary agent in physics and
thermodynamics
that helped to sort molecules, stating, "We fancifully began to use the word daemon to describe background processes that worked tirelessly to perform system chores".
Unix
systems inherited this terminology. Maxwell's demon is consistent with Greek mythology's interpretation of a
daemon
as a supernatural being working in the background.
In the general sense, daemon is an older form of the word "demon", from the
Greek
δαίμων. In the
Unix System Administration Handbook
Evi Nemeth
states the following about daemons:
Many people equate the word "daemon" with the word "demon", implying some kind of
satanic
connection between UNIX and the
underworld
. This is an egregious misunderstanding. "Daemon" is actually a much older form of "demon"; daemons have no particular bias towards good or evil, but rather serve to help define a person's character or personality. The
ancient Greeks
' concept of a "personal daemon" was similar to the modern concept of a "guardian angel"—
eudaemonia
is the state of being helped or protected by a kindly spirit. As a rule, UNIX systems seem to be infested with both daemons and demons.
Alternative terms include
service
(used in Windows, from Windows NT onwards, and later also in Linux),
started task
(IBM
z/OS
),
and
ghost job
(XDS
UTS
). Sometimes the more general term
server
or
server process
is used, particularly for daemons that operate as part of
client-server systems
A daemon that connects to a computer network is a
network service
After the term was adopted for computer use, it was incorrectly
rationalized as a
backronym
for
isk
nd
xecution
mon
itor.
Implementations
edit
Unix-like systems
edit
In a
Unix-like
system, the
parent process
of a daemon is often, but not always, the
init
process. A daemon is usually created either by the init process directly launching the daemon, by the daemon being run by an initialization script run by init, or by the daemon being launched by a
super-server
launched by init.
The init process in
Research Unix
and
BSD
starts daemons from an initialization script. A daemon started as a command in an initialization script must either
fork
a child process and then immediately exit, or must be run as a background process using
, so that the shell running the initialization script can continue after starting the daemon. In the former case, the daemon process run from the shell exits, thus causing init to adopt the child process that runs as the daemon; in the latter case, when the shell running the initialization script exits, the child daemon process is adopted by init.
The versions of init in
System III
and in
System V
can run arbitrary commands and can be configured to run them once or to restart them when they terminate.
10
11
12
13
The former mechanism can be used to run initialization scripts; daemons started from those scripts behave the same as in Research Unix and BSD. The latter mechanism can be used to run daemons directly from init.
A daemon can also be launched from a user's command line. However, daemons launched in that fashion typically must perform other operations, such as dissociating the process from any controlling
terminal
(tty). Such procedures are often implemented in various convenience routines such as
daemon(3)
. A daemon launched by an initialization script need not do these steps, but doing so allows the daemon to be restarted by a user if it exits;
14
init itself would not restart them.
15
Operations such a daemon must do include:
Optionally removing unnecessary variables from environment.
Executing as a
background task
by
forking
and
exiting
(in the parent "half" of the fork). This allows daemon's parent (shell or startup process) to receive exit notification and continue its normal execution.
Detaching from the invoking session, usually accomplished by a single operation,
setsid()
Dissociating from the controlling
tty
Creating a new session and becoming the session leader of that session.
Becoming a
process group
leader.
If the daemon wants to ensure that it will not acquire a new controlling tty even by accident (which happens when a session leader without a controlling tty opens a free tty), it may fork and exit again. This means that it is no longer a session leader in the new session, and cannot acquire a controlling tty.
Setting the
root directory
) as the current
working directory
so that the process does not keep any directory in use that may be on a
mounted
file system (allowing it to be unmounted).
Changing the
umask
to 0 to allow
open()
creat()
, and other operating system calls to provide their own permission masks and not to depend on the umask of the caller.
Redirecting
file descriptors
0, 1 and 2 for the
standard streams
stdin
stdout
and
stderr
) to
/dev/null
or a
logfile
, and closing all the other file descriptors inherited from the parent process.
If the process is started by a
super-server
daemon, such as
inetd
launchd
, or
systemd
, the super-server daemon will perform those functions for the process,
16
17
18
except for old-style daemons not converted to run under
systemd
and specified as
Type=forking
18
and "multi-threaded" datagram servers under
inetd
16
MS-DOS
edit
In
MS-DOS
, daemon-like functionality was implemented as a
terminate-and-stay-resident program
(TSR).
Windows
edit
In
Windows
, a
Windows service
provides the functionality of a daemon. It runs as a process, usually does not interact with the user (i.e. via monitor, keyboard, or mouse) and may be launched by the operating system at boot time. In
Windows 2000
and later versions, a Windows service is configured and controlled via various interfaces including the
Control Panel
, the
Service Control Manager
sc
command, the
net start
and
net stop
commands,
PowerShell
, or a custom program.
However, any Windows application can perform the role of a daemon, not just a service, and some Windows daemons have the option of running as a normal process.
Mac
edit
In
classic Mac OS
, optional features and services were provided by
system extensions
and
control panels
– files loaded at startup time that patched the operating system. Later versions of classic Mac OS augmented these with
faceless background applications
: regular applications that ran in the background. To the user, these were still described as regular system extensions.
The more modern
macOS
, which is
Unix
-based, uses daemons but uses the term "services" to designate software that performs functions selected from the
Services menu
, rather than using that term for daemons, as Windows does.
See also
edit
Beastie, the
BSD Daemon
List of computer term etymologies
List of Unix daemons
Service wrapper
Software bot
User space
Web service
Windows service
References
edit
Eric S. Raymond
"daemon"
The Jargon File
. Retrieved
2008-10-22
"The Origin of the word Daemon"
"The BSD Daemon"
. Freebsd.org
. Retrieved
2008-11-15
"Glossary of z/OS terms and abbreviations"
IBM Documentation
IBM
. 31 January 2006.
"sshd: OpenSSH server process | SSH Academy"
www.ssh.com
. Retrieved
2023-12-04
"Daemon Definition"
www.linfo.org
"What is a daemon?"
UCLA Knowledge Base
"Daemon"
Kayshav.com
"(Unix) daemonization turns out to be quite old"
. October 3, 2024.
UNIX User's Manual, Release 3.0
(PDF)
. June 1980. INIT(8).
UNIX User's Manual, Release 3.0
(PDF)
. June 1980. INITTAB(5).
UNIX System Administrator's Manua Release 5.0
(PDF)
. June 1982. INIT(1M).
UNIX User's Manual System V
(PDF)
. January 1983. INITTAB(4).
"Daemonization in Unix programs is probably about restarting programs"
. October 5, 2024.
"Traditionally, init on Unix was not a service manager as such"
. October 4, 2024.
inetd(8)
FreeBSD
System Manager's
Manual
launchd.plist(5)
Darwin
and
macOS
File Formats
Manual
"systemd.service"
. freedesktop.org
. Retrieved
August 25,
2012
External links
edit
Unix Daemon Server Programming
at the
Wayback Machine
(archived 2019-10-30)
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