Linux® has a lot of processes that run in the background. This means that they don’t disturb you when they run, they sit back there and, well, run. For example, the syslog process, the process that sends email, init, the great granddaddy of processes, they all sit back there and do their job without bothering you. They’re actually all called daemons.

Daemon in the flesh

That little guy is kind of cute, isn’t he?

The name came from a thought experiment known as Maxwell’s demon. In this experiment, an imaginary daemon moved things around in the background. They also come from Greek mythology, where the daemon is a guy working in the background but who isn’t necessarily up to no good. He’s not necessarily up to good either, he’s just back there doing his thing. The Greek Mythology spelling is the one that stuck for the daemons on the system.  That little guy is the mascot of the BSD® operating system.

A general rule is that daemons on the system end their name with ‘d’ so you know they’re daemons. syslogd is the daemon for syslog, dhcpd is the daemon that runs dhcp, and named is the daemon that runs DNS. This isn’t always true though, one of the main programs that handle email is called sendmail and so is it’s daemon. There’s no d at the end of that one.

I ran apropos daemon to see what I’d get:

arpd (8) - userspace arp daemon.
authorized_keys (5) - OpenSSH SSH daemon
blkmapd (8) - pNFS block layout mapping daemon
cron (8) - daemon to execute scheduled commands (Vixie Cron)
daemon (1) - turns other processes into daemons
daemon.conf (5) - turns other processes into daemons
dbus-daemon (1) - Message bus daemon
epmd (1) - Erlang Port Mapper Daemonepmd [-d|-debug] [DbgExtra...] [-port No] [-daemon] [-relaxed_command_c...
faked (1) - daemon that remembers fake ownership/permissions of files manipulated by fakeroot processes.
faked-sysv (1) - daemon that remembers fake ownership/permissions of files manipulated by fakeroot processes.
faked-tcp (1) - daemon that remembers fake ownership/permissions of files manipulated by fakeroot processe

This isn’t all of the output, I just chose the first few lines. You should do it on your system to see what you’ll get but there’s a lot of results, aren’t there?

We’ll talk about some of these daemons in the future, because most of them are important to keep your Linux® system running happily.