|Erik's linux page: Linux information for beginners and advanced users|
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A Linux system, as any UNIX system, has quite a few daemons hanging around, doing nice little things for you. This section will try to explain what a daemon is.
A daemon is a program executing in the background, doing a specific task, usually something important for the system. They are most often started at system boot, and stopped at system halt, running all the time the system is up. Normally, they don't interact directly with the user, instead you tell them how to behave with a configuration file. They are most often well written, so they won't consume much CPU power when they are just waiting for something to happend.
I'll describe some of the most common daemons, in no particular order.
The "internet super server" is one of the most common daemons. At startup, it reads it configuration file /etc/inetd.conf and then starts listening for incoming internet connections. When someone tries to access your system with, as an example, the telnet protocol, inetd checks it's configuration to see what to do if a telnet request comes, and executes the correct program.
Crond is a very useful daemon, making it possible to execute things at a specified time, or at an interval. You submit a configuration file to it, and every minute it wakes up from it's CPU friendly sleep, and checks if something should be done right this minute. Really useful, if you ask me.
This daemon takes care of logging information from various programs, and stores them to the correct place. It's configuration file, /etc/syslog.conf, makes it possible to have logs from pppd in one file, and logs from sendmail in another. It's also possible to have the log on a VT, or even on another machine via TCP/IP.
Takes care of messages from the kernel.
A daemon waking up every minute to see if something should be done at that time. Kind of similar to crond, but a job submitted to atd (with the command at) is only executed once, while a job maintained by cron is executed periodically.
This daemon acts a little like inetd for SUN RPC calls such as NFS. Sounded like greek ? Read the nfs man page :-)
These are various internet services, such as secure shell, http (web), nntp (news) running as separate daemons instead of being started by inetd. This makes them a bit faster, but they consume more memory/CPU.
Handling X server, remote or local. If you have a fancy login prompt in X, this is the daemon handling that.
Synchronising your time with a distant time server, keeping your clock within milliseconds to world time. This is very important in environments where you share files via NFS.