Daemon Process
What is daemon process. Highlight its features .
A daemon is a long-running background process that answers requests for services. The term originated with Unix, but most operating systems use daemons in some form or another. In Unix, the names of daemons conventionally end in "d". Some examples include inetd, httpd, nfsd, sshd, named, and lpd.
Daemons are processes that are often started when the system is bootstrapped and terminate only when the system is shut down. Because they don’t have a controlling terminal, they run in the background. UNIX systems have numerous daemons that perform day-to-day activities.
In simple language it's a process which continuously runs on your system till user specifically kills it. There are two ways to start this process.
1. User can manually start this daemon to run
2. We can initialize this daemon in init scripts supported by Linux which are run at the time of system start and runs continuously till user specifically kills the daemon or system itself stops.
Daemon Process
- It is the processes that constantly running without using an associated terminals or login shell
- It wait to get some instruction either in system or user and start performing the task
- It alive until process should shutdown
- The important process are swapper, init, cross, vhand etc
Feature of daemons
- They start running as soon as it initialized
- The lifetime of daemon is as long as system is running
- The daemon cannot kill prematurely
- Init process is one of the first program loaded after bootstrapping
- The scheduler process is used to manage and schedule other process
- Vhand (virtual memory handler) loaded into the system to swap active processes between memory and disk
- Run away process
- When the user set a process to run in the background by adding an & at the end of the command and logout without closing the program or killing the program such process is runaway process
Notes
- Most of the daemons run with superuser (root) privileges.
- None of the daemons has a controlling terminal: the terminal name is set to a question mark. The kernel daemons are started without a controlling terminal. The lack of a controlling terminal in the user-level daemons is probably the result of the daemons having called
setsid. Most of the user-level daemons are process group leaders and session leaders, and are the only processes in their process group and session. (The one exception isrsyslogd.) - The parent of the user-level daemons is the
initprocess.