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    Daemon is a program that continuously runs as a background process rather than being under the direct control of an interactive user. It exists for the purpose of handling periodic service requests that a computer system expects to receive. The term was coined by programmers at MIT’s Project MAC who took the name from Maxwell’s demon, which was an imaginary being from a thought experiment that continuously worked in the background. Unix systems then inherited this term, which is also consistent with Greek mythology’s interpretation of a daemon as a supernatural being that works in the background with neither a good or evil bias.

    While the term became well known with Unix, most operating systems use daemons in some form or another. Traditionally, the process names of a daemon end with the letter d, in order to clarify that the process is indeed a daemon and for differentiation between a daemon and a normal computer program.

    Unix daemon examples

    Below is a list of daemons found in Unix-like operating systems

    • biod: Works in cooperation with the remote nfsd to handle NFS requests
    • crond: Time-based job scheduler that runs jobs in the background
    • fingerd: Provides a network interface for the finger protocol. Used by the finger command
    • ftpd: Services FTP requests from a remote system
    • httpd: Web server daemon
    • lpd: The line printer daemon that manages printer spooling
    • ntpd: Network Time Protocol daemon that manages clock synchronization across the network.
    • sshd: Listens for secure shell requests from clients
    • syslogd: System logger process that collects various system messages
    • systemd: Replacement of init, the Unix program that spawns all other processes
    • syncd: Periodically keeps the file systems synchronized with system memory
    • vhand: Releases pages of memory for use by other processes.
    • ypbind: Finds the serves for an NIS domain and stores the information in a file