Processes which (re)start when you start your computer and run until you halt it are known as services
in the Microsoft world and daemons
in the Linux and Unix world. As well as providing support for the operating system and user, the service / daemon mechanism is used on server computers to provide the processes that listen out for requests, and handle them.
On the left of this article, you'll see a list of the more significant daemnon processes running on our server, as discuseed / taken from our Linux Web Server
. A memory caching daemon that lets us save the results of complex database requests to improve server throughput. Of all the daemons that I have listed on this page, this is the one you'll see the least often.
. The Apache Tomcat web server, which is a container for Java based applications written to the Servlet or JSP standards. You'll find Tomcat used for high traffic web applications, where re-loading the page on the server every time it is called up would be inefficient.
. This provides the service that lets other applications read and write data to / from the MySQL relational database.
. The Apached httpd web server - this is the daemon that turns your computer into a web server ... listening for connections from browsers, and sending out the appropriate pages in response. You'll normally find that you are running multiple
httpd processes on your server, as the daemon adjusts itself to have sufficient copies on hand to handle incoming traffic. On systems with both httpd and Tomcat, it's httpd than handles the incoming requests, which is passes on (proxies) to Tomcat as necessary if a Java program is to be run.
. The Secure Shell daemon, which allows the system administrator (and others if authorised) to log in to the server remotely to update settings, the web site, etc, and to run other programs on the server.
. Jobs which you schedule via the at
commands are started and controlled by this daemon. A good example of its use it to take a copy of your server log files in the middle of the night, and to start a new log file.
. If you have a printer attached to (or controlled from) your computer, lpd takes print requests and handles them for you, without you having to wait for the print request you've made to actually complete before you get on with your next job. It also allows several people to share a printer without any risk of their data getting mixed up.
. This daemon provides facilities for file uploads and downloads for authorised users using the File Transfer Protocol. Typically, FTP is used by web site maintainers to upload new pages, applications, images, etc, and by administrators to download backups and copies of log files.
. If your server is also an email server, sendmail is one of the daemons that you may use to handle the management, routing and transfers of email. (written 2008-11-23)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesA164 - Web Application Deployment - Services and Regular Jobs 
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Linux run states, shell special commands, and directory structures - (2007-08-03) 
Automatic startup and shutdown of Tomcat - (2008-02-24) 
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FTP server on Fedora Linux - (2008-07-06) 
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The Kernel, Shells and Daemons. Greek Gods in computing - (2012-07-01) 
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