UNIX Services and Daemons

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UNIX Services and Daemons
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Overview

• Daemons are
• background processes that perform a
  system-related task
• are independent processes rather than part of the
  kernel
• started at boot time or when there is a need for
  them.
• The historical meaning of the word daemon is an
  independent spirit, nor good or evil. To quote
  from the Handbook UNIX systems seem to be
  infested with both daemons and demons.

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Essential daemons

• Init (dont play with it if you are not an
  expert).
• is the first process to start after boot and
  places the system in single user mode of spawns a
  shell to read the startup files
• you can re-start the system by sending SIGHUP to
  init.
• cron
• runs commands at preset times
• accepts crontabs (schedule files) from both users
  and administrators
• inetd
• is a daemon that manages other daemons
• only works with network daemons

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inetd.conf and services

• /etc/inetd.conf
• list the services name, type of socket
  (streamTCP), protocol (tcp or udp), requests
  (multiplewait), username to run, fully qualified
  path name of daemon. See example.
• /etc/services
• list the services name, port/protocol, alias. See
  example.
• It is used by inetd to assign ports.

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The cron daemon

• It starts at boot and remains up.
• Almost anything you can do in a shell window can
  be done in cron.
• SGI /etc/cron.d directory for defaults (deny,
  allow) and /usr/var/spool/cron directory for
  crontabs.
• Crontabs are the scheduling files. See an example
  here. Are created by user, e.g. root, abento,
  etc.
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