daemons and cron

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Page 1: Daemons and Cron

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Tech Mahindra Limited confidential© Tech Mahindra Limited 2009

 Daemons and Cron

Prepared by:- EMAS Cheetah

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08/30/13 CONFIDENTIAL© Copyright 2009 Tech Mahindra Limited 2

Daemons and Cron

Purpose:

In Unix environment Daemons and Cron plays a vital role.

This presentation is helpful for everyone who works in UNIXenvironment

Understanding Daemons and Cron gives Unix applicationsupport people a fine edge.

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08/30/13 CONFIDENTIAL© Copyright 2009 Tech Mahindra Limited 3

Daemons

 A Daemon is a background processDaemon stands for Disk and Execution Monitor. A daemon is a long-

running background process that answers requests for services. Theterm originated with Unix, but most operating systems use daemons

in some form or another. In Windows NT, 2000, and XP, for 

example, daemons are called "services". In Unix, the names of 

daemons conventionally end in "d". Some examples include inetd,

httpd, nfsd, sshd, named, and lpd.

• Performs a specific system service• An NT service is similar 

Some Daemons• are started at boot-time and run continuously

• are started when needed, and die when completed• are started by the kernel• are kernel code dressed up to look like daemons• will fork themselves to run in the background

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init

•  The Primary, critical system daemon

Common system daemons 

• Paging daemon

Faults pages from disk into RAM as needed• Swapping Daemon

Moves entire processes out to disk• File system synchronization daemon

Flushes cached in-RAM disk blocks to disk

 

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Inetd

Networking super-daemon

Starts individual daemons as needed

Listens for activity on TCP/UDP ports

Configuration files• /etc/inetd.conf on most systems• /etc/services lists TCP/UDP port name/number pairs• /etc/protocols lists protocol name /number pairs

• /etc/rpc lists remote procedure call name /number pairs

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Xinetd

Enhanced Version of inetd

Used by Linux instead of inetd

Configuration files stored in directory /etc/xinetd.d

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NFS Daemons

nfsd

  Serves NFS filesMountd

  Handles NFS file system mount requests

Lockd

  Handles advisory locksStatd

  Monitors status of NFS servers

Biod  Caches NFS blocks – improves performance

read-ahead/write-behind

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Automount Daemons

Automount Automounter daemon for autofs-based systems

Amd

 AMD automounter daemon

Time Synchronization Daemons

  timed Xntp

Cron

Performs periodic, scheduled tasks for users

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NIS Daemons

 YpbindLocates NIS servers – runs on client and server 

 Ypserv

Handles NIS queries – runs on all NIS servers

 Ypxfrd

Transfers NIS database to slave servers

 Yppasswd

Listens on master for yppasswd password change requests

Rcp.nisd

Like ypserv, but for NIS+

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Boot and configuration Daemons

Dhcp

Dynamically provides IP,gateway, and name serversBootpd

Bootp daemon (precursor to DHCP)Tftpd

Trivial file transfer used for diskless bootingRarpd

Responds to reverse ARP requestsBootparamd

Assists booting of diskless clients

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Cron

Cron

Actually it is called ‘cron daemon’. Cron is an automatic task

machine. You will use it on your Unix or Linux operatingsystems for doing some tasks at specific intervals with outyour intervention every time. You set the clock and forget.The cron daemon runs the work for you.

What is cron tab?‘Cron tab(CRON TABle)’ is a text file that contains a series of cron functions.

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Cron (contd..)

Cron A Daemon Service that runs periodic jobs

Jobs specified in per-user configuration file – a crontab The Crontab configuration file

Lists one or more programs to run, and when to run them

one crontab file per user

The Crontab Program

The Crontab program is used to schedule jobs listed in auser’s crontab configuration file

  Stores a copy of the user’s crontab in its directory

Linux: /var/spool/cron

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Crontab Format

Each line contains six fields, separated by whitespace

minute || hour || day || month || weekday || command

Field Specifications:

Field Valid Values

minute 0-59

hour 0-23

Day 1-31

Month 1-12

Weekday 0-6(0=Sunday)

Command The command to be run

 

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Crontab Format

Metacharacters

The * wildcard can be used to mean all valuesRanges can be specified using dash

x-y means x through y, inclusivelyLists of values are separated with comma’s

x,y,z means x and y and z

A comment line starts with the # character as the first character can't be used whitin line

Environment Variables

A line may specify as environment variable setting

variable = value

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Cron

The Crontab Utility

The Commands

Crontab fileo Installs the crontab file into the system crontab directory

o Schedules the entries within file to run

Crontab –l

Lists the crontab for the current user

Crontab –e

Edits and re-schedules a user’s crontab

Crontab –r

Removes a users’ crontab

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Cron

Crontab Management

/etc/cron.allow

if it exists, specifically allows only the listed users to runcrontab

if it does not exist, everyone is allowed to run crontab

 /etc/cron.deny

if it exists, the listed users are not allowed to run crontab

Editing these files do not affect existing crontabs or readyscheduled jobs

The administrator must remove the desired crontabs eithermanually or by using crontab -r

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Thank You