202.1. Customizing SysV-init system startup

202.1 Customizing SysV-init system startup
Weight: 3
Description: Candidates should be able to query and modify the behaviour of system services at various targets / run levels. A thorough understanding of the systemd, SysV Init and the Linux boot process is required. This objective includes interacting with systemd targets and SysV init run levels.
Key Knowledge Areas:
Systemd
SysV init
Linux Standard Base Specification (LSB)
Terms and Utilities:
/usr/lib/systemd/
/etc/systemd/
/run/systemd/
systemctl
systemd-delta
/etc/inittab
/etc/init.d/
/etc/rc.d/
chkconfig
update-rc.d
init and telinit
Over View
During the previous lessons we talked about initrd/initramfs. When the kernel completly loaded it searchs for init process to start it. init process can be init, upstart or systemd. Traditionally System v init is used to start other services but it has some short comings. So other solutions invented like upstart and systemd.
/sbin/init can be linked to upstart or systemd. to check which system you are running, check each directory existence:
Directpry
Description [if exist]
/etc/init.d
Shows you have SysV in your linux box
/usr/share/upstart
You are on a Upstart Based system
/usr/lib/systemd
You are using Systemd-Based system
also try stat /proc/1/exe
link
Description
File: ‘/proc/1/exe’ -> ‘/sbin/init’
in SysV and upstart system
File: '/proc/1/exe' -> '/lib/systemd/systemd'
in Systemd box
Sys v
System "5" or Sys "v" is an ancient method of handling system services from unix world back to 1980s. SysV uses serial loading of services, in another word each service must be loaded in sequence (after each other). SysV uses runlevels concept to define which stat the server should boot in. In each runlevel specific amount of shell scripts is processed to reach the state we desire.
runlevels start from 0 upto 6 and they are different in Redhat based and Debian based systems.
runlevel
Redhat
Debian
0
System Halt (do not set as default)
System Halt (do not use as default)
1
Single User Mode
Single User Mode
2
Multi User without NFS
Full multi User mode with GUI(default)
3
Full Multi User Mode
--same as 2-- --unused--
4
---unused--
--same as 2-- --unused--
5
X11/Full Multi user Mode(default)
--same as 2-- --unused--
6
reboot (Do not set as initdefault)
rebbot (do not set as default)
/etc/inittab is SysV configuration file where default runlevel can be set, We use CentOS 5 for demonstration :
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#
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# inittab       This file describes how the INIT process should set up
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#               the system in a certain run-level.
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#
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# Author:       Miquel van Smoorenburg, <[email protected]>
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#               Modified for RHS Linux by Marc Ewing and Donnie Barnes
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#
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​
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# Default runlevel. The runlevels used by RHS are:
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#   0 - halt (Do NOT set initdefault to this)
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#   1 - Single user mode
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#   2 - Multiuser, without NFS (The same as 3, if you do not have networking)
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#   3 - Full multiuser mode
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#   4 - unused
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#   5 - X11
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#   6 - reboot (Do NOT set initdefault to this)
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# 
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id:5:initdefault:
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​
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# System initialization.
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si::sysinit:/etc/rc.d/rc.sysinit
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​
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l0:0:wait:/etc/rc.d/rc 0
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l1:1:wait:/etc/rc.d/rc 1
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l2:2:wait:/etc/rc.d/rc 2
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l3:3:wait:/etc/rc.d/rc 3
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l4:4:wait:/etc/rc.d/rc 4
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l5:5:wait:/etc/rc.d/rc 5
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l6:6:wait:/etc/rc.d/rc 6
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​
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# Trap CTRL-ALT-DELETE
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ca::ctrlaltdel:/sbin/shutdown -t3 -r now
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​
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# When our UPS tells us power has failed, assume we have a few minutes
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# of power left.  Schedule a shutdown for 2 minutes from now.
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# This does, of course, assume you have powerd installed and your
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# UPS connected and working correctly.  
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pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down"
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​
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# If power was restored before the shutdown kicked in, cancel it.
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pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled"
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​
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​
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# Run gettys in standard runlevels
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1:2345:respawn:/sbin/mingetty tty1
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2:2345:respawn:/sbin/mingetty tty2
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3:2345:respawn:/sbin/mingetty tty3
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4:2345:respawn:/sbin/mingetty tty4
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5:2345:respawn:/sbin/mingetty tty5
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6:2345:respawn:/sbin/mingetty tty6
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​
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# Run xdm in runlevel 5
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x:5:respawn:/etc/X11/prefdm -nodaemon
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By modifying id:5:initdefault: we can change the run level for the next time boot but there is another proper way :
init / telinit
init and telinit commands are the same.how ever telinit is recommended. They are both used to change current system runlevel
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[[email protected] ~]# telinit 3
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and for come back to previous run level:
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[[email protected] ~]# telinit 5
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and to see previous runlevel and current runlevel use runlevel command:
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[[email protected] ~]# runlevel
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3 5
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as you can see 3 was our previous runlevel and we have switched from it to 5.
/etc/init.d and /etc/rc.d
as we said SysV runs scripts in sequence to start services. But how and where they are managed? its simple but implementation is some how complicated.
all scripts are inside /etc/rc.d/init.d but there are symbolic links to desired rc folder. each rc folder specify one runlevel. so if you want to manually start a service in a run level you can create a symbolic link inside desired rc folder from init.d folder and put a name with sequence for that. "K" for Kill the service and "S" to Start it.
chkconfig
Exploring rc folders, creating symbolic links is a hard job. chkconfig is a great tool which let us turn on or off specific service or services in desired runlevel.lets start:
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[[email protected] ~]# chkconfig 
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chkconfig version 1.3.30.2 - Copyright (C) 1997-2000 Red Hat, Inc.
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This may be freely redistributed under the terms of the GNU Public License.
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​
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usage:   chkconfig --list [name]
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         chkconfig --add <name>
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         chkconfig --del <name>
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         chkconfig [--level <levels>] <name> <on|off|reset|resetpriorities>
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[[email protected] ~]# chkconfig --list
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NetworkManager  0:off   1:off   2:off   3:off   4:off   5:off   6:off
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acpid           0:off   1:off   2:on    3:on    4:on    5:on    6:off
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anacron         0:off   1:off   2:on    3:on    4:on    5:on    6:off
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atd             0:off   1:off   2:off   3:on    4:on    5:on    6:off
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auditd          0:off   1:off   2:on    3:on    4:on    5:on    6:off
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autofs          0:off   1:off   2:off   3:on    4:on    5:on    6:off
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avahi-daemon    0:off   1:off   2:off   3:on    4:on    5:on    6:off
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avahi-dnsconfd  0:off   1:off   2:off   3:off   4:off   5:off   6:off
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bluetooth       0:off   1:off   2:on    3:on    4:on    5:on    6:off
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conman          0:off   1:off   2:off   3:off   4:off   5:off   6:off
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cpuspeed        0:off   1:on    2:on    3:on    4:on    5:on    6:off
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crond           0:off   1:off   2:on    3:on    4:on    5:on    6:off
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cups            0:off   1:off   2:on    3:on    4:on    5:on    6:off
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dnsmasq         0:off   1:off   2:off   3:off   4:off   5:off   6:off
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dund            0:off   1:off   2:off   3:off   4:off   5:off   6:off
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firstboot       0:off   1:off   2:off   3:on    4:off   5:on    6:off
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gpm             0:off   1:off   2:on    3:on    4:on    5:on    6:off
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haldaemon       0:off   1:off   2:off   3:on    4:on    5:on    6:off
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hidd            0:off   1:off   2:on    3:on    4:on    5:on    6:off
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httpd           0:off   1:off   2:off   3:off   4:off   5:off   6:off
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ip6tables       0:off   1:off   2:on    3:on    4:on    5:on    6:off
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ipmi            0:off   1:off   2:off   3:off   4:off   5:off   6:off
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iptables        0:off   1:off   2:on    3:on    4:on    5:on    6:off
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irda            0:off   1:off   2:off   3:off   4:off   5:off   6:off
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irqbalance      0:off   1:off   2:on    3:on    4:on    5:on    6:off
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iscsi           0:off   1:off   2:off   3:on    4:on    5:on    6:off
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iscsid          0:off   1:off   2:off   3:on    4:on    5:on    6:off
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kudzu           0:off   1:off   2:off   3:on    4:on    5:on    6:off
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lvm2-monitor    0:off   1:on    2:on    3:on    4:on    5:on    6:off
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mcstrans        0:off   1:off   2:on    3:on    4:on    5:on    6:off
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mdmonitor       0:off   1:off   2:on    3:on    4:on    5:on    6:off
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mdmpd           0:off   1:off   2:off   3:off   4:off   5:off   6:off
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messagebus      0:off   1:off   2:off   3:on    4:on    5:on    6:off
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microcode_ctl   0:off   1:off   2:on    3:on    4:on    5:on    6:off
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multipathd      0:off   1:off   2:off   3:off   4:off   5:off   6:off
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netconsole      0:off   1:off   2:off   3:off   4:off   5:off   6:off
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netfs           0:off   1:off   2:off   3:on    4:on    5:on    6:off
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netplugd        0:off   1:off   2:off   3:off   4:off   5:off   6:off
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network         0:off   1:off   2:on    3:on    4:on    5:on    6:off
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nfs             0:off   1:off   2:off   3:off   4:off   5:off   6:off
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nfslock         0:off   1:off   2:off   3:on    4:on    5:on    6:off
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nscd            0:off   1:off   2:off   3:off   4:off   5:off   6:off
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ntpd            0:off   1:off   2:off   3:off   4:off   5:off   6:off
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oddjobd         0:off   1:off   2:off   3:off   4:off   5:off   6:off
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pand            0:off   1:off   2:off   3:off   4:off   5:off   6:off
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pcscd           0:off   1:off   2:on    3:on    4:on    5:on    6:off
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portmap         0:off   1:off   2:off   3:on    4:on    5:on    6:off
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psacct          0:off   1:off   2:off   3:off   4:off   5:off   6:off
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rawdevices      0:off   1:off   2:off   3:on    4:on    5:on    6:off
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rdisc           0:off   1:off   2:off   3:off   4:off   5:off   6:off
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readahead_early 0:off   1:off   2:on    3:on    4:on    5:on    6:off
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readahead_later 0:off   1:off   2:off   3:off   4:off   5:on    6:off
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restorecond     0:off   1:off   2:on    3:on    4:on    5:on    6:off
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rpcgssd         0:off   1:off   2:off   3:on    4:on    5:on    6:off
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rpcidmapd       0:off   1:off   2:off   3:on    4:on    5:on    6:off
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rpcsvcgssd      0:off   1:off   2:off   3:off   4:off   5:off   6:off
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saslauthd       0:off   1:off   2:off   3:off   4:off   5:off   6:off
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sendmail        0:off   1:off   2:on    3:on    4:on    5:on    6:off
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smartd          0:off   1:off   2:on    3:on    4:on    5:on    6:off
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sshd            0:off   1:off   2:on    3:on    4:on    5:on    6:off
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svnserve        0:off   1:off   2:off   3:off   4:off   5:off   6:off
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syslog          0:off   1:off   2:on    3:on    4:on    5:on    6:off
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vmware-tools    0:off   1:off   2:on    3:on    4:on    5:on    6:off
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vmware-tools-thinprint  0:off   1:off   2:on    3:on    4:on    5:on    6:off
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vncserver       0:off   1:off   2:off   3:off   4:off   5:off   6:off
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wdaemon         0:off   1:off   2:off   3:off   4:off   5:off   6:off
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wpa_supplicant  0:off   1:off   2:off   3:off   4:off   5:off   6:off
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xfs             0:off   1:off   2:on    3:on    4:on    5:on    6:off
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ypbind          0:off   1:off   2:off   3:off   4:off   5:off   6:off
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yum-updatesd    0:off   1:off   2:on    3:on    4:on    5:on    6:off
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to change see some usefull examples:
command
description
chkconfig <service name> on
start service on any unspecial run level(3,4,5 centOS)
chkconfig <service name> off
turn service off in all runlevels
chkconfig  --list <service name>
List service condition in all unlevels
chkconfig --level  345 <service name> on/off
Turn serve on/off in runlevel 3,4 and 5
update-rc.d
in Debian based systems like ubuntu update-rc.d used as a command instead of chkconfig command
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[email protected]:/etc# update-rc.d
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usage: update-rc.d [-n] [-f] <basename> remove
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       update-rc.d [-n] <basename> defaults [NN | SS KK]
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       update-rc.d [-n] <basename> start|stop NN runlvl [runlvl] [...] .
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       update-rc.d [-n] <basename> disable|enable [S|2|3|4|5]
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        -n: not really
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        -f: force
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​
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The disable|enable API is not stable and might change in the future.
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​
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[email protected]:/etc# update-rc.d apparmor disable
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update-rc.d: warning: apparmor start runlevel arguments (none) do not match LSB Default-Start values (S)
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 Disabling system startup links for /etc/init.d/apparmor ...
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 Removing any system startup links for /etc/init.d/apparmor ...
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   /etc/rc0.d/K63apparmor
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 Adding system startup for /etc/init.d/apparmor ...
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   /etc/rc0.d/K63apparmor -> ../init.d/apparmor
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upstart
The first serious attempt to replace systemV was upstart, created by ubuntu. upstart is reactionary, means it takes events and based on them run jobs. In comparison with SysV upstart is more flexible but still it uses scripts and like SysV has some shortages. Although upstart is backward compatible and lets us to use SysV commands. if your system has /etc/init directory it us using upstart.
upstart keeps all previous SysV Folders and uses it, We use Ubuntu 14 machine :
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[email protected]:/etc# cd /etc/init.d/ && ls
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acpid          dns-clean          procps      single
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anacron        friendly-recovery  pulseaudio  skeleton
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apparmor       grub-common        rc          speech-dispatcher
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apport         halt               rc.local    sudo
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avahi-daemon   irqbalance         rcS         thermald
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bluetooth      kerneloops         README      udev
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brltty         killprocs          reboot      umountfs
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console-setup  kmod               resolvconf  umountnfs.sh
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cron           lightdm            rsync       umountroot
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cups           networking         rsyslog     unattended-upgrades
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cups-browsed   ondemand           saned       urandom
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dbus           pppd-dns           sendsigs    x11-common
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​
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[email protected]:~# cd /etc/rc6.d/ && ls -l
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total 4
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lrwxrwxrwx 1 root root  29 Dec  9 02:07 K10unattended-upgrades -> ../init.d/unattended-upgrades
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lrwxrwxrwx 1 root root  20 Dec  9 02:07 K20kerneloops -> ../init.d/kerneloops
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lrwxrwxrwx 1 root root  15 Dec  9 02:07 K20rsync -> ../init.d/rsync
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lrwxrwxrwx 1 root root  27 Dec  9 02:07 K20speech-dispatcher -> ../init.d/speech-dispatcher
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-rw-r--r-- 1 root root 351 Mar 12  2014 README
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lrwxrwxrwx 1 root root  18 Dec  9 02:07 S20sendsigs -> ../init.d/sendsigs
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lrwxrwxrwx 1 root root  17 Dec  9 02:07 S30urandom -> ../init.d/urandom
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lrwxrwxrwx 1 root root  22 Dec  9 02:07 S31umountnfs.sh -> ../init.d/umountnfs.sh
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lrwxrwxrwx 1 root root  18 Dec  9 02:07 S40umountfs -> ../init.d/umountfs
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lrwxrwxrwx 1 root root  20 Dec  9 02:07 S60umountroot -> ../init.d/umountroot
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lrwxrwxrwx 1 root root  16 Dec  9 02:07 S90reboot -> ../init.d/reboot
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The configuration files of native upstart services are in /etc/init/ directory :
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[email protected]:/etc/init# ls 
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acpid.conf                   mtab.sh.conf
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alsa-restore.conf            networking.conf
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alsa-state.conf              network-interface.conf
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alsa-store.conf              network-interface-container.conf
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anacron.conf                 network-interface-security.conf
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apport.conf                  network-manager.conf
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avahi-cups-reload.conf       passwd.conf
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avahi-daemon.conf            plymouth.conf
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bluetooth.conf               plymouth-log.conf
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bootmisc.sh.conf             plymouth-ready.conf
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checkfs.sh.conf              plymouth-shutdown.conf
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checkroot-bootclean.sh.conf  plymouth-splash.conf
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checkroot.sh.conf            plymouth-stop.conf
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console.conf                 plymouth-upstart-bridge.conf
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console-font.conf            procps.conf
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console-setup.conf           pulseaudio.conf
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container-detect.conf        rc.conf
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control-alt-delete.conf      rcS.conf
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cron.conf                    rc-sysinit.conf
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cups-browsed.conf            resolvconf.conf
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cups.conf                    rfkill-restore.conf
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dbus.conf                    rfkill-store.conf
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dmesg.conf                   rsyslog.conf
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failsafe.conf                setvtrgb.conf
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failsafe-x.conf              shutdown.conf
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flush-early-job-log.conf     startpar-bridge.conf
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friendly-recovery.conf       systemd-logind.conf
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gpu-manager.conf             thermald.conf
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hostname.conf                tty1.conf
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hwclock.conf                 tty2.conf
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hwclock-save.conf            tty3.conf
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irqbalance.conf              tty4.conf
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kmod.conf                    tty5.conf
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lightdm.conf                 tty6.conf
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modemmanager.conf            udev.conf
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mountall-bootclean.sh.conf   udev-fallback-graphics.conf
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mountall.conf                udev-finish.conf
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mountall-net.conf            udevmonitor.conf
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mountall-reboot.conf         udevtrigger.conf
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mountall.sh.conf             ufw.conf
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mountall-shell.conf          upstart-file-bridge.conf
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mountdevsubfs.sh.conf        upstart-socket-bridge.conf
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mounted-debugfs.conf         upstart-udev-bridge.conf
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mounted-dev.conf             ureadahead.conf
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mounted-proc.conf            ureadahead-other.conf
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mounted-run.conf             usb-modeswitch-upstart.conf
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mounted-tmp.conf             vmware-tools.conf
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mounted-var.conf             vmware-tools-thinprint.conf
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mountkernfs.sh.conf          wait-for-state.conf
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mountnfs-bootclean.sh.conf   whoopsie.conf
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as an example let take a look at inside of ufw.conf:
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# ufw - Uncomplicated Firewall
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#
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# The Uncomplicated Firewall is a front-end for iptables, to make managing a
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# Netfilter firewall easier.
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​
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description     "Uncomplicated firewall"
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​
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# Make sure we start before an interface receives traffic
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start on (starting network-interface
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          or starting network-manager
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          or starting networking)
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​
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stop on runlevel [!023456]
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​
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console output
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​
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pre-start exec /lib/ufw/ufw-init start quiet
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post-stop exec /lib/ufw/ufw-init stop
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How upstart keeps backward compatibility and live beside old SysV? the secret is inside /etc/init/rcS.conf :
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# rcS - System V single-user mode compatibility
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#
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# This task handles the old System V-style single-user mode, this is
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# distinct from the other runlevels since running the rc script would
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# be bad.
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​
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description    "System V single-user mode compatibility"
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author        "Scott James Remnant <[email protected]>"
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​
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start on runlevel S
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stop on runlevel [!S]
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​
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console owner
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exec /sbin/sulogin
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​
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post-stop script
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    # Don't switch runlevels if we were stopped by an event, since that
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    # means we're already switching runlevels
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    if [ -n "${UPSTART_STOP_EVENTS}" ]
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    then
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    exit 0
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    fi
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​
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    # Switch, passing a magic flag
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    start --no-wait rc-sysinit FROM_SINGLE_USER_MODE=y
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end script
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bu using rcS.conf upstart can run SysV scripts which haven't been developed for upstart natively .
Systemd
Systemd is used in all modern linuxes. Its a new way of starting linux services but that is not all. The idea of Systemd project is scary! Systemd developers have this idea to create Systemd OS which runs on linux OS. So it seems usual if we know it has its own tool which work like cron, or fstab, rsyslog ... .
In Systemd world we have tragets and unit files, targets is like our goal which we want to reach. But for reaching targets we need to specified what ever is needed to be load in unit files. As Systemd doing variety of thing, there for different types of unit files exists:
service : unit file to start a service considering its dependencies
mount : replace the mount in /etc/fstab
timer : replacement for cron 
automount : mount a directory when needed
target : as we said a bunch of unit files :), target is an end point, tragets can be used as runlevel .
path :observ activities on a path and start a service associated with that
... 
/usr/lib/systemd/system and /etc/systemd/system
Orginal systemd ubit files are in /usr/lib/systemd/system directory but they should not be modified by administrators. Modification should be done in /etc/systemd/system and its good to know that they are linked. Inorde to do any modification we should copy service files from /usr/lib/systemd/system to /etc/systemd/system and then set our settings, we use Ubuntu 16:
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[[email protected] system]# cd /usr/lib/systemd/system && ls 
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abrt-ccpp.service                        plymouth-poweroff.service
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abrtd.service                            plymouth-quit.service
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abrt-oops.service                        plymouth-quit-wait.service
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abrt-pstoreoops.service                  plymouth-read-write.service
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abrt-vmcore.service                      plymouth-reboot.service
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abrt-xorg.service                        plymouth-start.service
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accounts-daemon.service                  plymouth-switch-root.service
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alsa-restore.service                     polkit.service
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alsa-state.service                       postfix.service
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alsa-store.service                       poweroff.target
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anaconda-direct.service                  poweroff.target.wants
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anaconda-nm-config.service               printer.target
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anaconda-noshell.service                 proc-fs-nfsd.mount
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anaconda-pre.service                     proc-sys-fs-binfmt_misc.automount
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anaconda.service                         proc-sys-fs-binfmt_misc.mount
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[email protected]                  psacct.service
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anaconda-sshd.service                    qemu-guest-agent.service
19
anaconda.target                          quotaon.service
20
[email protected]                   radvd.service
21
arp-ethers.service                       rc-local.service
22
atd.service                              rdisc.service
23
auditd.service                           rdma-ndd.service
24
auth-rpcgss-module.service               rdma.service
25
autofs.service                           realmd.service
26
[email protected]                          reboot.target
27
avahi-daemon.service                     reboot.target.wants
28
avahi-daemon.socket                      remote-fs-pre.target
29
basic.target                             remote-fs.target
30
basic.target.wants                       rescue.service
31
blk-availability.service                 rescue.target
32
bluetooth.service                        rescue.target.wants
33
bluetooth.target                         rhel-autorelabel-mark.service
34
brandbot.path                            rhel-autorelabel.service
35
brandbot.service                         rhel-configure.service
36
brltty.service                           rhel-dmesg.service
37
canberra-system-bootup.service           rhel-domainname.service
38
canberra-system-shutdown-reboot.service  rhel-import-state.service
39
canberra-system-shutdown.service         rhel-loadmodules.service
40
certmonger.service                       rhel-readonly.service
41
cgconfig.service                         rngd.service
42
cgdcbxd.service                          rpcbind.service
43
cgred.service                            rpcbind.socket
44
[email protected]                   rpcbind.target
45
[email protected]                     rpc-gssd.service
46
chronyd.service                          rpcgssd.service
47
chrony-wait.service                      rpcidmapd.service
48
colord.service                           rpc-rquotad.service
49
[email protected]               rpc-statd-notify.service
50
console-getty.service                    rpc-statd.service
51
console-shell.service                    rsyncd.service
52
[email protected]                 [email protected]
53
cpupower.service                         rsyncd.socket
54
crond.service                            rsyslog.service
55
cryptsetup-pre.target                    rtkit-daemon.service
56
cryptsetup.target                        runlevel0.target
57
ctrl-alt-del.target                      runlevel1.target
58
cups-browsed.service                     runlevel1.target.wants
59
cups.path                                runlevel2.target
60
cups.service                             runlevel2.target.wants
61
cups.socket                              runlevel3.target
62
dbus-org.freedesktop.hostname1.service   runlevel3.target.wants
63
dbus-org.freedesktop.import1.service     runlevel4.target
64
dbus-org.freedesktop.locale1.service     runlevel4.target.wants
65
dbus-org.freedesktop.login1.service      runlevel5.target
66
dbus-org.freedesktop.machine1.service    runlevel5.target.wants
67
dbus-org.freedesktop.timedate1.service   runlevel6.target
68
dbus.service                             saslauthd.service
69
dbus.socket                              [email protected]
70
dbus.target.wants                        [email protected]
71
debug-shell.service                      shutdown.target
72
default.target                           shutdown.target.wants
73
default.target.wants                     sigpwr.target
74
dev-hugepages.mount                      sleep.target
75
dev-mqueue.mount                         -.slice
76
dm-event.service                         slices.target
77
dm-event.socket                          smartcard.target
78
dmraid-activation.service                smartd.service
79
dnsmasq.service                          sockets.target
80
dracut-cmdline.service                   sockets.target.wants
81
dracut-initqueue.service                 sound.target
82
dracut-mount.service                     speech-dispatcherd.service
83
dracut-pre-mount.service                 spice-vdagentd.service
84
dracut-pre-pivot.service                 spice-vdagentd.target
85
dracut-pre-trigger.service               sshd-keygen.service
86
dracut-pre-udev.service                  sshd.service
87
dracut-shutdown.service                  [email protected]
88
ebtables.service                         sshd.socket
89
emergency.service                        sssd-autofs.service
90
emergency.target                         sssd-autofs.socket
91
fcoe.service                             sssd-nss.service
92
final.target                             sssd-nss.socket
93
firewalld.service                        sssd-pac.service
94
firstboot-graphical.service              sssd-pac.socket
95
flatpak-system-helper.service            sssd-pam-priv.socket
96
fprintd.service                          sssd-pam.service
97
fstrim.service                           sssd-pam.socket
98
fstrim.timer                             sssd-secrets.service
99
gdm.service                              sssd-secrets.socket
100
geoclue.service                          sssd.service
101
[email protected]                           sssd-ssh.service
102
getty.target                             sssd-ssh.socket
103
graphical.target                         sssd-sudo.service
104
graphical.target.wants                   sssd-sudo.socket
105
gssproxy.service                         suspend.target
106
halt-local.service                       svnserve.service
107
halt.target                              swap.target
108
halt.target.wants                        sys-fs-fuse-connections.mount
109
hibernate.target                         sysinit.target
110
hybrid-sleep.target                      sysinit.target.wants
111
hypervfcopyd.service                     sys-kernel-config.mount
112
hypervkvpd.service                       sys-kernel-debug.mount
113
hypervvssd.service                       syslog.socket
114
initial-setup-graphical.service          syslog.target.wants
115
initial-setup-reconfiguration.service    sysstat.service
116
initial-setup.service                    systemd-ask-password-console.path
117
initial-setup-text.service               systemd-ask-password-console.service
118
initrd-cleanup.service                   systemd-ask-password-plymouth.path
119
initrd-fs.target                         systemd-ask-password-plymouth.service
120
initrd-parse-etc.service                 systemd-ask-password-wall.path
121
initrd-root-fs.target                    systemd-ask-password-wall.service
122
initrd-switch-root.service               [email protected]
123
initrd-switch-root.target                systemd-binfmt.service
124
initrd-switch-root.target.wants          systemd-bootchart.service
125
initrd.target                            systemd-firstboot.service
126
initrd.target.wants                      systemd-fsck-root.service
127
initrd-udevadm-cleanup-db.service        [email protected]
128
instperf.service                         systemd-halt.service
129
iprdump.service                          [email protected]
130
iprinit.service                          systemd-hibernate.service
131
iprupdate.service                        systemd-hostnamed.service
132
iprutils.target                          systemd-hwdb-update.service
133
ipsec.service                            systemd-hybrid-sleep.service
134
irqbalance.service                       systemd-importd.service
135
iscsid.service                           systemd-initctl.service
136
iscsid.socket                            systemd-initctl.socket
137
iscsi.service                            systemd-journal-catalog-update.service
138
iscsi-shutdown.service                   systemd-journald.service
139
iscsiuio.service                         systemd-journald.socket
140
iscsiuio.socket                          systemd-journal-flush.service
141
kdump.service                            systemd-kexec.service
142
kexec.target                             systemd-localed.service
143
kexec.target.wants                       systemd-logind.service
144
kmod-static-nodes.service                systemd-machined.service
145
kpatch.service                           systemd-machine-id-commit.service
146
ksm.service                              systemd-modules-load.service
147
ksmtuned.service                         [email protected]
148
libstoragemgmt.service                   systemd-poweroff.service
149
libvirtd.service                         systemd-quotacheck.service
150
lldpad.service                           systemd-random-seed.service
151
lldpad.socket                            systemd-readahead-collect.service
152
local-fs-pre.target                      systemd-readahead-done.service
153
local-fs.target                          systemd-readahead-done.timer
154
local-fs.target.wants                    systemd-readahead-drop.service
155
lvm2-lvmetad.service                     systemd-readahead-replay.service
156
lvm2-lvmetad.socket                      systemd-reboot.service
157
lvm2-lvmpolld.service                    systemd-remount-fs.service
158
lvm2-lvmpolld.socket                     [email protected]
159
lvm2-monitor.service                     systemd-shutdownd.service
160
[email protected]                     systemd-shutdownd.socket
161
machine.slice                            systemd-suspend.service
162
machines.target                          systemd-sysctl.service
163
[email protected]             systemd-timedated.service
164
[email protected]               systemd-tmpfiles-clean.service
165
[email protected]                 systemd-tmpfiles-clean.timer
166
mdmonitor.service                        systemd-tmpfiles-setup-dev.service
167
[email protected]                           systemd-tmpfiles-setup.service
168
messagebus.service                       systemd-udevd-control.socket
169
microcode.service                        systemd-udevd-kernel.socket
170
ModemManager.service                     systemd-udevd.service
171
multipathd.service                       systemd-udev-settle.service
172
multi-user.target                        systemd-udev-trigger.service
173
multi-user.target.wants                  systemd-update-done.service
174
netcf-transaction.service                systemd-update-utmp-runlevel.service
175
NetworkManager-dispatcher.service        systemd-update-utmp.service
176
NetworkManager.service                   systemd-user-sessions.service
177
NetworkManager-wait-online.service       systemd-vconsole-setup.service
178
network-online.target                    system.slice
179
network-online.target.wants              system-update.target
180
network-pre.target                       system-update.target.wants
181
network.target                           target.service
182
nfs-blkmap.service                       tcsd.service
183
nfs-client.target                        [email protected]
184
nfs-config.service                       timers.target
185
nfs-idmapd.service                       timers.target.wants
186
nfs-idmap.service                        time-sync.target
187
nfs-lock.service                         tmp.mount
188
nfslock.service                          tuned.service
189
nfs-mountd.service                       udisks2.service
190
nfs-rquotad.service                      umount.target
191
nfs-secure.service                       upower.service
192
nfs-server.service                       [email protected]
193
nfs.service                              usbmuxd.service
194
nfs-utils.service                        user.slice
195
nss-lookup.target                        var-lib-nfs-rpc_pipefs.mount
196
nss-user-lookup.target                   vgauthd.service
197
ntpdate.service                          virt-guest-shutdown.target
198
ntpd.service                             virtlockd.service
199
numad.service                            virtlockd.socket
200
oddjobd.service                          virtlogd.service
201
packagekit-offline-update.service        virtlogd.socket
202
packagekit.service                       vmtoolsd.service
203
paths.target                             [email protected]
204
plymouth-halt.service                    wpa_supplicant.service
205
plymouth-kexec.service                   zram.service
206
​
207
​
208
​
209
[[email protected] system]# cd /etc/systemd/system/ && ls -l
210
total 4
211
drwxr-xr-x. 2 root root   31 Oct 28 11:21 basic.target.wants
212
drwxr-xr-x. 2 root root   31 Oct 28 11:20 bluetooth.target.wants
213
lrwxrwxrwx. 1 root root   41 Oct 28 11:20 dbus-org.bluez.service -> /usr/lib/systemd/system/bluetooth.service
214
lrwxrwxrwx. 1 root root   41 Oct 28 11:20 dbus-org.fedoraproject.FirewallD1.service -> /usr/lib/systemd/system/firewalld.service
215
lrwxrwxrwx. 1 root root   44 Oct 28 11:21 dbus-org.freedesktop.Avahi.service -> /usr/lib/systemd/system/avahi-daemon.service
216
lrwxrwxrwx. 1 root root   44 Oct 28 11:21 dbus-org.freedesktop.ModemManager1.service -> /usr/lib/systemd/system/ModemManager.service
217
lrwxrwxrwx. 1 root root   46 Oct 28 11:20 dbus-org.freedesktop.NetworkManager.service -> /usr/lib/systemd/system/NetworkManager.service
218
lrwxrwxrwx. 1 root root   57 Oct 28 11:20 dbus-org.freedesktop.nm-dispatcher.service -> /usr/lib/systemd/system/NetworkManager-dispatcher.service
219
lrwxrwxrwx. 1 root root   36 Oct 28 11:25 default.target -> /lib/systemd/system/graphical.target
220
drwxr-xr-x. 2 root root   87 Oct 28 11:20 default.target.wants
221
drwxr-xr-x. 2 root root   38 Oct 28 11:21 dev-virtio\x2dports-org.qemu.guest_agent.0.device.wants
222
lrwxrwxrwx. 1 root root   35 Oct 28 11:21 display-manager.service -> /usr/lib/systemd/system/gdm.service
223
drwxr-xr-x. 2 root root   32 Oct 28 11:20 getty.target.wants
224
drwxr-xr-x. 2 root root   65 Oct 28 11:27 graphical.target.wants
225
drwxr-xr-x. 2 root root 4096 Oct 28 11:27 multi-user.target.wants
226
drwxr-xr-x. 2 root root   26 Oct 28 11:21 printer.target.wants
227
drwxr-xr-x. 2 root root   31 Oct 28 11:20 remote-fs.target.wants
228
drwxr-xr-x. 2 root root  188 Oct 28 11:21 sockets.target.wants
229
drwxr-xr-x. 2 root root   36 Oct 28 11:21 spice-vdagentd.target.wants
230
drwxr-xr-x. 2 root root   26 Oct 28 11:20 sssd.service.d
231
drwxr-xr-x. 2 root root  169 Oct 28 11:20 sysinit.target.wants
232
drwxr-xr-x. 2 root root   44 Oct 28 11:20 system-update.target.wants
233
drwxr-xr-x. 2 root root   29 Oct 28 11:20 vmtoolsd.service.requires
Copied!
As you have probably seen Systemd works with different dependencies:
Dependency
Description
Requires
define unit files must beloaded, if not load it
wants
[seen in targets], specify unit files which must be loaded but if not, takes it easy and continue
requisite
must be already loaded, if not loaded fails
confilicts
unit files which never be activated with current unit file
before
current unit files activated before listed unit files
after
current unit file activated after listed unit file
systemctl
systemctl is systemd magic command to work with services:
systemctl useful commands
​
systemctl start <ServiceName>
to start a service
systemctl stop <ServiceName>
to stop
systemctl disable <ServiceName>
disable it, won't be activated even after reboot
systemctl enable <ServiceName>
enable a service
systemctl relaod <ServiceName>
reload a service by reading its conf file,[might not work]
systemctl restart <ServiceName>
stop and start a service
systemctl list-unit-files --type=service
same as chkconf --list but in systemd environment
systemctl daemon-reload
Reload systemd Daemon, used after unit files modification
systemctl list-depencencies
List targets and services dependencies
For the History:
Distro
Pre 2006
2006 (14.04)-2019
2015(15.10)-????
ubuntu
SysV
Upstart
Systemd
​
2007
2011-2020
2014-????
centOS
SysV (centOS 5)
Upstart (centOS6)
systemd (centOS 7)
.
.
.
201.3. Kernel runtime management and troubleshooting
202.2. System Recovery
Last modified 2yr ago
Copy link
Contents
202.1 Customizing SysV-init system startup
Over View
Sys v
init / telinit
/etc/init.d and /etc/rc.d
/usr/lib/systemd/system and /etc/systemd/system
systemctl
For the History: