101.3. Change runlevels / boot targets and shutdown or reboot system

101.3 Change runlevels / boot targets and shutdown or reboot system
Weight:3
Description: Candidates should be able to manage the SysVinit runlevel or systemd boot target of the system. This objective includes changing to single user mode, shutdown or rebooting the system. Candidates should be able to alert users before switching runlevels / boot targets and properly terminate processes. This objective also includes setting the default SysVinit runlevel or systemd boot target. It also includes awareness of Upstart as an alternative to SysVinit or systemd.
Key Knowledge Areas:
Set the default runlevel or boot target
Change between runlevels / boot targets including single user mode
Shutdown and reboot from the command line
Alert users before switching runlevels / boot targets or other major system events
Properly terminate processes
Terms and Utilities:
/etc/inittab
shutdown
init
/etc/init.d/
telinit
systemd
systemctl
/etc/systemd/
/usr/lib/systemd/
wall
In previous lesson we explain the sequence of system boot in liunx, then we get introduced to Sysv, Upstart and system as different service managers in linux world. In this lesson we learn how to take control over our linux system services using these service managers.
Lets start with SysV. SysV is is nothing more than many executable scripts which are run after init process. How we can define which service should be run when computer starts ? I'm glad you asked. It seems that huge text configuration files would be needed, there is where runlevels come to play.
runlevels
A runlevel is one of the modes that a Unix -based operating system will run in. Each runlevel has a certain number of services stopped or started, giving the user control over the behavior of the machine. runlevels avoid of having a few large files to edit by hand.
Conventionally, seven runlevels exist, numbered from zero to six. And there are some differences between Debian and RedHat based systems:
Runlevel
Debian
RedHat
0
Halt
Halt
1
Single User Mode
Single User Mode
2
Full,Multi-User,GUI
Multi-User, No Net
3
Nothing
Multi-User, with Net, No GUI
4
Nothing
Not used
5
Nothing
Full,Multi-User,GUI
6
Reboot
Reboot
CentsOS 5 was the last version which used SysV and from CentOS 7 Systemd is used.
runlevel
To find the current and previous runlevels , the runlevel command is used:
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[[email protected] ~]# runlevel
2
N 5
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In the above output, the letter ‘N’ indicates that the runlevel has not been changed since the system was booted. And, 5 is the current runlevel.
Now that we now about runlevels, How we can switch between them? well just tell 'init' what runlevel you like.
telinit
telinit is used to change the SysV system runlevel.
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telinit  RUNLEVEL
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Lets try it on a CentOS5 machine:
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[[email protected] ~]# runlevel
2
N 5
3
[[email protected] ~]# telinit 3
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and system goes to runlevel 3 lets go back to runlevel 5:
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[[email protected] ~]# telinit 5
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init
init (as a command) is a process control initialization like telinit.
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[[email protected] ~]# init 3
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telinit vs init
telinit is a smaller tool that informs init when it needs to switch runlevels. So we can use "telinit" to "tell init" that it needs to switch runlevel. telinit is actually linked to init command and it is possible to use init command instead but it is not recommanded.
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[[email protected] ~]# ls -l /sbin/telinit 
2
lrwxrwxrwx 1 root root 4 Aug 26 23:20 /sbin/telinit -> init
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There are several ways to change runlevels. To make a permanent change, we can edit /etc/inittab and change the default level that we just saw above.
/etc/inttab
After the Linux kernel has booted, the init program reads the /etc/inittab file to determine the behavior for each runlevel. Unless the user specifies another value as a kernel boot parameter, the system will attempt to enter (start) the default runlevel.(CentOs5)
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[[email protected] ~]# cat /etc/inittab 
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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.
5
#
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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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#
9
​
10
# 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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​
24
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
31
​
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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"
40
​
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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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​
44
​
45
# 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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​
53
# Run xdm in runlevel 5
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x:5:respawn:/etc/X11/prefdm -nodaemon
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the default runlevel is determined from theid:entry in /etc/inittab. How run levels are set up by default and how they are configured depends in part on the particular distribution you are running.
The format of each line in inittab file is as follows:
id:runlevel:action:process
Here is a description of these fields:
id (identification code) – consists of a sequence of one to four characters that identifies its function.
runlevels – lists the run levels to which this entry applies.
action – specific codes in this field tell init how to treat the process. Possible values include: initdefault, sysinit, boot, bootwait, wait, and respawn.
process – defines the command or script to execute.
Now lets see how SysV implements the concept of run levels.
/etc/init.d
/etc/init.d contains scripts used by the System V init tools (SysVinit).
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[[email protected] ~]# ls  /etc/init.d/
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acpid               hidd           netconsole       rpcidmapd
3
anacron             httpd          netfs            rpcsvcgssd
4
atd                 ip6tables      netplugd         saslauthd
5
auditd              ipmi           network          sendmail
6
autofs              iptables       NetworkManager   single
7
avahi-daemon        irda           nfs              smartd
8
avahi-dnsconfd      irqbalance     nfslock          sshd
9
bluetooth           iscsi          nscd             svnserve
10
conman              iscsid         ntpd             syslog
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cpuspeed            killall        oddjobd          vmware-tools
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crond               krb524         pand             vmware-tools-thinprint
13
cups                kudzu          pcscd            vncserver
14
cups-config-daemon  ldap           portmap          wdaemon
15
dnsmasq             lvm2-monitor   psacct           wpa_supplicant
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dund                mcstrans       rawdevices       xfs
17
firstboot           mdmonitor      rdisc            ypbind
18
functions           mdmpd          readahead_early  yum-updatesd
19
gpm                 messagebus     readahead_later
20
haldaemon           microcode_ctl  restorecond
21
halt                multipathd     rpcgssd
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As we said, in SysV, init program is the first process that is run and consequently some infrastructure services are started. Files in /etc/init.d are shell scripts that respond to start, stop, restart, and (when supported) reload commands to manage a particular service. But how SysV determine which services inside /etc/init.d should be started or stopped depend on default runlevel ?Lets draw a picture:
/etc/rc.d/
SysV uses grouping. Scripts of each runlevel are grouped and placed in /etc/rc{runlevel}.d/ where runlevel is the runlevel. 
As many services might be existed in different runlevels, the real script files are hold in /etc/init.d and /etc/rc{runlevel}.d/just point to required ones.
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[[email protected] etc]# ls | grep rc.
2
rc0.d
3
rc1.d
4
rc2.d
5
rc3.d
6
rc4.d
7
rc5.d
8
rc6.d
9
rc.d
10
rc.local
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rc.sysinit
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Lets take a look at rc.5 for example:
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[[email protected] etc]# ls -l rc5.d/
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total 288
3
lrwxrwxrwx 1 root root 17 Aug 26 23:21 K01dnsmasq -> ../init.d/dnsmasq
4
lrwxrwxrwx 1 root root 24 Aug 26 23:21 K02avahi-dnsconfd -> ../init.d/avahi-dnsconfd
5
lrwxrwxrwx 1 root root 24 Aug 26 23:23 K02NetworkManager -> ../init.d/NetworkManager
6
lrwxrwxrwx 1 root root 17 Aug 26 23:23 K02oddjobd -> ../init.d/oddjobd
7
lrwxrwxrwx 1 root root 16 Aug 26 23:20 K05conman -> ../init.d/conman
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lrwxrwxrwx 1 root root 19 Aug 26 23:21 K05saslauthd -> ../init.d/saslauthd
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lrwxrwxrwx 1 root root 17 Aug 26 23:20 K05wdaemon -> ../init.d/wdaemon
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lrwxrwxrwx 1 root root 16 Aug 26 23:20 K10psacct -> ../init.d/psacct
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lrwxrwxrwx 1 root root 15 Aug 26 23:21 K15httpd -> ../init.d/httpd
12
lrwxrwxrwx 1 root root 18 Aug 26 23:22 K15svnserve -> ../init.d/svnserve
13
lrwxrwxrwx 1 root root 13 Aug 26 23:23 K20nfs -> ../init.d/nfs
14
lrwxrwxrwx 1 root root 14 Aug 26 23:23 K24irda -> ../init.d/irda
15
<output has been truncated>
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lrwxrwxrwx 1 root root 23 Aug 26 23:20 S00microcode_ctl -> ../init.d/microcode_ctl
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lrwxrwxrwx 1 root root 22 Aug 26 23:28 S03vmware-tools -> ../init.d/vmware-tools
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lrwxrwxrwx 1 root root 25 Aug 26 23:20 S04readahead_early -> ../init.d/readahead_early
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lrwxrwxrwx 1 root root 15 Aug 26 23:23 S05kudzu -> ../init.d/kudzu
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lrwxrwxrwx 1 root root 16 Aug 26 23:20 S07iscsid -> ../init.d/iscsid
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lrwxrwxrwx 1 root root 19 Aug 26 23:19 S08ip6tables -> ../init.d/ip6tables
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lrwxrwxrwx 1 root root 18 Aug 26 23:19 S08iptables -> ../init.d/iptables
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lrwxrwxrwx 1 root root 18 Aug 26 23:21 S08mcstrans -> ../init.d/mcstrans
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lrwxrwxrwx 1 root root 17 Aug 26 23:21 S10network -> ../init.d/network
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lrwxrwxrwx 1 root root 16 Aug 26 23:20 S11auditd -> ../init.d/auditd
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lrwxrwxrwx 1 root root 21 Aug 26 23:21 S12restorecond -> ../init.d/restorecond
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lrwxrwxrwx 1 root root 16 Aug 26 23:21 S12syslog -> ../init.d/syslog
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lrwxrwxrwx 1 root root 18 Aug 26 23:19 S13cpuspeed -> ../init.d/cpuspeed
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lrwxrwxrwx 1 root root 20 Aug 26 23:21 S13irqbalance -> ../init.d/irqbalance
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lrwxrwxrwx 1 root root 15 Aug 26 23:20 S13iscsi -> ../init.d/iscsi
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lrwxrwxrwx 1 root root 17 Aug 26 23:21 S13portmap -> ../init.d/portmap
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lrwxrwxrwx 1 root root 17 Aug 26 23:23 S14nfslock -> ../init.d/nfslock
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lrwxrwxrwx 1 root root 19 Aug 26 23:21 S15mdmonitor -> ../init.d/mdmonitor
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<output has been truncated>
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Each script in each runlevel is run with its startup or shutdown functions depending on if that runlevel is going up or going down. S means starting script and K shows that it is a killing script .The sequence of actions is defined by the numbers. try cat command to see what is inside :
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[[email protected] rc5.d]# cat S55sshd
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#!/bin/bash
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#
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# Init file for OpenSSH server daemon
5
#
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# chkconfig: 2345 55 25
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# description: OpenSSH server daemon
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#
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# processname: sshd
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# config: /etc/ssh/ssh_host_key
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# config: /etc/ssh/ssh_host_key.pub
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# config: /etc/ssh/ssh_random_seed
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# config: /etc/ssh/sshd_config
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# pidfile: /var/run/sshd.pid
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​
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# source function library
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. /etc/rc.d/init.d/functions
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​
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# pull in sysconfig settings
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[ -f /etc/sysconfig/sshd ] && . /etc/sysconfig/sshd
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​
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RETVAL=0
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prog="sshd"
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​
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# Some functions to make the below more readable
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KEYGEN=/usr/bin/ssh-keygen
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SSHD=/usr/sbin/sshd
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RSA1_KEY=/etc/ssh/ssh_host_key
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RSA_KEY=/etc/ssh/ssh_host_rsa_key
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DSA_KEY=/etc/ssh/ssh_host_dsa_key
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PID_FILE=/var/run/sshd.pid
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​
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runlevel=$(set -- $(runlevel); eval "echo \$#" )
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​
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do_rsa1_keygen() {
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        if [ ! -s $RSA1_KEY ]; then
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                echo -n quot;Generating SSH1 RSA host key: "
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                rm -f $RSA1_KEY
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                if $KEYGEN -q -t rsa1 -f $RSA1_KEY -C '' -N '' >&/dev/null; then
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                        chmod 600 $RSA1_KEY
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                        chmod 644 $RSA1_KEY.pub
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                        if [ -x /sbin/restorecon ]; then
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                            /sbin/restorecon $RSA1_KEY.pub
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                        fi
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                        success quot;RSA1 key generation"
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                        echo
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                else
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                        failure quot;RSA1 key generation"
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                        echo
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                        exit 1
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                fi
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        fi
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}
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​
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do_rsa_keygen() {
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        if [ ! -s $RSA_KEY ]; then
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                echo -n quot;Generating SSH2 RSA host key: "
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                rm -f $RSA_KEY
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                if $KEYGEN -q -t rsa -f $RSA_KEY -C '' -N '' >&/dev/null; then
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                        chmod 600 $RSA_KEY
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                        chmod 644 $RSA_KEY.pub
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                        if [ -x /sbin/restorecon ]; then
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                            /sbin/restorecon $RSA_KEY.pub
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                        fi
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                        success quot;RSA key generation"
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                        echo
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                else
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                        failure quot;RSA key generation"
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                        echo
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                        exit 1
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                fi
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        fi
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}
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​
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do_dsa_keygen() {
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        if [ ! -s $DSA_KEY ]; then
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                echo -n quot;Generating SSH2 DSA host key: "
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                rm -f $DSA_KEY
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                if $KEYGEN -q -t dsa -f $DSA_KEY -C '' -N '' >&/dev/null; then
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                        chmod 600 $DSA_KEY
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                        chmod 644 $DSA_KEY.pub
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                        if [ -x /sbin/restorecon ]; then
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                            /sbin/restorecon $DSA_KEY.pub
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                        fi
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                        success quot;DSA key generation"
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                        echo
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                else
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                        failure quot;DSA key generation"
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                        echo
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                        exit 1
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                fi
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        fi
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}
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​
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do_restart_sanity_check()
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{
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        $SSHD -t
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        RETVAL=$?
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        if [ ! "$RETVAL" = 0 ]; then
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                failure quot;Configuration file or keys are invalid"
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                echo
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        fi
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}
104
​
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start()
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{
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        # Create keys if necessary
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        do_rsa1_keygen
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        do_rsa_keygen
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        do_dsa_keygen
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​
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        cp -af /etc/localtime /var/empty/sshd/etc
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​
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        echo -n quot;Starting $prog: "
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        $SSHD $OPTIONS && success || failure
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        RETVAL=$?
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        [ "$RETVAL" = 0 ] && touch /var/lock/subsys/sshd
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        echo
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}
120
​
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stop()
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{
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        echo -n quot;Stopping $prog: "
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        if [ -n "`pidfileofproc $SSHD`" ] ; then
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            killproc $SSHD
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        else
127
            failure quot;Stopping $prog"
128
        fi
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        RETVAL=$?
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        # if we are in halt or reboot runlevel kill all running sessions
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        # so the TCP connections are closed cleanly
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        if [ "x$runlevel" = x0 -o "x$runlevel" = x6 ] ; then
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            killall $prog 2>/dev/null
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        fi
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        [ "$RETVAL" = 0 ] && rm -f /var/lock/subsys/sshd
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        echo
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}
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​
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reload()
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{
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        echo -n quot;Reloading $prog: "
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        if [ -n "`pidfileofproc $SSHD`" ] ; then
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            killproc $SSHD -HUP
144
        else
145
            failure quot;Reloading $prog"
146
        fi
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        RETVAL=$?
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        echo
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}
150
​
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case "$1" in
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        start)
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                start
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                ;;
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        stop)
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                stop
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                ;;
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        restart)
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                stop
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                start
161
                ;;
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        reload)
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                reload
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                ;;
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        condrestart)
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                if [ -f /var/lock/subsys/sshd ] ; then
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                        do_restart_sanity_check
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                        if [ "$RETVAL" = 0 ] ; then
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                                stop
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                                # avoid race
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                                sleep 3
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                                start
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                        fi
174
                fi
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                ;;
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        status)
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                status -p $PID_FILE openssh-daemon
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                RETVAL=$?
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                ;;
180
        *)
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                echo quot;Usage: $0 {start|stop|restart|reload|condrestart|status}"
182
                RETVAL=1
183
esac
184
exit $RETVAL
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And all of these places were places that SysV uses to manage scripts and runlevels. In Sysv system we can manage services with servicecommand andchkconfigcommand to define how and when services are started. We will talk about them later.
rc.local
But what about /etc/rc.local ? This file runs after all other init level scripts have run, so it's safe to put various commands that you want to have issued upon startup.This is also a good place to place "troubleshooting" scripts in. But do not forget rc.local may not work properly in Upstart and Systemd. Test it, search it and do required configuration before using in production environment.
Lets go back to our topic and get familiar with runlevels equivalent in systemd. Although /etc/inittab still exist in Systemd systems but that is not part of configuration, we have something called "Targets".
systemd targets
Like runlevels there are some modes in systemd system that our system can run in, systemd runlevels are referred to as "targets". "targets" are described as a collection of services. Look at the equivalents: 
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   ┌─────────┬───────────────────┐
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   │Runlevel │ Target            │
3
   ├─────────┼───────────────────┤
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   │0        │ poweroff.target   │
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   ├─────────┼───────────────────┤
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   │1        │ rescue.target     │
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   ├─────────┼───────────────────┤
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   │2, 3, 4  │ multi-user.target │
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   ├─────────┼───────────────────┤
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   │5        │ graphical.target  │
11
   ├─────────┼───────────────────┤
12
   │6        │ reboot.target     │
13
   └─────────┴───────────────────┘
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In order to switch between Boot Targets we use systemctl tool.
systemctl
Systemctl is a systemd utility which is responsible for Controlling the systemd system and service manager. It can does lots of thing but what we need here and use it for is changing boot targets.
To see the current boot target use systemctl get-default command:
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[[email protected] ~]# systemctl get-default 
2
graphical.target
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To change the target use systemctl isolate xxxxx.target , for example:
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[[email protected] ~]# systemctl isolate rescue.target
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1
[[email protected] ~]# systemctl isolate multi-user.target
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To set the default target, run systemctl set-default xxxxxx.target command :
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[[email protected] ~]# systemctl set-default graphical.target 
2
Removed symlink /etc/systemd/system/default.target.
3
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/graphical.target.
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But how systemd knows what to do and how to do things ? We have something called "Unit". The concept of "Unit Files" replaces the SysV init scripts for services.
systemd unit files
There are different types of unit files and the best way to describe unit files, is 'that is a thing which should be started'.Yes that is a thing because there are different kinds of unit files. Each unit file is a simple text file describing a unit, what it does, what needs to run before or afterward, and other details
Unit files can be stored in a few different places on your system. systemd looks for system unit files in this order:
1.
/etc/systemd/system:directory stores unit files that extend a service. This directory will take precedence over unit files located anywhere else in the system.
2.
/run/systemd/system:directory is the runtime location for unit files.
3.
/usr/lib/systemd/system:directory is the default location where unit files are installed by packages. Unit files in the default directory should not be altered.
Unit files in the earlier directories override later ones. Lets take a look at them:
1./etc/systemd/system
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[[email protected] ~]# ls -l /etc/systemd/system
2
total 4
3
drwxr-xr-x. 2 root root   31 Oct 28  2017 basic.target.wants
4
drwxr-xr-x. 2 root root   31 Oct 28  2017 bluetooth.target.wants
5
lrwxrwxrwx. 1 root root   41 Oct 28  2017 dbus-org.bluez.service -> /usr/lib/systemd/system/bluetooth.service
6
lrwxrwxrwx. 1 root root   41 Oct 28  2017 dbus-org.fedoraproject.FirewallD1.service -> /usr/lib/systemd/system/firewalld.service
7
lrwxrwxrwx. 1 root root   44 Oct 28  2017 dbus-org.freedesktop.Avahi.service -> /usr/lib/systemd/system/avahi-daemon.service
8
lrwxrwxrwx. 1 root root   44 Oct 28  2017 dbus-org.freedesktop.ModemManager1.service -> /usr/lib/systemd/system/ModemManager.service
9
lrwxrwxrwx. 1 root root   46 Oct 28  2017 dbus-org.freedesktop.NetworkManager.service -> /usr/lib/systemd/system/NetworkManager.service
10
lrwxrwxrwx. 1 root root   57 Oct 28  2017 dbus-org.freedesktop.nm-dispatcher.service -> /usr/lib/systemd/system/NetworkManager-dispatcher.service
11
lrwxrwxrwx. 1 root root   36 Oct 28  2017 default.target -> /lib/systemd/system/graphical.target
12
drwxr-xr-x. 2 root root   87 Oct 28  2017 default.target.wants
13
drwxr-xr-x. 2 root root   38 Oct 28  2017 dev-virtio\x2dports-org.qemu.guest_agent.0.device.wants
14
lrwxrwxrwx. 1 root root   35 Oct 28  2017 display-manager.service -> /usr/lib/systemd/system/gdm.service
15
drwxr-xr-x. 2 root root   32 Oct 28  2017 getty.target.wants
16
drwxr-xr-x. 2 root root   65 Oct 28  2017 graphical.target.wants
17
drwxr-xr-x. 2 root root 4096 Oct 28  2017 multi-user.target.wants
18
drwxr-xr-x. 2 root root   26 Oct 28  2017 printer.target.wants
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drwxr-xr-x. 2 root root   31 Oct 28  2017 remote-fs.target.wants
20
drwxr-xr-x. 2 root root  188 Oct 28  2017 sockets.target.wants
21
drwxr-xr-x. 2 root root   36 Oct 28  2017 spice-vdagentd.target.wants
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drwxr-xr-x. 2 root root   26 Oct 28  2017 sssd.service.d
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drwxr-xr-x. 2 root root  169 Oct 28  2017 sysinit.target.wants
24
drwxr-xr-x. 2 root root   44 Oct 28  2017 system-update.target.wants
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drwxr-xr-x. 2 root root   29 Oct 28  2017 vmtoolsd.service.requires
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2./run/systemd/system 
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[[email protected] ~]# ls -l /run/systemd/system/
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total 8
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-rw-r--r-- 1 root root  17 Jan  2  2019 session-1.scope
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drwxr-xr-x 2 root root 160 Jan  2  2019 session-1.scope.d
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-rw-r--r-- 1 root root  17 Jan  2  2019 user-1000.slice
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drwxr-xr-x 2 root root 120 Jan  2  2019 user-1000.slice.d
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3./usr/lib/systemd/system
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[[email protected] ~]# ls  /usr/lib/systemd/system
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​
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<output has been trancuated>
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​
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​
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cryptsetup-pre.target                    runlevel0.target
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cryptsetup.target                        runlevel1.target
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ctrl-alt-del.target                      runlevel1.target.wants
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cups-browsed.service                     runlevel2.target
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cups.path                                runlevel2.target.wants
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cups.service                             runlevel3.target
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cups.socket                              runlevel3.target.wants
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dbus-org.freedesktop.hostname1.service   runlevel4.target
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dbus-org.freedesktop.import1.service     runlevel4.target.wants
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dbus-org.freedesktop.locale1.service     runlevel5.target
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dbus-org.freedesktop.login1.service      runlevel5.target.wants
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dbus-org.freedesktop.machine1.service    runlevel6.target
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​
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<output has been trancuated>
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​
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nfs-rquotad.service                      umount.target
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nfs-secure.service                       upower.service
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nfs-server.service                       [email protected]
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nfs.service                              usbmuxd.service
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nfs-utils.service                        user.slice
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nss-lookup.target                        var-lib-nfs-rpc_pipefs.mount
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nss-user-lookup.target                   vgauthd.service
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ntpdate.service                          virt-guest-shutdown.target
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ntpd.service                             virtlockd.service
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numad.service                            virtlockd.socket
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oddjobd.service                          virtlogd.service
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packagekit-offline-update.service        virtlogd.socket
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packagekit.service                       vmtoolsd.service
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paths.target                             [email protected]
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plymouth-halt.service                    wpa_supplicant.service
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plymouth-kexec.service                   zram.service
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plymouth-poweroff.service
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lets follow one of target unit files.Try ls -al *.target to see all of target files. As an instance default.target:
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[[email protected] ~]# ls -al /usr/lib/systemd/system/default.target
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lrwxrwxrwx. 1 root root 16 Oct 28  2017 /usr/lib/systemd/system/default.target -> graphical.target
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Lets take a look at that graphical.target:
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[[email protected] ~]# cat  /usr/lib/systemd/system/graphical.target
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#  This file is part of systemd.
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#
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#  systemd is free software; you can redistribute it and/or modify it
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#  under the terms of the GNU Lesser General Public License as published by
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#  the Free Software Foundation; either version 2.1 of the License, or
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#  (at your option) any later version.
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​
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[Unit]
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Description=Graphical Interface
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Documentation=man:systemd.special(7)
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Requires=multi-user.target
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Wants=display-manager.service
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Conflicts=rescue.service rescue.target
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After=multi-user.target rescue.service rescue.target display-manager.service
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AllowIsolate=yes
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And what it requires is multi-user.target, lets see:
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[[email protected] ~]# cat  /usr/lib/systemd/system/multi-user.target
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#  This file is part of systemd.
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#
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#  systemd is free software; you can redistribute it and/or modify it
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#  under the terms of the GNU Lesser General Public License as published by
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#  the Free Software Foundation; either version 2.1 of the License, or
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#  (at your option) any later version.
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​
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[Unit]
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Description=Multi-User System
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Documentation=man:systemd.special(7)
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Requires=basic.target
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Conflicts=rescue.service rescue.target
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After=basic.target rescue.service rescue.target
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AllowIsolate=yes
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and it requires basic.target :
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[[email protected] ~]# cat  /usr/lib/systemd/system/basic.target
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#  This file is part of systemd.
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#
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#  systemd is free software; you can redistribute it and/or modify it
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#  under the terms of the GNU Lesser General Public License as published by
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#  the Free Software Foundation; either version 2.1 of the License, or
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#  (at your option) any later version.
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​
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[Unit]
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Description=Basic System
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Documentation=man:systemd.special(7)
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​
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Requires=sysinit.target
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After=sysinit.target
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Wants=sockets.target timers.target paths.target slices.target
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After=sockets.target paths.target slices.target
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Finally sysinit.target :
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[[email protected] ~]# cat  /usr/lib/systemd/system/sysinit.target
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#  This file is part of systemd.
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#
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#  systemd is free software; you can redistribute it and/or modify it
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#  under the terms of the GNU Lesser General Public License as published by
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#  the Free Software Foundation; either version 2.1 of the License, or
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#  (at your option) any later version.
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​
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[Unit]
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Description=System Initialization
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Documentation=man:systemd.special(7)
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Conflicts=emergency.service emergency.target
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Wants=local-fs.target swap.target
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After=local-fs.target swap.target emergency.service emergency.target
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And sysint.target does not require any thing. It seems that we have reached top level target in our tree and we can go back.
Using unit files beside Targets concept make Systemd more flexible in comparison to SysV.
wall
There are times when multiple users are logged in to a server computer, and we need to, say, restart the server to perform some maintenance task. Of course, the correct way is to inform all those who are logged in about the maintenance activity.
wall (an abbreviation of write to all) is a Unix command-line utility that displays the contents of a file or standard input to all logged-in users. It is typically used by root to send out shutting down message to all users just before poweroff.(Ubuntu16)
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wall [-n] [-t timeout] [-g group] [message | file]
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1
OPTIONS         
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       -n, --nobanner
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              Suppress the banner.
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​
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       -t, --timeout timeout
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              Abandon the write attempt to the terminals after timeout
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              seconds.  This timeout must be a positive integer.  The
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              default value is 300 seconds, which is a legacy from the time
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              when people ran terminals over modem lines.
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​
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       -g, --group group
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              Limit printing message to members of group defined as a group
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              argument.  The argument can be group name or GID.
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​
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       -V, --version
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              Display version information and exit.
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​
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       -h, --help
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              Display help text and exit.
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Lets try sending out "we are going down" message:
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[[email protected] ~]# wall "we are going down"
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​
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Broadcast message from [email protected] (pts/1) (Sun Nov 11 00:46:58 2018):
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​
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we are going down
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and what will user1 receive:
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[[email protected] ~]$ 
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Broadcast message from [email protected] (pts/1) (Sun Nov 11 00:46:58 2018):
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​
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we are going down
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options might not work is some old distributions.
shutdown
shutdown - Halt, power-off or reboot the machine
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shutdown [OPTIONS...] [TIME] [WALL...]
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OPTIONS         
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       The following options are understood:
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​
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       --help
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           Print a short help text and exit.
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​
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       -H, --halt
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           Halt the machine.
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​
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       -P, --poweroff
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           Power-off the machine (the default).
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​
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       -r, --reboot
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           Reboot the machine.
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​
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       -h
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           Equivalent to --poweroff, unless --halt is specified.
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​
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       -k
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           Do not halt, power-off, reboot, just write wall message.
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​
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       --no-wall
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           Do not send wall message before halt, power-off, reboot.
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​
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       -c
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           Cancel a pending shutdown. This may be used cancel the effect of
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           an invocation of shutdown with a time argument that is not "+0"
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           or "now".
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       -t sec
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           Tell init to wait sec seconds between sending processes the warning 
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           and the kill signal, before changing to another runlevel.
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[TIME]
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           The time argument specifies when to perform the shutdown operation.
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​
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[WALL]        
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           A message to be sent to all users, along with the standard shutdown notification.
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shutdown does its job by signalling the init process, asking it to change the runlevel. According to previous title, runlevel 0 is used to halt the system, runlevel 6 is used to reboot the system, and runlevel 1 is used to put the system into a state where administrative tasks can be performed (single-user mode).
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[email protected]:~# shutdown -r -t 15 +5 "Server will restart in 5 minutes. Please save your work."
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Shutdown scheduled for Sun 2018-11-11 03:19:56 PST, use 'shutdown -c' to cancel.
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[email protected]:~# shutdown -c
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We may specify a time string (which is usually “now” or “hh:mm” for hour/minutes):
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[email protected]:~# shutdown -r 10:10 "Server will be rebooted at 10:10am"
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Shutdown scheduled for Tue 2018-11-13 10:10:00 PST, use 'shutdown -c' to cancel.
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[email protected]:~# shutdown -c
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​
halt vs poweroff ! it's a bit historical
halt was used before ACPI (Advanced Configuration and Power Interface)which today will turn off the power for us. It would halt the system and then print a message to the effect of "it's ok to power off now". Back then there were physical on/off switches, rather than the combo ACPI controlled power button of modern computers.
poweroff, naturally will halt the system and then call ACPI power off.
These days halt is smart enough to automatically call poweroff if ACPI is enabled. In fact, they are functionally equivalent now.
reboot
reboot command can be used to shutdown or reboot linux.
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[[email protected] ~]# reboot --help
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reboot [OPTIONS...] [ARG]
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​
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Reboot the system.
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​
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     --help      Show this help
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     --halt      Halt the machine
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  -p --poweroff  Switch off the machine
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     --reboot    Reboot the machine
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  -f --force     Force immediate halt/power-off/reboot
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  -w --wtmp-only Don't halt/power-off/reboot, just write wtmp record
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  -d --no-wtmp   Don't write wtmp record
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     --no-wall   Don't send wall message before halt/power-off/reboot
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To reboot linux just call the reboot command directly without any options.
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[[email protected] ~]# reboot
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This will perform a graceful shutdown and restart of the machine. This is what happens when we click restart from your menu.
-f option will forcefully reboot the machine. This is similar to pressing the power button of the CPU. No shutdown takes place. The system will reset instantly.
halt
The next command is the halt command. This can shutdown a system but has some other options:
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[[email protected] ~]# halt --help
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halt [OPTIONS...]
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​
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Halt the system.
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​
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     --help      Show this help
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     --halt      Halt the machine
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  -p --poweroff  Switch off the machine
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     --reboot    Reboot the machine
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  -f --force     Force immediate halt/power-off/reboot
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  -w --wtmp-only Don't halt/power-off/reboot, just write wtmp record
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  -d --no-wtmp   Don't write wtmp record
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     --no-wall   Don't send wall message before halt/power-off/reboot
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the halt command also has force option but try not to use it, because it might put your system in an in consistant state.
poweroff
There is another command exactly same as the halt command. It does the same things and takes the same options.
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[[email protected] ~]# poweroff --help
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poweroff [OPTIONS...]
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​
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Power off the system.
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​
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     --help      Show this help
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     --halt      Halt the machine
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  -p --poweroff  Switch off the machine
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     --reboot    Reboot the machine
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  -f --force     Force immediate halt/power-off/reboot
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  -w --wtmp-only Don't halt/power-off/reboot, just write wtmp record
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  -d --no-wtmp   Don't write wtmp record
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     --no-wall   Don't send wall message before halt/power-off/reboot
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and we are done.
.
.
.
​
Sources:
​https://www.linuxjournal.com/article/1274​
​https://en.wikipedia.org/wiki/Runlevel​
​https://www.geeksforgeeks.org/run-levels-linux/​
​https://www.ostechnix.com/check-runlevel-linux/​
​https://geek-university.com/linux/etc-inittab/​https://developer.ibm.com/tutorials/l-lpic1-101-3/​
​https://www.liquidweb.com/kb/linux-runlevels-explained/​
​https://linux.die.net/man/8/telinit​
​https://unix.stackexchange.com/questions/434560/what-differences-it-will-make-if-i-use-telinit-6-instead-of-reboot-command​
​https://askubuntu.com/questions/5039/what-is-the-difference-between-etc-init-and-etc-init-d​
​https://www.systutorials.com/239880/change-systemd-boot-target-linux/​
​https://www.tecmint.com/change-runlevels-targets-in-systemd/​
​https://fedoramagazine.org/systemd-getting-a-grip-on-units/​
​https://www.linode.com/docs/quick-answers/linux-essentials/what-is-systemd/​
​https://www.howtoforge.com/linux-wall-command/​
​https://en.wikipedia.org/wiki/Wall_(Unix)​
​http://man7.org/linux/man-pages/man1/wall.1.html​
​http://man7.org/linux/man-pages/man8/shutdown.8.html​
​https://www.computerhope.com/unix/ushutdow.htm​
​https://unix.stackexchange.com/questions/42572/is-halt-the-same-as-shutdown-h-and-poweroff-the-same-as-shutdown-p/42581​
​https://www.tecmint.com/shutdown-poweroff-halt-and-reboot-commands-in-linux/​
​https://www.binarytides.com/linux-command-shutdown-reboot-restart-system/​