Transcript Unix System Admin

System Startup & Shutdown
• Objectives
– to interpret the Unix startup and shutdown configuration files
– to be able to create a customised run level
• Contents
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bootstrap procedure
single and multi-user run levels
system startup files (rc files)
standard system processes
clean shutdown procedure
maintenance mode
• Practicals
– to modify the startup sequence of a Unix system
• Summary
Standard Boot Process
• The system boot process is hardware-specific
– often an automatic boot from hardware into multi-user Unix
– high security systems require PROM password for manual boot
• In order to boot a system boot loader is needed
– SysLINUX
– LILO (ctrl + x = boot prompt)
– GRUB (select kernel and press e = edit boot)
boot: linux root=/dev/hda1
boot: linux single
• Boot prompt arguments is optional
– Used to boot into single user mode whenever needed
– To set kernel arguments at boot like boot disks, network, kernelfile
• Once loaded the Unix kernel starts running
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initialise devices, virtual memory, etc.
initialise internal tables (processes, files, etc.)
creates scheduler process (number 0 invisable in all linux dialects)
runs the first process (number 1) /sbin/init
• System initialisation performed by /sbin/init is user configurable
Startup Flow Control
run level 0
boot
run level 1
single-user
maintenance
run level 2
multi-user
run level 3
multi-user
& DFS
run level 4
multi-user
user defined
Gentoo / SuSE / RedHat Run-Levels
• Run levels are standard under Gentoo Unix:
0
1
2
3
4
5
6
s/S
a/b/c
System HALT
Single user mode or administration mode
Local multiuser without remote network (e.g. NFS)
Full multi-user with networking, gentoo also x-windows
not used
Full multi-user with networking xdm (X-Windows), exept gentoo
System reboot
single-user mode
pseudo states (rarely used)
• Use the -r option to who to get the current run level
# who -r
run-level 5 Sep 25 10:35
last=S
# shutdown -h +10 "Memory upgrade. Please log off ”
clean HALT in 10 minutes with warning message: Memory upgrade….
The init Control file: /etc/inittab
• The init process uses /etc/inittab as its control file
# Default runlevel. The runlevels used by LINUX are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 – unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
#
id:3:initdefault:
# Console Text Mode
#id:5:initdefault:
# Console GUI Mode, exept gentoo
• Processes run from /etc/inittab are daemons
– standard I/O attached to /dev/null
– I/O usually be redirected to a log file or perhaps the console
Structure of /etc/inittab
• Each line has 4 fields separated by colons
id : level : action : process
id
unique identifier for line - up to four alphanumerics
level
run level(s) to activate process
action
keyword for how to run process
command
full pathname and parameters of command to be executed
• Main inittab action keywords are:
off
wait
once
respawn
don't run the command
run command and wait for completion
run command, but don't wait
run command. If process exits, then repeat the command
sysinit
boot
bootwait
initdefault
run command at first init
run command at boot-time
but don't wait
like boot, but wait for completion
defines default boot-level
Exercise - /etc/inittab
• What processes are run at system startup
• What processes are run at system shutdown
# more /etc/inittab
si::sysinit:/sbin/rc sysinit
rc::bootwait:/sbin/rc boot
id:3:initdefault:
l0:0:wait:/sbin/rc shutdown
l1:S1:wait:/sbin/rc single
l2:2:wait:/sbin/rc nonetwork
l3:3:wait:/sbin/rc default
l4:4:wait:/sbin/rc default
l5:5:wait:/sbin/rc default
l6:6:wait:/sbin/rc reboot
ca::ctrlaltdel:/sbin/shutdown -r -t 4 now
pf::powerwait:/etc/init.d/powerfail
pn::powerfailnow:/etc/init.d/powerfail now
po::powerokwait:/etc/init.d/powerfail
c1:12345:respawn:/sbin/agetty 38400 tty1 linux
c2:12345:respawn:/sbin/agetty 38400 tty2 linux
X:a:once:/etc/X11/startDM.sh
Run Command Scripts
• The run command scripts invoked by init via /etc/inittab
• Each rc attribute controls changes to named run level
rc
rc
rc
rc
rc
rc
rc
sysinit
boot
shutdown
single
nonetwork
default
reboot
internal runlevel at boot is activated
looks in /etc/runlevel/boot at booting
looks in /etc/runlevel/shutdown for level 0
looks in /etc/runlevel/single for level 1 & S
looks in /etc/runlevel/nonetwork for level 2
looks in /etc/runlevel/default for levels 3, 4 & 5
looks in /etc/runlevel/reboot for level 6
• Script runs startup programs in subdirectory of /etc/runlevel
• Startup program names is formed like:
Startup in alphanumeric order or by script internal logics.
• All startup programs in runlevels are links to scripts in
/etc/init.d
RC Script Details (Start and Stop)
# sh sshd start
# sh sshd stop
# more /etc/init.d/sshd
case "$1" in
start) ...
...;;
stop) ...
...;;
esac
-Adding sshd
# cd /etc
# ln init.d/sshd runlevels/default/sshd
# init 3
# init 0
Set Programs Run At Each runlevel
• Check, set and move program’s to various runlevels
# rc-status -a
Runlevel: boot
keymaps
. . .
Runlevel: default
sshd
Local
. . .
Runlevel: nonetwork
Local
. . .
Runlevel: single
Runlevel: UNASSIGNED
[ started ]
[ stopped ]
[ started ]
[ started ]
• Switch Off sshd Starting Up In gentoo runlevel default
# rc-update –d sshd default
• Switch On sshd Starting Up In gentoo runlevel default
# rc-update –a sshd default
Manually start and stop services
• Check if service is running
# /etc/init.d/sshd status
* status: started
• Stop service
# /etc/init.d/sshd stop
Stopping sshd
[ok]
• Start service
# /etc/init.d/sshd start
Starting sshd
[ok]
• Reload service after reconfiguring
# /etc/init.d/sshd reload
Reloading sshd [ok]
• Also note that other start/stop scripts can have:
restart
condrestart
graceful
Exercise - System Startup
• What subsystems are started up at run level 3
• What subsystems are stopped going from level 3 to 2
# grep '[23]' /etc/inittab
id:3:initdefault:
l2:2:wait:/sbin/rc nonetwork
l3:3:wait:/sbin/rc default
ca:12345:ctrlaltdel:/sbin/shutdown -t3 -r now
pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure!"
pr:12345:powerokwait:/sbin/shutdown -c "Power Restored"
1:2345:respawn:/sbin/agetty 38400 tty1 linux
2:2345:respawn:/sbin/agetty 38400 tty2 linux
# ls /etc/runlevels/nonetwork /etc/runlevels/default
/etc/runlevels/default:
local
net.eth0
netmount
syslog-ng vixie-cron
/etc/runlevels/nonetwork:
local
Changing run levels
• Use init to change run levels if users not logged on
– use to switch to multi-user from single user
– use to switch between multi-user levels
• Use shutdown to close down the system
– warns users what is happening
– performes a lot of housekeeping routines
– issues the init command
# init 2
switch to run level 2
# init 3
now switch to run level 3
# shutdown
default system shutdown
System Shutdown
• Use /sbin/shutdown when closing down a Unix system
shutdown [-tN ] [+M ] [-h ] [-r ] [-c ] time message
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+M
wait period in seconds (now=0)
-tN wait period in seconds between message and action
-h
shutdown to halt
-c
Cancel shutdown
-r
shutdown and reboot
time absolute time hh:mm
message
shutdown message
wall is used to send broadcasts at set intervals until system shutdown
shutdown is started by init changing to new run level
any process left running are sent a TERM signal
any still running are sent a KILL signal
# shutdown –r now
reboot the system now
# shutdown +3600 -r
halt system in one hour
• WARNING: databases cannot normally close down quickly
enough when receiving a TERM signal during shutdown
BSD Derived Systems (AIX, OSF/1)
• Only one multi user level
– boot up direct to multi-user
– boot to single user and go to multi-user on exit from single user root
shell
– cannot use init to change levels
• Use shutdown and specify time to exit from multi user
• Startup/shutdown controlled by command scripts in /etc
– scripts must be modified using a text editor
– require knowledge of script programming
– look for files: rc, rc.local, rc.single, rc.tcpip, rc.boot, rc.shutdown
# shutdown -r now
reboot the system now
# shutdown -h 17:00
halt system at 5pm
# shutdown now
password:
# sync
# sync
# halt
# halt
# poweroff
# reboot
Maintenance mode
• Maintenance mode is used to get out of trouble
– the system won't boot due to password/kernel/init problems
• If the system is alive and well, use shutdown to run level 1
• Otherwise reboot to single user mode
– if you have to power cycle the system wait for the disk activity lights to
stop blinking
• If the system won’t boot, insert the installation (boot) media,
and boot from this instead
– ensure that you do not attempt to reinstall the system at this point
– this will give you a system where Unix is running in memory using the
installation media as a filesystem. The hard disk may now be able to be
fixed (if you are lucky!)
– Gentoo uses the live CD or DVD
• Most unix serves has a boot prompt, in INTEL plateform
it does not exist, there you have bios settings and grub
or lilo or syslinux
Summary
• Unix systems start up through a clearly defined
sequence
• The first configurable stage is with the init
process using /etc/inittab
• Run levels provide additional configuration
through the run command scripts
• Link and remove files from the /etc/init.d/rc*.d
directories to enable/disable sub-systems for
different run levels
• Shutdown Unix using the shutdown command
• AIX and OSF/1 (BSD) use a different mechanism
to SVR4 (SVR3.2) systems