Exam weight: 3 — LPIC-1 v5, Exam 101
What you need to know
- Set the default runlevel or boot target
- Switch between runlevels and targets, including single-user mode
- Shut down and reboot the system from the command line
- Notify users before a runlevel change or other major event
- Properly terminate running processes
- Know the basics of acpid
Service managers: three approaches
When the kernel finishes loading, it launches the first process in the system. That process always receives PID 1. Depending on the distribution, it can be SysVinit, systemd or Upstart. All three can start and stop services, but they do it differently.
SysVinit and runlevels
SysVinit divides system states into runlevels numbered 0 through 6.
| Runlevel | Purpose |
|---|---|
| 0 | Halt |
| 1, s, single | Single-user mode, no network (maintenance mode) |
| 2, 3, 4 | Multi-user mode with console or network login |
| 5 | Same as 3, plus graphical login |
| 6 | Reboot |
The only runlevels with a consistent meaning across all distributions are 0, 1 and 6.
/etc/inittab
/etc/inittab controls init behaviour. Line format:
id:runlevels:action:process
The default runlevel is set like this:
id:3:initdefault:
Never set 0 or 6 as the default — the system will immediately halt or reboot after loading.
Available action values:
| Action | Behaviour |
|---|---|
boot | Runs during system initialisation; runlevels field is ignored |
bootwait | Same, but init waits for the process to finish |
sysinit | Runs after initialisation, regardless of runlevel |
wait | Runs for the specified runlevels; init waits for completion |
respawn | Process is restarted when it exits |
ctrlaltdel | Executed when Ctrl+Alt+Del is pressed |
Service scripts
Scripts live in /etc/init.d/. For each runlevel there is a directory /etc/rcN.d/ (N is the runlevel number) containing symbolic links to scripts from /etc/init.d/. A link name begins with K (kill, stop) or S (start), followed by a two-digit priority and the service name. For example, /etc/rc1.d/K90network stops the network service when entering runlevel 1.
SysVinit commands
Show the current runlevel:
runlevel
# Output: N 3 (N means the runlevel has not changed since boot)
Switch runlevel without rebooting:
telinit 3
telinit 1 # single-user mode
telinit 6 # reboot
telinit 0 # halt
init N does the same thing as telinit N.
After every change to /etc/inittab, run:
telinit q # or telinit Q
This forces init to re-read its configuration. Without this step, a bad entry in /etc/inittab can cause the system to hang on the next reboot.
systemd and boot targets
systemd is used by the majority of distributions today. Instead of runlevels, it operates with units and targets.
Unit types
| Type | Purpose |
|---|---|
service | An active service that can be started and stopped |
socket | A network or filesystem socket |
device | A hardware device registered through udev |
mount | A mount point (similar to an /etc/fstab entry) |
automount | Same, but mounted automatically on access |
target | A group of units managed together |
snapshot | A saved state of the systemd manager |
Runlevel to target mapping
| SysV runlevel | systemd target |
|---|---|
| 0 | poweroff.target |
| 1 | rescue.target |
| 3 | multi-user.target |
| 5 | graphical.target |
| 6 | reboot.target |
Managing units with systemctl
systemctl start unit.service # start
systemctl stop unit.service # stop
systemctl restart unit.service # restart
systemctl status unit.service # detailed status
systemctl is-active unit.service # active or inactive
systemctl enable unit.service # enable at boot
systemctl disable unit.service # disable at boot
systemctl is-enabled unit.service # check autostart status
If there is only one unit with a given name in the system, the .service suffix can be omitted. For example, systemctl status sshd works the same as systemctl status sshd.service.
Switching targets
systemctl isolate multi-user.target
systemctl isolate rescue.target # single-user mode
Default target
Show the current default:
systemctl get-default
Change it:
systemctl set-default multi-user.target
The default target is stored as a symlink at /etc/systemd/system/default.target. Never set shutdown.target as the default — it is the equivalent of runlevel 0. You can also set the target via a kernel parameter at boot:
systemd.unit=multi-user.target
systemd configuration files
/lib/systemd/system/— vendor unit files (primary location)/usr/lib/systemd/— alternative location on some distributions/etc/systemd/— user overrides/etc/systemd/logind.conf— power event settings (suspend, hibernate, etc.)/etc/systemd/logind.conf.d/— drop-in config files (alternative to a single logind.conf)
List units:
systemctl list-unit-files
systemctl list-unit-files --type=service
systemctl list-unit-files --type=target
systemctl list-units
On systemd-based systems, /sbin/init is a symlink to /lib/systemd/systemd.
Shutdown and reboot
The shutdown command
shutdown [options] time [message]
The time argument is required. Formats:
| Format | Meaning |
|---|---|
hh:mm | Specific time, e.g. 23:00 |
+m | In m minutes, e.g. +10 |
now or +0 | Immediately |
Options:
| Option | Action |
|---|---|
-h | Halt after shutdown |
-r | Reboot |
-c | Cancel a scheduled shutdown |
Examples:
shutdown -h now
shutdown -r +5 "Rebooting in 5 minutes"
shutdown -r 23:00
shutdown -c
After shutdown runs, all processes receive SIGTERM first, then SIGKILL, after which the system switches runlevel or powers off. Running shutdown without -h or -r drops the system into single-user mode (runlevel 1).
On SysV systems you can restrict which users can reboot via Ctrl+Alt+Del. Add the -a option to the shutdown command on the ctrlaltdel line in /etc/inittab. Then only users listed in /etc/shutdown.allow will be able to trigger a reboot that way.
systemctl for power management
systemctl reboot
systemctl poweroff
systemctl suspend # sleep mode, data stays in RAM
systemctl hibernate # hibernation, data written to disk
On some distributions poweroff and reboot are symlinks to systemctl.
Notifying users: wall
The wall command broadcasts a message to the terminal sessions of all logged-in users.
wall "Server going down for maintenance in 10 minutes"
wall /path/to/message.txt
The shutdown command automatically sends a warning via wall when a message argument is provided.
acpid
acpid (Advanced Configuration and Power Interface daemon) is the main power management daemon in Linux. It handles hardware events: lid close, battery level changes, power button press.
If acpid is running, systemd power management functions (suspend, hibernate) configured in logind.conf will not work, because acpid intercepts those events first.
Upstart (for reference)
Upstart was developed for Ubuntu as a SysVinit replacement with parallel service startup. Ubuntu switched to systemd in 2015 and Upstart is now rarely seen.
Upstart scripts were stored in /etc/init/. Main commands:
initctl list # list services
start tty6 # start
stop tty6 # stop
status tty6 # status
Exam command reference
| Command / file | Purpose |
|---|---|
runlevel | Show current runlevel (SysVinit) |
telinit N | Switch runlevel (SysVinit) |
telinit q | Reload /etc/inittab |
init N | Same as telinit N |
/etc/inittab | SysVinit configuration file |
/etc/init.d/ | Service scripts |
/etc/rcN.d/ | Symlinks per runlevel (K*/S* naming) |
systemctl isolate X.target | Switch to a target immediately |
systemctl get-default | Show default target |
systemctl set-default X.target | Set default target |
/etc/systemd/system/default.target | Symlink pointing to the default target |
shutdown -h now | Halt immediately |
shutdown -r +5 | Reboot in 5 minutes |
shutdown -c | Cancel scheduled shutdown |
wall "message" | Broadcast message to all users |
/sbin/init | Symlink to /lib/systemd/systemd on systemd systems |
Typical exam questions
Q: How do you reboot the system using telinit?
telinit 6 — runlevel 6 means reboot.
Q: What happens to a service linked as /etc/rc1.d/K90network when entering runlevel 1?
The service is stopped. Names starting with K cause the script to be called with the stop argument.
Q: How do you check whether sshd.service is running?
systemctl status sshd.service for detailed output, or systemctl is-active sshd.service for a one-word answer.
Q: How do you enable sshd.service to start at boot?
systemctl enable sshd.service (as root). It creates a symlink that systemd picks up at boot.
Q: A SysV system always boots into runlevel 1, even though /etc/inittab says 3. Why?
A kernel parameter 1 or S is set in the bootloader configuration. Kernel parameters override the initdefault value in /etc/inittab.
Q: What does /sbin/init point to on a systemd system?
/lib/systemd/systemd.
Q: How do you cancel a scheduled shutdown?
shutdown -c
Q: How do you check the default boot target in systemd?
systemctl get-default or inspect the symlink: ls -l /etc/systemd/system/default.target.
Related topics
- 101.2 Boot the System — bootloader, initramfs, dmesg
- 101.1 Determine and Configure Hardware Settings — BIOS, hardware resources
Exercises
Exercise 1 — Reboot with telinit
How do you reboot the system using the telinit command?
Answer
telinit 6
Runlevel 6 means reboot.
Exercise 2 — K90network at runlevel 1
What happens to the service behind /etc/rc1.d/K90network when the system enters runlevel 1?
Answer
The service is stopped. The filename starts with K (kill), so when runlevel 1 is entered, the script is called with the stop argument.
Runlevel 1 is single-user mode without a network connection, so all network services in
/etc/rc1.d/have names starting with K.
Exercise 3 — Check sshd status
How do you use systemctl to check whether sshd.service is running?
Answer
systemctl status sshd.service
Or more concisely:
systemctl is-active sshd.service
is-activeprintsactiveif the service is running andinactiveotherwise.statusgives full details: PID, recent log lines, start time.
Exercise 4 — Enable sshd at boot
What command enables sshd.service to start automatically on a systemd system?
Answer
systemctl enable sshd.service
Run as root. The command creates a symbolic link that systemd picks up during boot.
Exercise 5 — Wrong runlevel despite inittab
A SysV system always boots into runlevel 1, even though /etc/inittab specifies runlevel 3. What is the most likely cause?
Answer
A kernel parameter 1 or S is set in the bootloader configuration. Kernel parameters take precedence over the initdefault value in /etc/inittab.
Check the parameters of the current boot with
cat /proc/cmdline. Persistent parameters are set in the bootloader config, for example inGRUB_CMDLINE_LINUXinside/etc/default/grub.
Exercise 6 — /sbin/init on systemd
The file /sbin/init exists on systemd systems but is only a symbolic link. What does it point to?
Answer
/lib/systemd/systemd
Exercise 7 — Check the default target
How do you check the default boot target on a systemd system?
Answer
systemctl get-default
Or by inspecting the symlink directly:
ls -l /etc/systemd/system/default.target
Both methods show the same information.
systemctl get-defaultreads the symlink and prints the target name in a convenient form.
Exercise 8 — Cancel a scheduled shutdown
How do you cancel a reboot scheduled with the shutdown command?
Answer
shutdown -c
LPIC-1 Study Notes | Topic 101: System Architecture