Linux Runlevels Explained

There are times when Linux system boots either into a Graphical User Interface (GUI) or a command line. A Linux system may also boot as a single-user system, which means that it is available only to a single user, the superuser used for system maintenance.

The way the Linux system boots and operates is known as the system’s state, known as the runlevel.

Objectives

In this tutorial, we will learn about the various runlevels that exist within a Linux system.

Prerequisites

• Access to a Linux machine. In this tutorial we’ll use an Ubuntu Virtual Machine with a GUI.
• Acting as a non-root sudo user to ensure a secure environment

Runlevel Overview

When we boot a Linux system, it boots with the predefined conditions, which define its state. For example, we may boot a Linux system, such as Ubuntu, into a command line and work from there. This is typically done by expert Linux users. On the other hand, we may want to boot into the GUI. These are different states, which we call runlevels.

There are two key reasons for defining a runlevel in a Linux system:

• Defines the programs or applications that need to execute when the system boots
• Defines the state of the system when it boots, such as whether the GUI is available or not

Before we proceed further, we can quickly find the runlevel of the Ubuntu installation by using the following command:

runlevel

Notice that the output displays a number, which is 5 in this case. This is the default runlevel in most Linux distributions. We can, however, change it to a different runlevel.

N 5

Another method we can use to find the runlevel is by using the who command with the -r parameter:

who -r

We get two different runlevels here. We get the current runlevel along with another runlevel, which was the last runlevel. If we had ever changed the runlevel from the default, it would be listed with the last parameter in the output. In our scenario, the last runlevel was 3, and the current one is 5.

run-level 5  2021-08-06 18:54

We can also use the systemctl command, which we will look at later in this tutorial.

Before using any command, it is always advisable to read through the help to understand its parameters. We can get the help on runlevels by using the following command:

runlevel --h

Notice that the help also contains a reference to the man pages. We should be able to get more details on the runlevels.

runlevel [OPTIONS...]

Prints the previous and current runlevel of the init system.

Options:
     --help      Show this help

See the runlevel(8) man page for details.

Let’s quickly see what the man pages hold by executing the following command:

man runlevel

The man pages display a total of 7 runlevels, which range from 0 to 6. Each runlevel is designated to perform a specific task.

RUNLEVEL(8)                        runlevel                       RUNLEVEL(8)

NAME
       runlevel - Print previous and current SysV runlevel

SYNOPSIS
       runlevel [options...]

OVERVIEW
       "Runlevels" are an obsolete way to start and stop groups of services
       used in SysV init. systemd provides a compatibility layer that maps
       runlevels to targets, and associated binaries like runlevel.
       Nevertheless, only one runlevel can be "active" at a given time, while
       systemd can activate multiple targets concurrently, so the mapping to
       runlevels is confusing and only approximate. Runlevels should not be
       used in new code, and are mostly useful as a shorthand way to refer
       the matching systemd targets in kernel boot parameters.

       Table 1. Mapping between runlevels and systemd targets
       ┌─────────┬───────────────────┐
       │Runlevel │ Target            │
       ├─────────┼───────────────────┤
       │0        │ poweroff.target   │
       ├─────────┼───────────────────┤
       │1        │ rescue.target     │
       ├─────────┼───────────────────┤
       │2, 3, 4  │ multi-user.target │
       ├─────────┼───────────────────┤
       │5        │ graphical.target  │
       ├─────────┼───────────────────┤
       │6        │ reboot.target     │
       └─────────┴───────────────────┘

...

So, what does this mean, let’s proceed further to understand the numbers used to define the runlevels. The man pages displayed a total of 7 runlevels, which are:

Location for Runlevel Files

Now that we know what runlevels are, we must understand that each runlevel is a specific environment executed through a specific set of scripts. So, where are these scripts stored? Let’s find that out.

In a Linux system, we have the /etc directory, which contains the system-wide configuration files. The /etc directory contains several subdirectories as well as configuration files. The runlevel files are stored in their respective directories, which are named as:

• rc0.d
• rc1.d
• rc2.d
• rc3.d
• rc4.d
• rc5.d
• rc6.d

Each of the subdirectories is designed to keep the configuration files for a specific runlevel. Let’s first view these subdirectories by executing the following command:

ls /etc

The output of the ls command is now listed. We need to scroll back up to view the rc set of subdirectories.

cpi                           hdparm.conf          pnm2ppa.conf
adduser.conf                   host.conf            polkit-1
alsa                           hostid               popularity-contest.conf
alternatives                   hostname             ppp
anacrontab                     hosts                profile
apg.conf                       hosts.allow          profile.d
apm                            hosts.deny           protocols
apparmor                       hp                   pulse
apparmor.d                     ifplugd              python2.7
apport                         ImageMagick-6        python3
appstream.conf                 init                 python3.8
apt                            init.d               rc0.d
avahi                          initramfs-tools      rc1.d
bash.bashrc                    inputrc              rc2.d
bash_completion                insserv.conf.d       rc3.d
bash_completion.d              iproute2             rc4.d
bindresvport.blacklist         issue                rc5.d
binfmt.d                       issue.net            rc6.d
bluetooth                      kernel               rcS.d
brlapi.key                     kernel-img.conf      resolv.conf
brltty                         kerneloops.conf      rmt
brltty.conf                    ldap                 rpc

...

Let’s view the files in /etc/rc0.d subdirectory, which is used for runlevel 0:

ls /etc/rc0.d/

The output of the ls command is now listed. A set of configuration scripts and files are stored in this subdirectory.

K01alsa-utils    K01kerneloops                   K01saned
K01avahi-daemon  K01open-vm-tools                K01speech-dispatcher
K01bluetooth     K01openvpn                      K01spice-vdagent
K01cups-browsed  K01plymouth                     K01udev
K01gdm3          K01pulseaudio-enable-autospawn  K01unattended-upgrades
K01irqbalance    K01rsyslog                      K01uuidd

Changing Runlevel

Just as a quick recap, we now know what a runlevel is, along with the directory and subdirectory where its configuration files are stored. We need to now move ahead and change the runlevel.

There are two ways to change a runlevel. The first method is to change it using the init or telinit commands. We will first use the init command:

init 6

This command reboots the system. We get the same output if we execute the telinit command:

telinit 6

Let’s now look at the systemctl command that helps us find the current runlevel and helps us change it as well. Let’s execute this command to first find the runlevel:

systemctl get-default

Notice that the output displays a graphical.target, which is the runlevel 5, as seen in the table above.

graphical.target

Now, it is time to change the runlevel using the systemctl command. To do this, execute the following command:

sudo systemctl isolate multi-user.target

The moment we press Enter, we are taken to the command line to provide the user credentials. After successful authentication, we are now on the command line.

Let’s check the runlevel once again. We will simply use the runlevel command:

runlevel

Notice that two runlevels are displayed. The first one, which is 5, is the last runlevel that was used. The next number, which is 3, is the current runlevel.

Let’s get back to the GUI using the following command:

sudo systemctl isolate graphical.target

We are prompted for authentication for the user edxd because we are using sudo. After successful authentication, the system boots into the GUI environment, where we need to select the appropriate user and log on to the system. After successful authentication, we are back on the GUI mode.

Conclusion

Well done. Hopefully, this tutorial helped us understand the Linux runlevels. If you encountered any issues, or have any questions, please feel free to leave a comment or contact us, and we’ll get back to we as soon as we can.