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Saturday, June 19, 2021

How To Check The Runlevel In Linux

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This brief guide explains what is a runlevel, how many runlevels are there in Linux and how to check the runlevel in Linux operating systems.

What is runlevel?

runlevel is one of the modes that a Unix-based operating system will run in. In other words, a run level is a state of init and the whole system that defines what system services are operating.

In Linux Kernel, there are 7 runlevels exists, starting from 0 to 6. The system can be booted into only one runlevel at a time.

By default, a system boots either to runlevel 3 or to runlevel 5. Runlevel 3 is CLI, and 5 is GUI. The default runlevel is specified in /etc/inittab file in most Linux operating systems.

Using runlevel, we can easily find out whether X is running, or network is operational, and so on.

1. Check the Runlevel In Linux (SysV init)

Here is the list of runlevels in Linux distributions,which were distributed with SysV init as default service manager.

  • 0 – Halt
  • 1 – Single-user text mode
  • 2 – Not used (user-definable)
  • 3 – Full multi-user text mode
  • 4 – Not used (user-definable)
  • 5 – Full multi-user graphical mode (with an X-based login screen)
  • 6 – Reboot

Now, let us see how to find the runlevels.

To find out the system runlevel, open your Terminal and run the following command:

$ runlevel

Sample output:

N 3

In the above output, the letter 'N' indicates that the runlevel has not been changed since the system was booted. And, 3 is the current runlevel i.e the system is in CLI mode.

In newer versions of init, you can find the current and previous runlevel details using the environment variables called RUNLEVEL and PREVLEVEL.

Say for example, to find out the current runlevel, you need to run:

$ echo $RUNLEVEL

To find the previous runlevel:

$ echo $PREVLEVEL

If you’d like to change the RunLevel to something else, edit /etc/inittab file:

$ sudo vi /etc/inittab

Find and edit entry initdefault to the runlevel of your choice. Say for example, to set the runlevel to multi-user graphical mode (runlevel 5), modify it as shown below.

id:5:initdefault:

Save and close the file. Reboot your system to login to your new runlevel.

For more details about runlevels, refer man pages.

$ man runlevel

2. Check the Runlevel In Linux (Systemd)

In recent versions of Linux systems (starting from RHEL 7, Ubuntu 16.04 LTS), the concept of runlevels has been replaced with systemd targets.

Here is the list of Systemd targets in Linux distributions,which were distributed with Systemd as default service manager.

  1. runlevel0.target, poweroff.target – Halt
  2. runlevel1.target, rescue.target – Single-user text mode
  3. runlevel2.target, multi-user.target – Not used (user-definable)
  4. runlevel3.target, multi-user.target – Full multi-user text mode
  5. runlevel4.target, multi-user.target – Not used (user-definable)
  6. runlevel5.target,graphical.target – Full multi-user graphical mode (with an X-based login screen)
  7. runlevel6.target,reboot.target – Reboot

In Linux systems that are using Systemd as default service manager, you can find the current target using command:

$ systemctl get-default

Sample output:

graphical.target

As you see in the above output, my current runlevel (target in other words) is 5, which is graphical mode.

To view all currently loaded targets, run:

$ systemctl list-units --type target

Sample output:

   UNIT                   LOAD   ACTIVE SUB    DESCRIPTION                             
   basic.target           loaded active active Basic System
   bluetooth.target       loaded active active Bluetooth
   cryptsetup.target      loaded active active Local Encrypted Volumes
   getty.target           loaded active active Login Prompts
   graphical.target       loaded active active Graphical Interface
   local-fs-pre.target    loaded active active Local File Systems (Pre)
   local-fs.target        loaded active active Local File Systems
   multi-user.target      loaded active active Multi-User System
   network-online.target  loaded active active Network is Online
   network-pre.target     loaded active active Network (Pre)
   network.target         loaded active active Network
   nfs-client.target      loaded active active NFS client services
   nss-lookup.target      loaded active active Host and Network Name Lookups
   nss-user-lookup.target loaded active active User and Group Name Lookups
   paths.target           loaded active active Paths
   remote-fs-pre.target   loaded active active Remote File Systems (Pre)
   remote-fs.target       loaded active active Remote File Systems
   rpc_pipefs.target      loaded active active rpc_pipefs.target
   rpcbind.target         loaded active active RPC Port Mapper
   slices.target          loaded active active Slices
   sockets.target         loaded active active Sockets
   sound.target           loaded active active Sound Card
   swap.target            loaded active active Swap
   sysinit.target         loaded active active System Initialization
   timers.target          loaded active active Timers
   veritysetup.target     loaded active active Local Verity Integrity Protected Volumes
 LOAD   = Reflects whether the unit definition was properly loaded.
 ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
 SUB    = The low-level unit activation state, values depend on unit type.
 26 loaded units listed. Pass --all to see loaded but inactive units, too.
 To show all installed unit files use 'systemctl list-unit-files'.
Check the Runlevel In Linux (Systemd)

The above command will show only the active targets.

To view all loaded targets (active and inactive), run:

$ systemctl list-units --type target --all

If you’d like to change the RunLevel to something else, for example runlevel3.target, set it as shown below:

$ sudo systemctl set-default runlevel3.target

To change to a different target unit in the current session only, run the following command:

$ sudo systemctl isolate runlevel3.target

For more details, refer Systemd man pages.

$ man systemd

Check Linux Runlevel Using ‘who’ command

As one of the reader suggested in the comment section below, we can also find the runlevel using who command like below:

$ who -r

Sample output:

     run-level 5  2021-06-09 10:21

Hope this helps.

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