Linux Run Level

Linux: How to change the default Run Level?

To list the current run level
$ who -r
run-level 5 2008-07-22 21:35 last=5

To change the default Run level edit /etc/inittab

You can also switch to another run level number by issuing
$ init

For example,
$ init 3

Now the system switches from Run level 5 to Run level 3

To list the current run level
$ who -r
run-level 3 2008-07-22 22:35 last=5

In the above result notice the last part which states “last=5”. This indicates that the last run level was 5

When the system is running and if we want to apply patches or alter certain configurations, the above command comes in handy.

The other option to change the current level is
$ telinit <run level number>

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Linux: How to find the current Run Level?

Default init level is set in /etc/inittab.

$ cat /etc/inittab (to display the init levels)

Inside that file, you can note a line which will state the run level number like
id:5:initdefault: (indicating run level 5 which means that the system is running with x windows)

To find the current run level you can execute the following commands

$ who -r
run-level 5 2008-07-22 21:35 last=5
which indicates that the system is in run-level 5

Other option to find the run level is
$ /sbin/runlevel
N 5

To find out what sub processes are associated with each run level you can go to
$ cd /etc/rcN.d/ (Where N denotes the run level number)

For example
$ cd /etc/rc1.d/
$ ls -l

This will list symbolic links to various processes associated with run level 1.

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Linux: How many Run Levels are there and what are they?

Linux systems have 7 Run Levels and they are as follows.

0 – system halt
1 – single user mode (system maintenance mode)
2 – multiuser mode without network access
3 – full multiuser mode with network access
4 – not used
5 – multiuser mode with X windows
6 – reboot or restart

Each run level in linux has a very specific purpose.

Run level 1 is also known as system maintenance mode and at this level the system prompt appears without root level password. It should be used only when it is absolutely necessary for system maintenance.

Run level 4 is not used and it can be used by the user to customize his own level.

Note: Solaris Run Level is different from Linux run levels.

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Linux: What is Run Level?

Run Level is a term to indicate your computer’s state of operation. When your computer starts it initially starts the the first process called the init process. Init process in turn starts associated sub-processes for that state of operation.

Run Level is the state of the machine – indicating the processes and sub-processes that needs to be started in order for the Operating System to be in that state.

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