Linux Runlevels.

Only one runlevel is executed on startup; run levels are not executed one after another, i.e. either the runlevel 2, 3 or 4 is executed, not more of them sequentially or in any other order.

LSB 4.1.0
ID Name Description
0 Halt Shuts down the system.
1 Single-user mode Mode for administrative tasks.
2 Multi-user mode Does not configure network interfaces and does not export networks services.
3 Multi-user mode with networking Starts the system normally.
4 Not used/user-definable For special purposes.
5 Start the system normally with appropriate display manager (with GUI) Same as runlevel 3 +  display manager
6 Reboot Reboots the system.

In standard practice, when a computer enters runlevel zero, it halts, and when it enters runlevel six, it reboots. The intermediate runlevels (1-5) differ in terms of which drives are mounted, and which network services are started. Default runlevels are typically 3, 4, or 5. Lower run levels are useful for maintenance or emergency repairs, since they usually don’t offer any network services at all. The particular details of runlevel configuration differ widely among operating systems, and also among system administrators.

In various Linux distributions, the traditional /etc/rc script used in the Version 7 Unix was first replaced by runlevels and then by systemd states on major distributions.

 

In each run-level you will find a series of if links pointing to start-up scripts located in /etc/init.d. The names of these links all start as either K or S, followed by a number. If the name of the link starts with an S, then that indicates the service will be started when you go into that run level. If the name of the link starts with a K, the service will be killed (if running).

The number following the K or S indicates the order the scripts will be run. Here is a sample of what an /etc/init.d/rc3.d may look like.

# ls -l /etc/init.d/rc3.d
lrwxrwxrwx  1 root root 10 2004-11-29 22:09 K12nfsboot -> ../nfsboot
lrwxrwxrwx  1 root root  6 2005-03-29 13:42 K15xdm -> ../xdm
lrwxrwxrwx  1 root root  9 2004-11-29 22:08 S01pcmcia -> ../pcmcia
lrwxrwxrwx  1 root root  9 2004-11-29 22:06 S01random -> ../random
lrwxrwxrwx  1 root root 11 2005-03-01 11:56 S02firewall -> ../firewall
lrwxrwxrwx  1 root root 10 2004-11-29 22:34 S05network -> ../network
lrwxrwxrwx  1 root root  9 2004-11-29 22:07 S06syslog -> ../syslog
lrwxrwxrwx  1 root root 10 2004-11-29 22:09 S08portmap -> ../portmap
lrwxrwxrwx  1 root root  9 2004-11-29 22:07 S08resmgr -> ../resmgr
lrwxrwxrwx  1 root root  6 2004-11-29 22:09 S10nfs -> ../nfs
lrwxrwxrwx  1 root root 12 2004-11-29 22:40 S12alsasound -> ../alsasound
lrwxrwxrwx  1 root root  8 2004-11-29 22:09 S12fbset -> ../fbset
lrwxrwxrwx  1 root root  7 2004-11-29 22:10 S12sshd -> ../sshd
lrwxrwxrwx  1 root root  8 2005-02-01 09:24 S12xntpd -> ../xntpd
lrwxrwxrwx  1 root root  7 2004-12-02 20:34 S13cups -> ../cups
lrwxrwxrwx  1 root root  6 2004-11-29 22:09 S13kbd -> ../kbd
lrwxrwxrwx  1 root root 13 2004-11-29 22:10 S13powersaved -> ../powersaved
lrwxrwxrwx  1 root root  9 2004-11-29 22:09 S14hwscan -> ../hwscan
lrwxrwxrwx  1 root root  7 2004-11-29 22:10 S14nscd -> ../nscd
lrwxrwxrwx  1 root root 10 2004-11-29 22:10 S14postfix -> ../postfix
lrwxrwxrwx  1 root root  6 2005-02-04 13:27 S14smb -> ../smb
lrwxrwxrwx  1 root root  7 2004-11-29 22:10 S15cron -> ../cron
lrwxrwxrwx  1 root root  8 2004-12-22 20:35 S15smbfs -> ../smbfs

How run levels start are configured in /etc/inittab by lines like the following:

l2:2:wait:/etc/init.d/rc 2

The first field is an arbitrary label, the second one means that this applies for run level 2. The third field means that init should run the command in the fourth field once, when the run level is entered, and that initshould wait for it to complete. The /etc/init.d/rc command runs whatever commands are necessary to start and stop services to enter run level 2.

The command in the fourth field does all the hard work of setting up a run level. It starts services that aren’t already running, and stops services that shouldn’t be running in the new run level any more. Exactly what the command is, and how run levels are configured, depends on the Linux distribution.

When init starts, it looks for a line in /etc/inittab that specifies the default run level:

id:2:initdefault:

You can ask init to go to a non-default run level at startup by giving the kernel a command line argument of single or emergency. Kernel command line arguments can be given via LILO, for example. This allows you to choose the single user mode (run level 1).

While the system is running, the telinit command can change the run level. When the run level is changed, init runs the relevant command from /etc/inittab.

 

The runlevel command can be used to find both the current runlevel and the previous runlevel by merely typing the following and pressing the Enter key:

/sbin/runlevel

 

Linux Find Out Current Run Level Command

Type the following command:
$ who -r