Linux Run-Levels
By Joshua Erdman
Digital Foundation, inc.
Linux has 7 different run levels (or operating modes):
rc0.d - System Halted
rc1.d - Single User Mode
rc2.d - Single User Mode with Networking
rc3.d - Multi-User Mode - boot up in text mode
rc4.d - Not yet Defined
rc5.d - Multi-User Mode - boot up in X Windows
rc6.d - Shutdown & Reboot
Each mode has it's own list of settings for what services to start and what services to shutdown. Not only does this list contain what is supposed to be running, but also what order each service should be started in - a feature many Linux lovers like to point out that Windows lacks..
chkconfig - will list all the registered services and display thier status at each different run level.
Most of the time when your server boots up it will start in runlevel 3 or 5 depending on what it is set to. This setting can be found in the file /etc/inittab
If you ever want to change what mode your server defaults to on bootup, just edit that file, but NEVER change it to runlevel 6 or 0. This will cause your server to shutdown on bootup or if you set it to runlevel 6 it will continuously reboot. I guess this could make a great prank on a friend's workstation.
RC.D Directory Explanation
In the folder: /etc/rc.d are all the run level folders starting from rc1.d to rc6.d and including init.d
It is in each of these rc#.d folders where the service run settings are kept. If you change directory into /etc/rc.d/rc3.d all the files that start with a capital S are the services that will start at this runlevel. All the files that start with a K are the services that will be killed at that runlevel.
The reason why services that are not supposed to run are still listed is because it is popular to switch a server from one run level to another instead of just booting into the needed run level. That way you can take your server offline (single user mode) and then apply a patch or do a file restore.
Let's take a closer look at these files that are inside the rc#.d folders. Notice that they are symbolic links to the script file located in /etc/rc.d/init.d This makes things very easy. All the commands to start, stop, or reload a service are handled by these scripts. So when a server is to go into a specific run level it can find the correct script and knows whether to start it based on its name and link path.
*we can easily change root password by running in single user mode
Boot Linux into single-user mode
Reboot the machine.
Press the ESC key while GRUB is loading to enter the menu.
If there is a 'recovery mode' option, select it and press 'b' to boot into single user mode.
Otherwise, the default boot configuration should be selected. Press 'e' to edit it.
Highlight the line that begins with 'kernel'. Press 'e' again to edit this line.
At the end of the line, add an additional parameter: 'single'. Hit return to make the change and press 'b' to boot.
Change the admin password
The system should load into single user mode and you'll be left at the command line automatically logged in as root. Type 'passwd' to change the root password or 'passwd someuser' to change the password for your "someuser" admin account.
