Difference between which command and whereis command in Linux For e.g. lets find where "samba" is located in a CentOS server $ which samba which command searches the list of programs listed down through the PATH settings $ whereis samba whereis command also searches for programs that are not present in the PATH setting

GRUB stands for GRand Unified Bootloader. This is to enable the listing of multiple operating systems so the users could choose the OS of their choice. To change the menu order we need to edit a file called menu.lst # vi /boot/grub/menu.lst Look for a line "default 0". This is the option to indicate that the first menu on the list should be loaded by default. In order for you to set the default menu content in the GRUB loader that needs to get started you can set this parameter. This is of the form "default <ID>" where ID = N -1. If there are 4 list items in the menu and if you want to load the 4th item by default, you need to set this configuration setting to "default 3". Another option for you is to have the menu list with the entry "savedefault = true". This would enable the last used menu entry to get loaded during the next boot.

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>

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.

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.

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.