Linux Miscs

Linux Miscs

Specail command line parameter --
Example: vi -- -12

Runlevels
http://www.comptechdoc.org/os/linux/howlinuxworks/linux_hlrunlevels.html
http://www.networkclue.com/os/Linux/run-levels.aspx
A runlevel is a software configuration of the system that allows only a selected group of processes to exist. Init can run the system in one of eight runlevels. These runlevels are 0-6 and S or s. The system runs in only one of these runlevels at a time. Typically these runlevels are used for different purposes. Runlevels 0, 1, and 6 are reserved. For Redhat Linux version 6, the runlevels are:
0 halt
1 Single user mode
2 Multiuser, without NFS (The same as 3, if you don't have networking)
3 Full multiuser mode
4 unused
5 X11
6 Reboot
Each mode has it's own list of settings for what services to start and what services to shutdown.
The inittab file
The "/etc/inittab" file tells init which runlevel to start the system at and describes the processes to be run at each runlevel. An entry in the inittab file has the following format:
id:runlevels:action:process
action - Describes which action should be taken. Valid actions are listed below
* respawn - The process will be restarted whenever it terminates.
* wait - The process will be started once when the specified runlevel is entered and init will wait for its termination.
* once - The process will be executed once when the specified runlevel is entered
* boot - The process will be executed during system boot. The runlevels field is ignored.
* bootwait - Same as "boot" above, but init waits for its termination.
* off - This does nothing.
* ondemand - This process will be executed whenever the specified ondemand runlevel is called.
* initdefault - Specifies the runlevel which should be entered after system boot. If none exists, init will ask for a runlevel on the console. The process field is ignored.
* sysinit - The process will be executed during system boot. It will be executed before any boot or bootwait entries. The runlevels field is ignored.
* powerwait - The process will be executed when init receives the SIGPWR signal. Init will wait for the process to finish before continuing.
* powerfail - Same as powerwait but init does not wait for the process to complete.
* powerokwait - The process will be executed when init receives the SIGPWR signal provided there is a file called "/etc/powerstatus" containing the word "OK". This means that the power has come back again.
* ctrlaltdel - This process is executed when init receives the SIGINT signal. This means someone on the system console has pressed the "CTRL-ALT-DEL" key combination.
* kbrequest - The process will be executed when init receives a signal from the keyboard handler that a special key combination was pressed on the console keyboard.
* process - Specifies the process to be executed. If the process starts with the '+' character, init will not do utmp and wtmp accounting for that process. This is needed for gettys that insist on doing their own utmp/wtmp housekeeping (a historic bug).


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
Switching Run Levels
init <Run Level number>
telinit [number of runlevel]
Which runlevel are you in?
sudo runlevel
who -r


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