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.

A quick note on the links related where to find the ubuntu releases and how to do a checksum to ensure that the downloaded copy If you are downloading Ubuntu from the official site, MD5/SHA1 checksum (ubuntu hashes) for latest releases may not be listed there. If it is not there, you can visit the site http://releases.ubuntu.com/. This site lists a comprehensive list of ubuntu releases and their checksums. Checksums ensure that the download is complete without corruption and is 100% accurate. So, it is always advisable to do a checksum after each download. (more…)

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.

There are times when you would like to do some changes in your web server configuration file which might necessitate the server to be restarted in order for the changes to take effect. Issuing a apache restart command will forcefully close all the connections made to the web server which will end any user related activity on the server. If you would want to avoid such scenarios then issue the following command $ sudo /etc/init.d/apache2 graceful (in ubuntu)

Linux: History command Each bash sessions stores the history of commands executed in .bash_history file which in turn in stored under each user's home directory usr100@dev01:/tmp/user$ ls 07-14.txt a.txt b.txt d.txt usr100@dev01:/tmp/user$ cat ~/.bash_history clear su root rm -i d.txt ls -ltr touch c.txt