What are the main differences between RHEL4 & RHEL5? 

Answer:
Difference between RHEL 4 and RHEL 5 :
1) In RHEL 4 SELinux Block only 13 services, But on RHEL 5 SElinux Block 80 services.

2) RHEL 4 have ext2 filesystem, but RHEL 5 we have ext3 filesystem that support Journal.

3) RHEL 4 have no virtualization Feature, but in RHEL 5 we have virtualization with Hypervisor-V.

4) In RHEL 4 we have no Yum, But in RHEL 5 we have Yum available.

Any many more....... need more time to describe it

(or)

   rhel4                                    rhel5
no yum server                       yum server
selinux                                  Advanced selinux
secure                                   more secure
no cdkey                               cd key
---                                        virtualization
mim 256 mb ram                   512 ram
---                                        better support for 
                                            Red Hat Directory
                                            Ext4 (on latest version)

What is the difference between Telnet and SSH? 

Answer: 
SSH using PORT 22 and telnet using Port 23 ssh is a secured shell, where telnet is not a secured one.when you ssh to trasnfer data between a system, the data will be send in the encrypted form, where the hacker cannot encode or decode it. While you telnet,the data send between the system is alphabetical format(ASCII), where every one can understand. More over as per network security, telnet and ftp are prohibited. Always, trust SSL based data transfer.

How to recover a deleted file in linux ?

Answer:
Determine the partition in which the file you wish to recover was located by using the command pwd (present work directory) and unmount the current directory by using the command umount. Use the "debugfs" command. The "debugfs" command is used to handle most major errors and bugs that can occur in Linux by using #debugfs /usr/directory name.Use the "lsdel" once you have begun the debug of the file system.  Recover those files you wish to undelete and take backup of that file.

What is the difference between home directory and working directory?

Answer:
Home directory is one over which user have complete control and it is its default working directory when its logs in.  While the working directory is the users current directory which may or may not be his home directory.

What is the minimum number of partitions you need to install Linux?

Answer:
Well in my opinion is case if minimum nukber of partition is asked then better answer would be 
/
/boot
/swap

You can install linux with minimum of three partitions.

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