Any Computer system consists of

     * Hardware
     * Operating System
     * Applications and Utilities

Unix is an operating system which was first developed in the 1960s in Bell Labs. Unix is a popular OS because of
multi user, multi tasking, stability and portability capabilities.

what is an Operating System?

   Operating system is a resource manager, which manages resources like CPU, Memory, Disks, printers, modems, network cards etc etc. Operating system acts as a host for the application programs which runs on the machine.
Unix/Linux Architecture:

      (1) kernel:

            Kernel is key part of Operating system which manages the kernel subsystems like process management, memory management, network management, filesystem, devices etc. This is heart of operating system which manages system and processes. Kernel provides an execution environment where applications runs.

       (2) Shell:

           The most generic term of shell means program where users run commands. Its a command interpreter. It takes input from the user and interacts with the kernel. It acts as an interface between kernel and user.

As far as i know, These are the following important shells.

               (a) Bourne Shell(sh)                                      

               (b) C shell(csh)

               (c) korne shell(ksh)
                
               (d) Bourne-again shell (bash)

               (f) z shell (zsh)

      To find out on what shell you are working depends on which shell the system administrator has specified as your default shell when your system was first installed.   All shells have their own initialization files.

To determine your login shell:

$ echo $SHELL

find the following to determine your default shell.

/bin/sh - Bourne shell

/bin/bash - Bourne Again shell

/bin/csh - C shell

/bin/ksh - Korn shell

/bin/zsh - Z shell

When you first login into the system, it runs the system profile file /etc/profile. after your system executes the system profile, then user profile will be run..User profile is the file where your environment is defined.

Depending on your default shell, user profile will be the following

.profile (Bourne and Korn shells)

.bash_profile (Bourne Again shell)

.login and .cshrc ( C shell)

User profile resides in each users home directory.

To find out your environment

Just type env command at your shell

$env

—————————————————

_=/usr/bin/env
SSH_TTY=/dev/pts/0
PATH=/oradata/app/oracle/product/10.2.0/bin:/usr/bin:/opt/ansic/bin:

/usr/ccs/bin:/usr/contrib/bin:/usr/contrib/Q4/bin:/opt/hparray/bin:

/opt/nettladm/bin:/opt

/fcms/bin:/usr/contrib/kwdb/bin:/usr/bin/X11:/opt/graphics/common/bin:

/opt/upgrade/bin:/opt/ipf/bin:/opt/resmon/bin:/opt/wbem/bin:

/opt/wbem/sbin:/opt/sas/bin

:/opt/sec_mgmt/bastille/bin:/opt/dsau/bin:/opt/dsau/sbin:/opt/firefox:

/opt/gnome/bin:/opt/ignite/bin:/opt/mozilla:/opt/perl/bin:

/opt/sec_mgmt/spc/bin:/opt/ssh/bin:/opt/hpsmh/bin:/opt/thunderbird:

/opt/gwlm/bin:/opt/sfm/bin:/usr/contrib/bin/X11:/opt/aCC/bin:

/opt/caliper/bin:/opt/cadvise/bin:/opt/langtools/bin:.

COLUMNS=179
EDITOR=vi
LOGNAME=oracle
MAIL=/var/mail/oracle
SFTP_UMASK=
SFTP_LOG_FACILITY=2
ORACLE_SID=PROD
ERASE=^H
SFTP_PERMIT_CHOWN=1
USER=oracle
SHELL=/sbin/sh
LOG_SFTP=0
SFTP_LOG_LEVEL=3
HOME=/home/oracle
TERM=xterm
ORACLE_HOME=/oradata/app/oracle/product/10.2.0
TZ=WAT-3
SFTP_PERMIT_CHMOD=1
LINES=40
$
———————————————————

above output is from my HP-UX Oracle user Environment

These are the main distributions of Unix, which are more popular:

                  (a) AIX ( IBM)   (Aix 6.1 is the latest version)
                  (b) HP-UX ( HP)  (11i V3 is the latest version)
                  (c) Solaris (Sun Microsystems) (Solaris 10 is the latest version)

we have following Linux Distributions, which are more popular

                 (a) RedHat Enterprise Linux (RHEL 5 is the latest version)
                 (b) SuSe Enterprise Linux   (SuSe 10 is the Latest Version)
                 (c) Oracle Enterprise Linux (Oracle Enterprise Linux 5 is the Latest Version)

For supported Unix/Linux versions for Oracle Applications 11i and Release 12, check the following link.
http://www.oracle.com/technology/support/metalink/index.html

Reference: 

           http://docs.sun.com/app/docs/prod/solaris.10
            http://docs.hp.com/en/oshpux11iv3.html

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