Implementing Global and Local variable using Set, Unset, env

The bash shell uses a feature called environment variables to store information about the shell session and the working environment. This feature also allows you to store data in memory that can be accessed by any program or script running from the shell. There are 2 types of variables Global and local.

    Global variables are visible from the shell session. Local variables are only available in the shell creates them.The linux system sets several global variables when you start your bash session. The system environment variables always use capital letters to differentiate them from normal user environmental variables

To view the global variables you can use the printenv command.
The set command displays all of the environmental variables set for a specific processes. How ever this also includes the global environment variables.

[root@tester ~]# set
BASH=/bin/bash
BASH_ARGC=()
BASH_ARGV=()
BASH_LINENO=()
BASH_SOURCE=()
BASH_VERSINFO=([0]="3" [1]="1" [2]="17" [3]="1" [4]="release" [5]="i686-redhat-linux-gnu")
BASH_VERSION='3.1.17(1)-release'
COLORS=/etc/DIR_COLORS.xterm
COLORTERM=gnome-terminal
COLUMNS=80
CVS_RSH=ssh
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-u7p0n37t4X,guid=5fc61850c666640beda0c84c08ab0a00
DESKTOP_SESSION=default
DESKTOP_STARTUP_ID=
DIRSTACK=()
DISPLAY=:0.0
EUID=0
GDMSESSION=default
GDM_XSERVER_LOCATION=local
GNOME_DESKTOP_SESSION_ID=Default
GNOME_KEYRING_SOCKET=/tmp/keyring-oY8mdS/socket
GROUPS=()
GTK_RC_FILES=/etc/gtk/gtkrc:/root/.gtkrc-1.2-gnome2
G_BROKEN_FILENAMES=1
HISTFILE=/root/.bash_history
HISTFILESIZE=1000
HISTSIZE=1000
HOME=/root
HOSTNAME=rac1.orainstance.com
HOSTTYPE=i686
IFS=$' \t\n'
INPUTRC=/etc/inputrc
LANG=en_US.UTF-8
LESSOPEN='|/usr/bin/lesspipe.sh %s'
LINES=24
LOGNAME=root
LS_COLORS='no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lzh=00;31:*.zip=00;31:*.z=00;31:*.Z=00;31:*.gz=00;31:*.bz2=00;31:*.bz=00;31:*.tz=00;31:*.rpm=00;31:*.cpio=00;31:*.jpg=00;35:*.gif=00;35:*.bmp=00;35:*.xbm=00;35:*.xpm=00;35:*.png=00;35:*.tif=00;35:'
MACHTYPE=i686-redhat-linux-gnu
MAIL=/var/spool/mail/root
MAILCHECK=60
OPTERR=1
OPTIND=1
OSTYPE=linux-gnu
PATH=/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/root/bin
PIPESTATUS=([0]="0")
PPID=5466
PROMPT_COMMAND='echo -ne "\033]0;${USER}@${HOSTNAME%%.*}:${PWD/#$HOME/~}"; echo -ne "\007"'
PS1='[\u@\h \W]\$ '
PS2='> '
PS4='+ '
PWD=/root
SESSION_MANAGER=local/rac1.orainstance.com:/tmp/.ICE-unix/4499
SHELL=/bin/bash
SHELLOPTS=braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor
SHLVL=2
SSH_AGENT_PID=4597
SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass
SSH_AUTH_SOCK=/tmp/ssh-nHuEYj4499/agent.4499
TERM=xterm
UID=0
USER=root
WINDOWID=31457361
XAUTHORITY=/tmp/.gdmRXFEIW
XMODIFIERS=@im=none
_=/etc/bashrc
consoletype=pty

[root@tester ~]# env
SSH_AGENT_PID=4597
HOSTNAME=rac1.orainstance.com
DESKTOP_STARTUP_ID=
SHELL=/bin/bash
TERM=xterm
HISTSIZE=1000
GTK_RC_FILES=/etc/gtk/gtkrc:/root/.gtkrc-1.2-gnome2
WINDOWID=31457361
USER=root
LS_COLORS=no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;31:*.tgz=00;31:*.arj=00;31:*.taz=00;31:*.lzh=00;31:*.zip=00;31:*.z=00;31:*.Z=00;31:*.gz=00;31:*.bz2=00;31:*.bz=00;31:*.tz=00;31:*.rpm=00;31:*.cpio=00;31:*.jpg=00;35:*.gif=00;35:*.bmp=00;35:*.xbm=00;35:*.xpm=00;35:*.png=00;35:*.tif=00;35:
GNOME_KEYRING_SOCKET=/tmp/keyring-oY8mdS/socket
SSH_AUTH_SOCK=/tmp/ssh-nHuEYj4499/agent.4499
SESSION_MANAGER=local/rac1.orainstance.com:/tmp/.ICE-unix/4499
MAIL=/var/spool/mail/root
DESKTOP_SESSION=default
PATH=/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/root/bin
GDM_XSERVER_LOCATION=local
INPUTRC=/etc/inputrc
PWD=/root
XMODIFIERS=@im=none
LANG=en_US.UTF-8
GDMSESSION=default
SSH_ASKPASS=/usr/libexec/openssh/gnome-ssh-askpass
HOME=/root
SHLVL=2
GNOME_DESKTOP_SESSION_ID=Default
LOGNAME=root
CVS_RSH=ssh
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-u7p0n37t4X,guid=5fc61850c666640beda0c84c08ab0a00
LESSOPEN=|/usr/bin/lesspipe.sh %s
DISPLAY=:0.0
G_BROKEN_FILENAMES=1
COLORTERM=gnome-terminal
XAUTHORITY=/tmp/.gdmRXFEIW
_=/bin/env

To display the value of an environmental variable, use the echo command.
You can set the environmental variables and remove it using unset command.


[root@tester ~]# test="krish"
[root@tester ~]# echo test
test
When referencing a environment variable , you must place a dollar sign before the environmental variable name
[root@tester ~]# echo $test
krish
[root@tester ~]# unset test
[root@tester ~]# echo $test

The PATH environment variables seems to cause the most problem on linux systes. It defines where the shell looks for commands you enter on the command line. If it cant find the command it produces an error message.
PATH=$PATH:/home/user/test
echo $PATH
/usr/kerberos/sbin:/usr/local/bin:/home/user/test
The individual directories separated by a colon.