PATH - An Environmental Variable

PATH is an environmental variable in Linux and other Unix-like operating systems that tells the shell which directories to search for executable files (i.e., ready-to-run programs) in response to commands issued by a user. It increases both the convenience and the safety of such operating systems and is widely considered to be the single most important environmental variable.
Environmental variables are a class of variables (i.e., items whose values can be changed) that tell the shell how to behave as the user works at the command line (i.e., in a text-only mode) or with shell scripts (i.e., short programs written in a shell programming language). A shell is a program that provides the traditional, text-only user interface for Unix-like operating systems; its primary function is to read commands that are typed
in at the command line and then execute (i.e., run) them.


Practical - Setting Path

Login as root

$id
$echo $PATH
$useradd john
$passwd john
$su - john
$id

Verify john's PATH

$echo $PATH

you cant find :/sbin:/usr/sbin so u cant run cmnd's fdisk, shred under the same.

$fdisk -l

will get command not found.

So u can set path, but it's temporary for the shell.

$PATH=$PATH=:/sbin:/usr/sbin

To set under environment run

$export PATH

For permanent you can locate the above two cmnds under /etc/profile file, which run's always after login.
Now chk you will get the above added dir under john’s path.

$echo $PATH

Now try

$ fdisk -l

Note-: The cmd is executed but fdisk binary will work only by uid 0(root), bcoz it's programmed like that.
So search for the cmd in /sbin & /usr/sbin , which can run by other uid's.
Now create a testscript under /opt and execute the script

$vi /opt/testscript
#Append the following
echo “ THIS IS MY SCRIPT”
#Save
$cd /opt

set execute permisson

$chmod +x /opt/testscript
$./testscript # (./ means current path execution)

But what if u want to run the script from any other directories under your filesystem hiriearchy.
Then set the /opt dir to the users path as mentioned above or copy the script under the following PATH . (which is already set)
set. 
For eg-:

$PATH=$PATH:/opt
$cd /
$testscript

or

$cp /opt/testscript /bin or /usr/local/bin etc...

Now try running the script

$cd /
$testscript