Shell Scripting Tutorial-39: The 'while' Loop Video Lecture | Shell Scripting: Discovering to Automate Command-Line Tasks - Back-End Programming

hello guys my name is mother and welcome
to my channel this is the 39th tutorial
in the series of tutorials on UNIX shell
scripting in this video we're gonna
learn to use the while loop and while
loop is one of the three iteration
statements that UNIX provides us and the
other two are the untrue loop and the
for loop and we'll be checking them out
too in this course but for the time
being let's just concentrate on while
now what is while loop and why do you
use it while loop is something that
comes in handy when you have to do
something in your script over and over
again for example if you are preparing a
script that displays a message to the
user on screen 100 times or 500 times
then you would want to use echo you know
100 different lines or 500 lines in your
script instead you would want to use a
construct that instructs the shell that
ok I'm gonna I'm gonna type me an echo
statement you know at one line in my
script and I want you to execute it you
know 100 times or 500 times or as many
times as you want so that's why we use
the while loop and let's learn the
syntax of while by creating a simple
shell script so I have my text editor
here and I've saved this file in the
home folder I've given it the name while
in the extension SH and what this script
file is going to do is it's going to
display numbers from 1 to 10 on screen
right so it's gonna do a very simple
task and there are three things that you
have to keep in mind while using the
while loop and the first thing is to
declare and initialize a control
variable and that's nothing but a
variable that the loop uses to do what
it does and the second thing is to put
the control variable in a test condition
that the loop evaluates before each
iteration and the last thing is to
increment or decrement the value of the
control variable so that the loop gets
fresh value for the control variable
before evaluating the test condition
each time right so on the second line in
my script I'm going to type in a
variable name and it's called count and
I'll give it the value 1 right I'm
giving it the value 1 because I want to
start printing numbers 1 onwards on line
3 I'm gonna type in the keyword while
and I'll forward off to the space and
then I type in my square brackets and
within these square brackets I'm going
to type in the test condition that the
while loop will
before each iteration and only if this
test condition will return and exits are
zero or you know if the stress condition
will evaluate to true will the body of
the loop get executed and if it
evaluates to false then the loop will do
nothing right so I'm gonna type in the
test condition for this script it's
gonna be dollar count less than equal to
ten and I'm typing in ten there because
I'm gonna print numbers from one to ten
if suppose you wanna print numbers from
1 to 100 then all you have to do is
change 10 here to 100 right so it's
gonna be that simple and online for I
type in another keyword and this one's
called do-right we are now inside the
body of the while loop and we can type
in the statements that we want the loop
to execute over and over again so the
first one of these statements is going
to be echo dollar count and this will
display the value of count on screen and
on line 6 we will increment the value of
count so that on the first line 1 gets
printed and on the next line 2 gets
printed and on the next line 3 gets
printed that is we get a fresh value on
each line right so the way I will
increment the control variable is I'm
going to use expr and for the space and
then type in the variable dollar count
the addition symbol and I'm going to add
one to its value and I'll assign this
value do my variable so count gets the
value exp R plus dollar count and since
I'm just concerned with the output of
this expression I'm gonna put this
expression within reverse codes right so
on line seven I'm going to type in
another keyword and this one's done and
this is the last keyword for the script
so these three keywords while do and
done are part of the while syntax and
you have to remember them right so let's
talk about how this loop works while
will exit evaluate the exit status of
this condition that is it will check
whether this is true or not and count
initially is one and one is obviously
less than ten that's why these two
statements will get executed but for the
next iteration count will not have the
value 1 instead it will have the value 2
why because we've incremented it by
using the
we are on line six right so two is still
less than ten so these statements will
get executed again so for the next
titration count will get three and three
is still less than equal to 10 and this
will keep on happening till count gets
the value 11 and that's why when we will
see the output of the script we see that
11 won't get printed and only numbers
from 1 to 10 will get printed so I save
the script I bring up my terminal and
execute the script by typing in SH space
wild or Sh and there you go in the
output you see the numbers from 1 to 10
are being printed so that's it I hope
you people have understood how the while
loop works the purpose behind this
tutorial was to make you guys familiar
with the syntax of while I could have
chosen a more complicated application
this is you know obviously pretty simple
displaying numbers 10 numbers on-screen
but you know we leave the complicated
part for some other tutorial in this
course I just want you guys to you know
practice this and that's a while for the
time being and once you master it you'll
be able to understand until and for
pretty easily right so in the next
tutorial we're going to check out the
anti loop till then thank you so much
for watching this tutorial and I'll see
you guys in the next one please
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