Shell Scripting Tutorial-51: Nested Loops 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 51st tutorial
in the series of tutorials on UNIX shell
scripting in this tutorial we're going
to talk about nested loops and just as
with the nested if-else construct in
lester loops you can have one loop
within the body of another loop right so
you have one while loop and within the
body of that loop you can have another
while loop and within the body of that
while loop you can have yet another
while loop so you know there is no
restriction or limitation on the level
of nesting that you can have in your
shell scripts you can have as many while
loops within as many while loops as you
want right so to demonstrate the concept
of nested loops I'm going to use a shell
script so I bring up my text editor and
i'm save this file in the home folder
i've given it the name pattern and has
the extension SH right so let's get
started what I'm going to do in this
script is I'm gonna make it display all
combinations of the numbers 1 2 & 3
right and there can be 27 combinations
like 1 1 1 and 1 to 2 and 1 to 3 and
then 2 to 2 and 3 3 3 right so there can
be like so many combinations and once we
prepare the shell script I'm going to
show you guys the output and you can see
for yourself so on line 1 I'm going to
declare and initialize the variable the
control variable for my first or the
outermost while loop and I'm gonna call
this one a and I'll give it the value 1
since we want to start our range from 1
and extended to 3 on line 2 I'm going to
type in my first while statement and the
text condition for this statement is
going to be while the owner a is less
than equal to 3 right because this is
going to be the range of my control
variable it's going to start from 1 and
it's going to end at 3 and this is going
to be true for the other two control
variables as well and we're going to use
3 while loops so on line 3 I'll type in
the keyword do you get inside the body
of the first while loop and now I'll
declare and initialize the control
variable for my second while loop I'm
going to call this one B and I'll give
it the value 1 right on line 5 I'll type
in my second while statement and the
test condition this time is going to be
while dollar b is less than equal to 3
right and on line 6 i'll type in the
keyword do to get inside the body of the
second while loop and now i declare and
initialize the variable for my third
while loop I'll call it C and I'll give
it the value 1 and online aid I'm going
to type in my last while statement and
the test condition for this one is going
to be while dollar C is less than equal
to 3 right and online I'll type in the
keyword to get inside the body of the
innermost while loop and now I'll use
the echo statement to display the 3
variables on screen so dollar a dollar B
and dollar C right so all three
variables will be displayed and now I'll
increment the control variable for the
innermost while loop by typing in expr
space Taurus II space addition symbol
space 1 and I'll put this with the
reverse codes and I'll assign see the
value of this mathematical expression
and then I'll terminate the body of my
innermost while loop by typing in the
keyword done on line 12 and then I'll
increment the control variable for my
second while loop and this I'm going to
do by typing in b is equal to e x PR
space dollar B space plus space 1 right
and the mathematical expression will
obviously go in reverse codes and then
I'll type in the keyword done to
terminate my second while loop and next
I'll increment the control variable for
my are uttermost while loop and that I'm
going to do by typing in a is equal to e
x PR space dollar a space addition
symbol space 1 right and the expression
will go in reverse codes and then on
line 16 I'll type
the key word done and our script is
ready right so now we can check out the
baton that we get I have my terminal
here I'll type in the command SH space
Patton thought SH and that's the name of
the script file that we just made and
when I press the Enter key
you see these 27 numbers on-screen and
the first one is 1 1 1 and the last one
is 3 3 3 and all the cases between these
two numbers have been covered right and
you know the number that you see on 1 1
2 online - sorry you won't see that
number on any other line so all these
numbers are different and we've covered
all possible combinations that you can
have with the numbers 1 2 and 3 and I'm
talking about three-digit combinations
so that's it
in this script we haven't really done
anything fancy but this was the best
example that I could think of to
demonstrate the concept of nested loops
and you would be using nested loops Lord
not only in this course but if you
intend to take up shell programming as a
career option then you know many times
you would be required to use nested
loops and they can be you know pretty
confusing sometimes and it's important
to comment your code you know if the
level of nesting is pretty high in your
shell scripts and in this case we didn't
really need comments but you know it's
all it always helps to have comments in
your shell scripts so thank you so much
for watching this tutorial I see you
guys in the next one in which we will
discuss something important again and
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