Shell Scripting Tutorial-24: Operator Precedence 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 twenty-fourth
tutorial in the series of tutorials on
UNIX shell programming now in this
tutorial I'm just gonna discuss a little
bit of theory with you and maybe if we
have time we'll prepare a script but I
don't think we will so what I'm gonna
talk about is operator precedence now
what does operator precedence mean in
the last tutorial when we learn to
perform arithmetic operations I
discussed five operators with you right
and let me first open my script file
this was it and there you go these are
my five operators I have the addition
operator I have the subtraction operator
I have the multiplication operator then
the division operator and the modular
division operator so these are the five
operators that I discussed now all these
operators if they appear in a single
mathematical expression then they have a
particular order a specific order in
which they would evaluate and the first
priority is given to the division
multiplication and modular division
operators that is we clear out this
section for you that is the division and
the multiplication and the modular
division have the same priority and the
addition and the subtraction have the
same priority but their priority is not
the same as the priority that these guys
enjoy so enjoy so what I mean by that is
if Ennis expression you have division
multiplication and subtraction first
then these operations will be performed
first and then addition and subtraction
would be performed right and you guys
can verify that at your own end you can
prepare shell script with you know a
random expression and you can execute it
and see how it goes now what if you wish
to change the order of precedence if
suppose you have an expression like this
one say this out I'm gonna create two
more variables as a matter of fact in
this script let me change the value of a
to 31st
see give it a value let's say do and
I'll have D now give it a value say five
right and if suppose I wish to evaluate
an expression like taller a multiplied
by the result of the addition of dollar
B and dollar C and if I wish to divide
the entire thing by dollar D now when I
would perform this evaluation this when
I would evaluate this expression using
expr the shell would say that okay I
have multiplication operator here and it
appears first so let me first multiply
dollar a with dollar B and then I'll
probably add it to daughter C and okay
no maybe I won't add it because division
has enjoys higher priority than the
addition symbol so I'll first divide
dollar C by dollar D and then I'll add
the output of dollar a plus dollar B and
daughter suppressed already but that's
not what we want what we want the shell
is to do is to first add dollar be to
dollar C and then to multiply the output
of the addition of these two operators
$2 a and then then divide the entire
thing by dollar D right so that's what
we wanted to do now how do we do that
what I'm gonna do is I'm gonna put these
I'm gonna put this middle expression in
parentheses or parents you can call them
whatever you want and what this is gonna
do is it's gonna instruct the shell to
perform this part of the expression
before anything else now deeper the
parents have special meaning to the
shell they cannot be used in you know in
in this way what we would have to do to
take the special meaning away is escape
them so that's what I'm gonna do I'm
gonna put the escape character here and
I'm also gonna put the escape character
here and I have to give a space between
all of my items in the expression and
this is very important you know
if you are performing a simple addition
like dollar a plus dollar B then you
have to give a space between dollar a
and the addition symbol and dollar B and
the relationship but if you don't do
that then exp I just want to evaluate
and it's a very stubborn tool actually
and you know it can be very frustrating
debugging your scripts when you have
errors like these so that's it I'm gonna
use expr and I'm gonna put this entire
thing in reverse codes right and I'm
gonna echo out the output on screen and
I'm gonna save this file and remember I
haven't created a new script so my file
name is SS 8 dot Sh I'll clear the
terminal for you guys and if we go we're
ready to execute SS 8 not Sh when I
press the Enter key I get the output
hundred and two and we can actually
check if you guys warned what has
happened is we first added 15 to two so
the output is 17 then we multiply it 17
with 30 and what would be the output
that would be 17 times 51 so 510 when
divided by 5 gives us the output 52 so
sorry hundred and two so you can check I
have my calculator here otherwise you
can do it on your own
510 divided by 5 is nothing but hardened
to so that's it that's all I wanted to
discuss in this tutorial I hope you got
the idea behind operator precedence and
why it is important and how you can get
around it if you want to basically do
anything that's against the rules that
have been specified and that's by using
parentheses and you'll have to escape
the parentheses just like you escape
your multiplication operator and yeah
that's pretty much it in the next
tutorial we will learn to perform
arithmetic operations on real numbers
studies numbers with decimal points in
them and yeah that's for the next
tutorial thank you so much for watching
this one and please subscribe to my
channel