Shell Scripting Tutorial-23: Math On Integers Using 'expr' 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-third
tutorial in the series of tutorials on
UNIX shell programming now in this
tutorial we're going to learn how to
perform arithmetic operations in our
shell scripts and what I mean by that is
if suppose you have two variables and
both of them have numbers then you can
add them you can subtract them you can
divide them multiply them and you can
also perform the modular division now
what I'm gonna do is I'm gonna open my
text editor because we're gonna do all
this through the to a shell script and
I'm gonna save this file first before
typing in anything and I'll give it the
name ss8 not SH I'll save it in my home
folder and I put a comment first
something like the medic operations and
now what I'm gonna do is I'm gonna
create two variables a and B and in a
I'm gonna store a number you can put
anything out put 30 and then B I'm gonna
store another number I'll put 15 and a
right and then on the next line I'm
gonna add these numbers and the way I'm
gonna do it is I'm gonna type in dollar
a and I give a space and I'll use the
addition symbol and then I'll type in
dollar B and on the next line I'm gonna
subtract B from a so or maybe yeah
that's what I'm gonna do so daughter a
space then minus sign then daughter be
the next line I'm gonna multiply these
two numbers and I use the asterisk or
the multiplication symbol to multiply
them but I told you guys that asterisk
is a symbol that has some special
meaning to the shell it basically refers
to all files or directories all items
that you have in your current directory
right so we would want to take the
special meaning away from the astrick
symbol before using it in the sense in
which we want to and the way you do that
is by using the backslash it's a
character that you would find under your
backspace on the keyboard and you put it
before asterisk and it would become
the normal multiplication operator right
and then I'll also divide the variable a
by B and I'll type in dollar a space the
division operator and then dollar B and
what else can I do I also perform the
modular division I'll tell you guys
about it what the modular division means
in just a second and there you go but
then we haven't used any command right
we haven't used any program to basically
perform these operations how would the
shell know what this means I mean for it
for the shell this could be just another
statement now there is a command that
would instruct the shell to do not just
treat this statement or these statements
as ordinary statements but as
mathematical expressions and the
statement or the command is expr so I
will type in expr before each and every
statement that I've gotten this program
there you go and there has to be a space
off for a experienced way and that is it
that's pretty much all you have to do
but we're not going to just evaluate
these expressions right we also have to
display them on screen and how do you
display something on screen using the
echo statement so I'm gonna echo out
these results for you on screen
that's it now my script is almost ready
it's like 99% of the work has been done
only 1% remains and that 1% is very
crucial now when I would execute the
statement in the shell then the shell
would probably think that ok I basically
want to echo out this on screen but how
would I know that it has to evaluate
this statement first and then output the
actual result of the statement on screen
and not the statement itself the shell
would you know think that ok you know
these are just words experience one word
dollar a is another world and then I
have the symbol here and then I have
daughter B as in other word so let me
just output these five or six words on
screen all right but we don't want it to
do that so what we would have to do is
we would have to use our reverse quotes
something that we studied in one of the
previous tutorials and what the reverse
codes would do is the reverse quotes
would you know instruct the shell to
first evaluate these statements and you
know find out the result of each of
these statements and then echo the
results out and not these statements
themselves all right so there you go
that's a my script is now ready and I
can execute it and let me first save it
it's gonna take some time
yeah it's done now so I have my terminal
here and I'm gonna simply use SH and
type in the name of my file which is SS
8 dot SH and when I press the Enter key
I have all these results now what is 30
plus 15 it's 45 what do we have we have
45 okay what is the next thing 30 minus
15 which is 15 so I have 15 on screen on
the next line we have the multiplication
so 30 times 15 is 450 we have 450 on the
next line we have 30 divided by 15 which
is 2 we have 2 and on the last line we
have modular division
now what modular dimension means is that
this shell would basically divide dollar
a with dollar be and it would see if
there is any remainder or not now 30 is
perfectly divisible by 15 so the
remainder is
but if I change 30 and if I put
something else like let's say if I put
37 here then 15 times 2 is 30 and 7
would remain as a remainder right so 7
would be the output of the last
statement let me clear the terminal for
you and when I execute it you see that
on the last statement I have 7 right now
one major drawback with expr is that it
works with only integer values which
means that if I would have thirty seven
point seven eight here then it would
basically not work it will not give me
the correct output now there is another
tool that you use to perform arithmetic
operations with your fractional numbers
and that tool is called BC and that's
something that will be checking out
later but in this tutorial that's all I
wanted to share with you guys so thank
you so much for watching this one it
really means a lot to me and hope to see
you guys in the next tutorials and
please subscribe to my channel