Shell Scripting Tutorial-37: The 'case' Statement 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 37th tutorial
in the series of tutorials on UNIX shell
scripting in this tutorial we're gonna
encounter the case selection statement
for the first time and if you guys
remember that when I discussed the if
statement with you I told you that there
are 4 selection constructs that UNIX
provides us and three of them we've
already discussed and those three are
the if statement and the if then else
statement and the if then eel if-else
statement and we've also studied the
nested if-else statement which is not a
separate statement but just a variation
of the standard if-else so what I'm
gonna do in this script is I'm going to
ask the user to enter character and then
I'll check whether that character is a
lowercase alphabet or an uppercase
alphabet or a digit or a special symbol
or a if it is not one character the user
is entered but more than one character
is right and in each of these cases we
display appropriate messages so let's
get started on the first line I'll use
the echo statement and ask the user to
enter a character and you know what I'll
probably save the script file before you
know typing in the code because I like
syntax highlighting feature of this text
editor so I'm gonna give it the name
case extension SH and I'll save it in my
home folder as I usually do and on the
next line I'll use read you store the
input in a variable called var write and
now we have to get inside the case block
and the way you do that is by typing in
the keyword CA se that is case and then
you have to use a control variable and
in this case since our character is
stored in the variable var var is going
to be our control variable and I'll use
the dollar symbol and var because I'm
interested in the value of var and not
just a string year and then you have to
give a space and type in another keyword
and this one is in right and this would
indicate the shell that we are now going
to enter inside the case block so
whatever commands you see or whatever
cases you see they all belong to you
know the case that's controlled by the
variable dollar pair right so I hope
there isn't any confusion up till now
and now we have to check whether the
character is a lowercase alphabet or not
and the way you do that is by not typing
in characters from A to Z like abcdefgh
after that you don't do that right
instead what we do is we use the square
brackets meta-characters and you simply
type in your two square brackets right
the opening bracket and the closed
bracket and in between these brackets
you type in the range of values with
which you want dollar var to be matched
and you know I'm gonna start with small
a of course because that's the first
lowercase alphabet and I'm gonna extend
it to small Z then that's the last
lowercase alphabet so this variable will
be matched with not just a and Z but all
the alphabets that you have all the
lowercase alphabets that you have
between a and Z right so isn't that
amazing I mean imagine if you would if
you were to do this using if-else then
you would obviously have to use the or
operator 24 times like you know check if
the order bar is equal to a - Oh
daughter bar is equal to small B - no
daughter bar is equal to small C so you
know that would obviously consume a lot
of time unnecessarily so after typing in
this I'm gonna type in the closing
pattern and what this is going to do is
this is going to instruct the shell now
get inside the case that is represented
by this range of values right here right
so first we got inside the case block
and now we are going to get inside the
block of range of values that are
represented by this case right here
right so inside this block what I'm
going to do is I'm going to just use
echo to display a message like you
entered a lower case alphabet right and
to get out of this case just this case
what I have to do is I'll have to use a
: twice right so if I type in semicolon
twice that wouldn't shut the shell that
you know we're out of this case now and
we can move on to something else so now
we will have to type in the code for the
next case and before that we have to
tell the shell what is the range of
values with which we want our variable
to be compared and in this case it will
be capital a to capital Z right and of
course I'll put my closing parent here
and on the next line I'll type in the
statement that I want to be executed if
the variable lies within this range here
so what do I print in you entered an
upper case alphabet right and on the
next line just as I did in the previous
case to move out I'll have to type in
semicolon twice okay
so now we have to cover our next case
which is going to be four digits so the
range of values would be from 0 - 9
because these are the digits known to us
get inside the case again the closing
pattern on the next line the echo
statement you entered a digit and let me
to stop there and on the next line to
move out of the case again two
semicolons and now if any of these cases
are not executed and why would that
happen dollar var would be first
compared to all the values that lie
between small a and small Z and if a
match would be found then this echo
statement would be executed but if a
match would not be found then we would
compare the value of the variable var
with the the values within the range
capital A - Capital Z and if a value if
a match would be found then this echo
statement would be executed but if not
we would get to this case but if suppose
we type in J as the input now J small J
lies between small a and small Z so this
echo statement would be executed and we
wouldn't really execute this case or
this case
is or any of the cases that would follow
and we would simply move out of the case
block right because our condition would
be met in the very first case so you
know we would get here only if none of
these cases have been executed so on the
next line I'm going to begin a new case
and this one will be executed if all
three cases above have failed right and
that's going to be to check if the
character is neither a small case
alphabet nor a capital case letter nor a
digit but what a special symbol so I'm
going to use the question mark metal
character and I haven't ever spoken
about radiate and what the question mark
represents is it it means one character
right so it substitutes it's basically a
substitution for one character and the
asterisk as you must be remembering is a
you know is a substitution for
everything you know if you have if you
use asterisk in a diet tree then it
means everything that you have in the
directory and if you use it for input
that it means all the inputs that have
been provided so you know if it's one
character then it would be in one
character but if there are more than one
characters and an asterisk will
represent all the characters so in this
case the echo statement would have to
print out echo you entered a special
symbol okay and move out again through
semicolons and on the next line we would
have to cover for more than one
characters right because that's the only
thing left now so the asterisk symbol
and the closing parent and on the next
line we'll use echo to display another
message and this one would be you
entered more than one characters right
and what this our script work is over
and what I'll have to do is firstly
obviously I'll have to exit this case
and after exiting this case I'll have to
close my entire case block and the way I
would do that is by typing in the
keyword case but this time in the
reverse order so it would be
CSAC right it's pretty it's analogous to
what we did for if right with if and fi
we have a case and a sack as you can
call it I call it a sack right so I'm
gonna save the script file and I'll uh
just a second so I have my terminal here
and let me first maximize it for you and
I'll execute the script by typing in SH
space the name of the script which is
case gone SH when I'd type in I press
the Enter key it asks me for a character
so if I type in small Y here then it
would tell me that I entered a lowercase
alphabet and if I would type in capital
G here then it would tell me that I
entered an uppercase alphabet and if I
would type in a number like five then it
would tell me that I entered a digit and
if I would enter a symbol such as the
pound sign or you know the person date
symbol then in both cases it tells me
that I entered a special symbol and if
in any case I would you know type in
more than one characters and press the
Enter key then it would tell me that I
entered more than one characters right
so that's what the script does I hope
you've got the basic idea behind it and
you know the purpose behind the script
was to just make you guys familiar with
the case statement and we will be you
know probably going through a few more
applications of case in the coming
tutorials so if something is not clear
to you guys in this script just hang on
and maybe in the coming scripts I'll you
know explain the case statement the
working of it in a better way and you'll
probably be able to understand it then
so thank you so much for watching this
one I'm running out of time otherwise I
would have walk you guys through the
script once again but in the next
tutorial I promise I'll clarify things
for you in case you have any doubts so
thanks for watching and see you in the
next tutorial please subscribe to my
channel