Shell Scripting Tutorial-40: The 'until' 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 fortieth
tutorial in the series of tutorials on
UNIX shell programming in this tutorial
we're going to check out the untell loop
which is only slightly different from
the while loop that we checked out in
the previous tutorial if you guys
remember in while the while block gets
executed if the test condition that's
placed within the square brackets
evaluates to true in the case of untell
however the --until block gets executed
if the test condition evaluates to false
so that's the only difference between
while and until and if you know while
and if you understood the script in the
previous tutorial then this is going to
be a minutes plate for you so let's not
waste any more time I have my text
editor here and I've saved this file in
the home folder giving it the name until
and the extension SH right so what the
script is going to do is it's going to
behave in the same way as the script in
the previous tutorial day that is it's
going to display numbers from one to ten
on screen so on the first line I'm going
to put a comment until loop in action ok
and this really isn't necessary I don't
even know why I do this in every
tutorial right but anyway so on the next
line I'm going to declare and initialize
my control variable for the loop and I'm
going to call it count
cou NT and I'm going to give it the
value one because we want to start
printing our numbers from 1 onwards
right on the next line I'm gonna put the
keyword until and I give a space and
within square brackets I'll put my test
condition now for while our test
condition was that dollar count
we checked if the or account was less
than equal to 10 or not right but in
this case since we want the test
condition to evaluate to false we are
not going to use the - le operator
instead we're going to use the - GE
operator right so we're going to check
if dollar count is greater than equal to
ten which for the first I train it's
definitely not because count initially
is one and it's not greater than equal
then of course so just as for while in
case of until we put all our executable
statements between the
keywords do and done so on the fourth
line I'm going to put the keyword do and
on the fifth line I'm going to put the
keyword done and all my statements for
the --until blog are going to go between
due and done right so the first
statement is going to be to display the
variable so dollar echo dollar account
that will be do the job for me and on
the next line I'm going to use expr to
increment the value of count so expr
dollar count plus one make sure to put a
space immediately before and after the
addition symbol if you're using expr
because you know if you won't do that
then expr will return annoyed and I'll
put the expression within reverse code
since we are concerned just with the
output of this expression and I'll
assign count the output of this
expression right so the script is
complete now and I'll save it and I'll
bring up my terminal and execute it
until @sh and there you go you see
numbers from 1 to 9 and the reason why
you don't see a 10 is because we've used
- GE if you would use a hyphen GT then
you would see 10 as well wait a second
let me first clear the screen there you
go now you see numbers from 1 to 10 so
let me talk you guys through the script
once again we have used - GT operator
here because we wanted this test
condition to evaluate to false and while
this condition will evaluate to false or
untell this condition gets a true value
the --until block will keep on getting
executed so count is equal 1 and it is
not greater than 10 so execute the unta
loop then we will increment the variable
and count for the next iteration will
get the value 2 2 is not greater than 10
well executed again execute the untrue
block then count gets 3 3 is not greater
than 10 execute the uncial block again
and this will keep on happening till
count we get the value 11
and once count gets the value 11 11 is
of course greater than 10 so you know
that would be the time when until you
will key or will stop executing its
statements
so thank you so much for watching this
tutorial I hope it was worth your time
and I'll see you guys in the next
tutorial where we discuss something
interesting please subscribe to my
channel