Shell Scripting Tutorial-28: 'if-then' Statement in Action 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 28th tutorial
in the series of tutorials on UNIX shell
scripting in this tutorial we're going
to learn to use the if-then statement in
our scripts to make decisions now before
I demonstrate the if-then statement to
you guys what I'm going to do is I'm
going to take a minute and talk about
exit status now I have my terminal here
and if I type in a command any command
even clear then what the shell does is
it associates of value to the command
and it basically uses two numbers it
uses zero if the command has been
executed successfully and it uses one if
the command has not been executed
successfully so if I would use mkdir
with the the argument new here and if I
would press the Enter key then a folder
called new would have been created in my
home folder and if I want to check out
whether this was done successfully or
not then I have a lot of options I can
either go to the home folder and find my
folder new there or I can execute the LS
command in my terminal and find the
directory named new in the output of LS
or I can check out the exit status of
the command mkdir space new and in order
to do that I would have to echo out the
system variable dollar question mark
on-screen so there you go you see that
the output is 0 which means that the
command was executed successfully if I
would execute the same command again
then this time mkdir would throw an
error message at me because the the
folder new already exists and it could
not create it again so if I would check
out the exit status of this command I
would see that the shell displays one to
me instead of zero which means that
there was some problem with the command
either the syntax was incorrect or the
argument that I passed to the command
was was faulty there was some problem
with the command and thus its existence
just one it would tell that me right so
now we are at the point where we can
begin our discussion on F so what I do
is I'll open my text editor and I save
this script file and I've become
more than sick and tired of the naming
convention that we've been following in
this course so I'll be a little more
elaborate and I'll give a name F if then
thought s edge that is if - then Lord s
H to be just a little more elaborate I
hope you guys appreciate that on the
first line I put a comment something
like if-then statement in action now
what I'm going to do in this script is
I'm going to ask the user to provide me
two names and the first name has to be
the name of a file that exists in the
home folder and if the file exists then
it would be renamed to the second name
that has been provided to the square
right so it's going to be a simple
simple file and we would use the MV
command for renaming and we've discussed
MV in the previous tutorials in case you
don't know how MV works then you can
check out the other tutorials that are
made and you would find MV in one of the
tutorials right so first I'll have to
ask the user to provide me - file names
and I will display a message in order to
ask him so I would type in enter source
and target file names there you go on
the next line I'll use Reed to grab
those names and I'll store them in the
variables source and target respectively
on the next line I'll use MV to rename
my source file to the target file ok now
what if I want to display a message if
this command was executed successfully
so let me first type in the message that
I would want to show and the message
would be your file has been successfully
renamed
right but I would want this message to
show up only if this operation was
performed successfully so what I would
have to do is I would have to use the if
statement in this case so if I simply
type in the keyword if before my MV
command then what the shell would do is
it would first execute this command and
it would find out the exit status of
this command right if the exit status
would be zero then it would execute the
echo statement and if the exercise would
be one then it would basically do
nothing right but the if statement
doesn't work alone it requires a two
more keywords the first keyword that it
requires is the den keyword and the
other keyword that it requires is the fi
keyword now fi is nothing but if spelled
backwards and it's something that you
have to put at the end of your if log to
indicate to the shell that we've done
working with our if statement and the
range of F is only between the then
keyword and the fi keyword right so I
hope you got this and the syntax is very
simple and the operation is even simpler
it checks whether the command that
follows the if has been executed
successfully or not it checks the exit
status if it is zero they considered
itself good to go if it's one then it
does nothing right so it's very simple
so I'll save the script file and what I
do now is I'll clear my terminal first
and I'll use touch to create a file in
my home folder and I'll create a file
called test right so test has been
created now I'll use SS to execute my
script file and it's called f5 and then
third SH and I have to enter two file
names and I'll input test as the first
name and I would input something else
like sample as the second name so what
the script would do is it would check if
the file test exists in my home folder
if it does then it would rename test to
sample and if it doesn't exist that is
if it won't be able to find test then it
would basically do nothing but the shell
would return an error message to me not
the script right so if I press the Enter
key here
you wouldn't see that I get the message
your file has been successfully renamed
so that's what the if statement does for
you it checks the exit status of Pokemon
and it tells you oh sorry and it
executes a piece of code anything that
you type in between the keywords then
and if spells backwards that's fi right
so that was our discussion on if then in
the next tutorial we will be checking
out the if-then-else statement and it
basically gives us another option in
this tutorial we did some work because
the exit status of our command was 0
what if we wanted to display another
message if the exit status was 1 and in
order to that you would have to use the
keyword else and I will leave that
discussion for the next tutorial
thank you so much for watching this one
please subscribe to my channel and I'll
see you guys in the next tutorial