Shell Scripting Tutorial for Beginners 25 - Function Example Video Lecture | Study Complete Linux Course: Become a Linux Professional - Software Development | Best Video for Software Development

hey guys welcome to the next video on
shell scripting tutorial for beginners
in the last two videos we have seen how
to use functions in shell scripting now
in this video to consolidate our
knowledge which we have gained in the
last two videos we are going to take an
example and then from this example some
more concept will be clearer to you so
let's start with the example and then we
will discuss about the concepts so what
we want is we want to check whether a
file exists or not using our script
so whenever user runs the script with
hand argument and this argument will be
a name of the file for example and the
script will check whether this file
exists or not and it will print now this
example we have seen in the last videos
also but in this video we will use first
of all functions and then we will use
some more concept so let's get started
so let's say our function name is does v
exist or is the file exists right so is
file exists and then we will just use
these brackets and these curly brackets
right right now we want to pass an
argument so the name of the file we want
to pass as an argument so what we are
going to do is we will just declare a
local variable we can say it's file
write a file name you can name it
anything and this is a first argument so
dollar 1 is the first argument to this
function write this we have learned in
the last video now we want to check
whether this file exists or not so we
can use these double square brackets for
that right now in the previous video you
also know that minus F flag can be used
to check whether the given you know file
is a really file or a directory or
whatever right so now we have this file
you know variable which is a local
variable which comes from this argument
so we will check whether this file is
really a file or not so we will just say
dollar file and then this will give us
true or false right now notice are not
giving any if conditions here right I am
going to use the return value so this if
you are familiar with some other
programming language you may know the
ternary operations so we are checking
whether one operation is true depending
upon the you know result if it's true we
will return some value otherwise we will
return some other value right so we will
just use double and operator and then we
will return zero I will explain you what
I am writing or I will return one okay
so let me explain you what's happening
with this line so we already know that
this can be used to you know check that
the file exists or not right so we will
either get our true or false or in the
terms of this script we will either get
1 or 0 so when in the condition a file
exists we will get 1 right now if you
know the program flow or how program
works whenever we get 1 in the first
condition and whenever there is end here
so for example there is end and we have
the other condition right or so
condition to I will just write Co and to
write so whenever they
is one right so it's true here and when
it's true then program flow is not going
to check this one or this will directly
jumps to the or condition because we
just want to see if this condition is
true or not right so in the end
condition we will check if it's zero
then it goes to the next and operator
which is this one so it would be zero
then it's going to go to the return zero
you know operation because here is zero
and then if we are not getting one here
then program cannot move forward so it's
going to check whether the other
condition is true or not and so on right
but here we know that first condition is
already to true so it's going to
directly go to the return one condition
right and if it's false for example if
it's false then it's going to check the
second condition which is return true
and it gets the written zero here and
then it's going to return zero right so
in case of file doesn't exist it will be
zero and then it going it's going to
check the other part of the end
operation if it's one it's already got
its true value so it's going to go to
the next operation which is in the or
operation so first of all we will check
whether user has given any argument or
not with the shell scripting right so we
will once again use these double
brackets and then if you remember this
dollar hashed which is a built-in
variable is going to return us the
number of arguments number of arguments
which are given with the script so this
is going to give us the number of
argument and if this number of argument
is equal
zero then we will say we want to do
something or we want to show some
message to the user so for this I am
going to you know create some usage
function for example so I'm going to
just write usage here and let's create
this usage function here for example
right so usage and then we will just use
this curly brace brackets and this just
simply for example print some message
and exists so we are going to print for
example so we will just echo this
message that you need to provide an
argument and we will just give the usage
of this script also so we need to
provide the name of the script and the
file name right and if the file name is
supplied and argument is for example
more than 0 then we will go forward
right and then what we are going to do
in the next step we will just check so
we will use if condition and then we
will call this function but let's first
give for example then and we want to use
else also here so we will write else and
then if I to end the condition so we
will call this function a file exists we
are going to get the true value and then
we will print the message so here we
will just give the first arguments or
dollar one now be eye in mind this is
the first argument of this function and
this is different this is the first
argument which is provided to the script
right this is the first argument because
it's inside a function this is the first
argument provided to the function right
and this is the first argument which is
provided to the script right so these
are two
declaration of dollar one are different
okay and then if pile exists we will
just echo that while is found or file
found otherwise we will just print file
not found right so let's print file not
found
okay now let's run the script let's
first do Alice here list all the files
which are there in my desktop directory
so this there is only one file which is
the script itself right so let's create
one file so I am going to just touch for
example file 1 dot txt here okay and
then I am going to press ENTER and then
I am going to to LS and now I have a
file name called file 1 dot txt so I can
just run by this my script and then
first of all let's not give any argument
here we know that we need to give a file
name as an argument with the script but
let's provide our wrong argument or no
argument here and then just press ENTER
and then it says you need to provide an
argument usage is the script name and
then the file name and I also see this
message that local command not found
because it's spelled wrong so it should
be locally not lack law local right so
this keyword should be local right so
now next attempt we will give the name
of the file and then press Enter and now
we will see file found so what we have
learned from this video from this
example two things we have learnt we
have already learned how to use
functions but we learned that this
dollar one and this taller one are
different this is for the argument which
is provided with this one right
with the calling of your scrip and this
is the argument which is provided to the
function right and we have learned how
we can use and and/or operation without
the if conditions right you could have
used this in different conditions also
so you could have you know written if
this is this and then return 0 or 1 this
could be also done but it can take more
number of lines right so if you are not
comfortable with this code then you can
always switch to if conditions like
normal life conditions and then also you
can you know get your result ok so I
hope you've enjoyed this video please
rate comment subscribe and bye for now