Shell Scripting Tutorial for Beginners 2 - using Variables and Comments Video Lecture | Complete Linux Course: Become a Linux Professional - Software Development

hello guys welcome to the next video on
shelf tripping tutorial for beginners in
the last video we have seen how we can
create our first script and how we can
execute our first hello world script now
in this video we will talk about
comments and variables so let's get
started so first of all let's talk about
comments because they are really easy to
use and understand so first of all what
are comments comments are the lines of
code which are not executed by a script
but are helpful to know some information
about your scripts so how we can write
comments we can write comments with a
hash before a line of comments so
whenever you start a line of code with
hash then it will become a comment right
so just write hash and then when I write
this is a comment then this is a comment
right and this will not be executed by a
script but it will give you some
information about your you know script
and then the second way of writing the
comment is like this also so you write
your code or line of code and then after
that start a hash and then this is also
a comment right so this will also be
your comment and it will also not be
executed right now these are the two
most common ways of using comments in
your script there are some other ways
but these are the most commonly used
ways right now let's talk about
variables so what are variables
variables are the container which stores
some data inside them right so whenever
you define a variable this store some
kind of data it can be a string data or
any number on any other kind of data
right now in shell there are or any UNIX
system there are two types of variables
one is the system variable
and other are called the user defined
variables
okay so first is what are a system
variable system variables are created
and maintained by your Linux or UNIX
operating system so these are the some
predefined variable which are defined by
your operating system and the standard
convention is they are these are the
variables which are defined in capital
cases right so whenever you see some
variables which are capital most likely
they are the system variables now the
other type of variables are user defined
variables and these types of variables
are created and maintained by the user
like us right and these are type of
variables are generally defined in lower
cases but there is no you know strict
rule that you need to write your
variables in lower cases you can even
write them in capital cases also so
let's see what are what are some system
variables which are we can use in
scripts so for example I will write echo
and then you write a dollar symbol
before a variable and then there is a
variable call for example bash and it
will give you your bash shell name right
and let's just save your script and
let's run our script so I'm going to
just run our script and what it gives
this variable this variable gives us the
name of the bash or shell right there
are some more variables for example echo
bash version so I'm going to just write
bash version and then it will give me
the bash version right and let me just
copy it few times and then there is a
one variable for example for your home
then you use home
this will give you the home directory
and then there is variable called PWD
present working directory it will give
you whatever where present working
directory you are working in right and
there are some more variables in you
know your operating systems but you just
need to you know search name or just
search the system variables right so I'm
going to just term execute the script
once again and what it gives but it
gives me the bash name or shell name
then it gives me the bash version this
is the version of bash I'm using and
then it gives me the home directory and
then it gives me the present working
directory which is desktop so I'm
working on my desktop directory you can
see here also right now let's talk about
the user defined variables right so how
we can define user defined so you can
define user defined variables really
simply by giving a name so for example I
want to create a variable called name so
I will just write the name here and then
I will just give a equal symbol here or
equal sign here and then the values so
for example mark okay and how you can
use this variable you can use this
variable with a dollar symbol before it
so I will just write echo and then I
will just write dollar here and then the
name of the variable so my variable name
is name itself right so it will give me
the name now with echo you can also
write for example you can just write the
name is and then the variable name right
so this will give me the name whatever
this name variable contains right and I
will just execute my script and print
the name is mark right so your variable
name can be used with your echo you know
string with a dollar symbol before it
right
so in the same way you can you know
write some text here also and it will
work fine so what we will write is for
example I will write our shall name is
this one right or our shell version name
is something like something else so our
shell version name is this right or for
our home directory we can just write our
home directory is home this is the
system variable right or for the present
working directory or current working
with current working directory we can
write something like this or current
working directory is this right it will
also work fine so let's execute our
script let me clear the terminal and
then I will just execute my script it
gives me the output so first of all
hello world and then our shell name is
this or shell version name is this our
directory is this home directory and
then our current working directory is
this one and then we have defined a
user-defined variable which is this one
right now let's define some other
variables so let me define 10 and then
this can be the variables right you can
think of this as a variable and then I
will name my variable as for example 10
well okay just random and then I will
give you the value give it the value of
10 here right and let's print this 10
here so value and then I'm going to just
give the name of the variable right and
let's see what happens and I'm going to
execute the script and gives me this
kind of result which is not the result
we are expecting right so the problem
here
is the variable name should not start
with the number right so if your
variable name starts with a number it
will not be considered as a variable so
what it prints here is this value here
right it just disregards this kind of
output because it's not considering this
value as a variable right and if you
start your variable name with a normal
letter small or capital it will be your
variable so let's print once again I
print the correct value of the variables
write your value can be or you can
define a variable in capital also so
something like well you something like
this also it's totally fine and let's
print it let me clear the terminal first
of all and let's execute our script
right and it executes the same result
right so in this way we have learned how
we can use comments and variables with
our script so we have used the system
variables also and we have used the user
defined variables also so I hope you
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