Shell Scripting Tutorial for Beginners 26 - Readonly command Video Lecture | Complete Linux Course: Become a Linux Professional - Software Development

hey guys welcome to the next video on
shell scripting tutorial for beginners
in this video we will learn how to use
read-only command in shell scripting now
read-only command can be used with
variables and functions and by the name
itself we can understand that
it can be used to make variables or
functions read-only so they cannot be
overwritten so let's take an example so
first of all we will take a variable for
example var and then we will assign some
value to it so for example 31 right and
now we can use a read-only command on
this variable so just write read-only
and then the name of the variable so
read-only space bar for example ok and
till now everything is ok but now after
this point whenever we want to reset a
reassign some value to the variable same
variable var then let's see what happens
so we will just run our script for
example and you can see on line number 7
var read only variable so it gives us a
warning that war is a read only variable
and we cannot change the value of it so
let's print the value of wire for
example so I'm going to just echo and
then bar is equal to let's give it
something like this and then let's print
war so let's print the value of war and
the value of war is 31 so even though we
have reassigned a value of var to 50 but
the var remains same as this one which
is 31 because we have made this war
variable read-only and we cannot
reassign any value to war because it's a
read-only very
now and you will also see this warning
so you will be able to understand that
this is our read only variable in a
similar way the functions can also be
made read-only so let's take an example
so I'm going to create a hello function
for example and then I will just print
hello world using echo for example hello
world okay so everything is okay right
now and whenever we call this function
hello
and let's run the script once again ad
prints hello world everything is okay
right now if we want to make this
function read-only then we use a
read-only first of all the read-only
keyword which is this one right before
the function name but this will not make
your function read-only for functions
you need to use minus F flag okay so you
need to use this minus F option in order
to make a function read-only right and
now whenever you want to overwrite your
functions for example I will once again
create a hello function and I will just
echo this time hello world again right
something like this and let's see what
happens so I am going to run the script
and you can see this warning at line 19
and it says hello read-only function
right so from here we know that this
error is coming from the line 19 right
it will give you the place where it's
coming from and it says that the
function is read on this we cannot
overwrite this function okay so
read-only command can be used to you
know create a variable or a function
read-only that means we cannot you know
reassign the value of variable in case
of variables and we cannot overwrite a
function in
okay so function if the function is
read-only function so sometimes we need
these kind of read-only variables and
functions in our script and this is how
we can achieve them okay now for example
we just write the keyword read-only here
at the last okay so what happens when we
write our read-only keyword only and
nothing in front of it so let's run the
script and let's see what happens and
you can see here after this warning you
will be able to see all the variables
which are read-only by default right -
let me make it a little bit clearer I'm
going to just you know remove everything
and I will just use a keyword read-only
and let me clear the terminal and run
the script once again and it gives us
the list of all read-only built-in
variables which are listed here you can
see that that bash opts are read-only
and we can see bash PID is read-only
bash version info is read-only UID is
read-only right so all the the you know
variables are built in and they are
read-only you can also use the option -
P with this thing and this will give you
almost the same result okay so if you
want to see the list of all read-only
variables
yet then you can use read-only only like
this or with the flag P okay and if you
want to see all the read-only functions
then you just need to use - app flags
okay so once again let me clear the
terminal and let me run the script and
right now there is no read-only function
right and we can create this kind of
function so once again I'm going to
create the hello world
and I make this function read only
something like this right and then I
just write read only minus F and let's
see what happens so now it gives us this
function which is read only okay so
minus f is for functions so if you write
read only minus F and nothing in front
of it
it will give you the list of all the
function which are read-only okay
so minus P for variables and minus F for
functions so I hope you've enjoyed this
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bye for now