Shell Scripting Tutorial-32 Append Text to a File Through Shell Script

hello guys my name is mother and welcome
to my channel this is the thirty second
tutorial in the series of tutorials on
UNIX shell scripting now over the past
few tutorials we've been learning a lot
of new concepts we learn to use the if
statement and the FL statement and then
we learn to use the dest statement to
run checks on numbers and on files and
we've been doing a lot of boring work so
what we're going to do in this tutorial
is we're going to utilize all the
knowledge that we've gained over the
past few tutorials and actually do
something meaningful so I have my text
editor here and I've saved this file in
my home folder I've given it the name
append and the extension SH now what
this script is going to do is it's going
to ask the user to input a name and then
it will check whether a file of the name
that has been input exists in the home
folder or not and if the file exists
then it would check whether the user has
write permission to that file or not and
if the user has write permission then it
will present the user the option to
append some text into the file and if
the user doesn't have write permission
then it will basically do nothing but
display a message to the user that he
cannot write text into the file because
he doesn't have permission right so it's
going to do a very basic work and it's
going to be a very easy shell script to
learn so let's get started on the first
line
I'll put an echo statement to ask the
user for a file name right
and these slashes escape sequence of
course go in there on the next statement
I'll use a read to grab the user's input
and I'll store it in the variable F name
on the next line I'll check whether a
file off the name that has been stored
in F name exists in the home folder or
not right so I'll put my square brackets
in there and then the option hyphen F to
check for the file and then the name of
the variable which is dollar F name and
the space after dollar F name and this
would go inside an if statement on the
next line I'll have my ten keyword and
now we would get inside the if block
only if the file has been found so now
we
have to do the next thing and that is to
check whether we have write permission
or not so that could happen by putting a
test condition inside square brackets
again dollar F name with the option
hyphen W to check for write permission
and this would also have to be in
another if statement on the next line
I'll have my key word then but this
belongs to the inner if block on the
next line I'll display a message this
can be thought of as an instructional
message to the user how he has to go
about entering the text right
so type matter to append and to quit
press control B and that's because we're
going to use the cat command to append
text to the file right so cat with the
double redirection operator since we
want to append the text not override it
and the name of the file which is store
F name right and if the user doesn't
have write permission then we would
display a message to the user inside the
else block of course we would use echo
for it you do not have permission to
write great and then we would have to
first end the inner if block by using
the keyword fi and then the outer if
lock by using fi again so I'll first
execute this script and show you guys
how it behaves and then talk you through
it once more
so in my home folder I have a text file
called test write and it just has a one
line and the line says life is like a
box of chocolates and this is a 1/2 of
the famous quotation from Forrest Gump
that life is like a box chopped leeks
you never know what you're going to get
and the second statement would be
appended to this file through the
terminal when we execute our script
now what I'm going to do first is I'm
going to take away right permission from
this file by using the chmod command and
the option minus W and of course I have
to type in the name of my file and when
I execute it and show you the file in my
home folder you see that now there is a
symbol of a lock on my file and if I
would check out the properties of the
file then you would see that the owner
just has read-only permission and it
doesn't have read and write permission
okay so that is there and now I would
execute my script append or Sh god
append not spend so enter file name and
when I type in test and press the Enter
key it tells me that I do not have
permission to write so this is being
displayed because the else block
executed now what I'm going to do is I'm
going to give the file write permission
write and get it back to its original
state and execute the script again but
let me first clear the terminal clear it
and profile name test and now I see that
the cursor is blinking and waiting for
me to type in some text so I'm going to
type in the remaining portion of that
quotation you never know what you are
going to get next line control D to exit
and now if I show you guys the output of
my test command sorry not the test
command but the test file you would see
that the first statement is the
statement that the file originally have
in the second statement is what we've
just input into the pile so now I'll
talk you guys through the script file
once again on the first line we have an
echo statement that asks the user to
input a file name the name is grabbed by
read and is stored in the variable F
name on the next statement we have an if
statement on the next line that is we
have an if statement that checks if a
file of the name that is stored in F
name exists in the home folder or not if
it does then we get inside the if block
we have another if statement that checks
for right permission to the file and if
the user has right permission then we
get inside the if block where we first
display a message using echo and then we
use the cat command to append text to
the file and if we not have write
permission then we get inside the else
block and we display a message that the
user has no right permission to the file
so there you go it's a very simple shell
script and the only new thing in the
shell script is that we've used a nested
if-else construct and what is meant by
that is that we've used a if else block
within another if block right so this
thing is called nested if-else and those
of you who have any sort of experience
in other programming languages would
understand how this thing works and even
if you don't have experience then I'm
pretty sure it's pretty simple for you
guys to handle so there you go that's
all I wanted to talk about in this
tutorial in the next tutorial we'll
discuss something interesting again so
thank you so much for watching this one
and please subscribe to my channel