Shell Scripting Tutorial-47: Reading From a File 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 47th tutorial
in the seeds of tutorials on UNIX shell
scripting in this tutorial we are going
to learn to switch the default input
stream from the keyboard to a text file
right and for the sake of this tutorial
I have created a text file in the home
folder and its called text and let me
just use cat to show the contents of it
so I have my terminal here and when I
type in cat space text and press the
Enter key
I see the contents of my file and there
are four sentences in the file at the
moment and the four sentences are I am
feeling dizzy tonight I didn't get
proper sleep during the day the dog in
my street seems to be at rest tonight he
is in Barking and I think I'll have some
Italian food for lunch tomorrow that's
the port statement so one thing that I
want you guys to observe here is that
when you use cat to see the output from
a text file on the terminal cat doesn't
display line numbers right and there is
no option that you can use with the cat
command that shows line numbers along
with the text from a text file right so
if you want to see line numbers in your
text file while you're using cat in the
terminal then you'll have to do
something else and we're going to build
a shell script that's gonna do the job
for us
so I have my text editor here now I've
saved this file in the home folder it's
called read and has the extension SH now
when you change the default input stream
from the keyboard to a text value you do
that using the command exec and that's e
x e C right and every time you do that
you have to type in exec and then the
lesson symbol that is the in body
Direction operator and then you have to
type in the name of the file from which
you wish to receive input right so what
I'm gonna do first in the script is I'm
going to use echo to ask the user to
input a file name to me enter a file
name right and I'm gonna store the input
in a variable called f name and the next
thing that I'm going to do is I'm gonna
validate the input and by that I mean
I'm gonna check if the IMP
that has been provided consists of more
than zero characters or not right so it
has to have more than zero key actors if
it is valid input and my test condition
for that is going to be - z space taller
F name and since F name is most likely
to have a string I'm gonna put the value
dollar F name within double quotation
marks and let me make this an if
statement and on line four I'll have to
type in the keyword then to get inside
the if law and if the number of
characters in the input that has been
provided is zero then we won't do
anything we'll just type in exit on line
five and what that would do is it would
it would just terminate these scripts
execution write on line six we'll get
out of the flock by typing in the
keywords fi and well if this if block
doesn't execute then we're good to go
and that's why on line six what I'm
gonna do first is I am gonna save the
current terminal settings in a variable
called terminal and maybe I've discussed
this in this course and I don't really
remember but ppy is the command that
contains information about the current
terminal settings right so I'll have to
type in TTY and I have to assign the
settings to a variable terminal and I do
that by typing in terminal is equal to
TDY but since TTY is a command and since
terminal is variable you can't simply
you know have this you have to put TTY
with in reverse codes and what this
would do is it would fetch the output of
the command TTY and it will assign the
output to the variable terminal right so
now we can change our input stream and I
told you guys that we do that using exec
so I'll type in exact here and then the
in word redirection operator and you
just have to type that once and then the
name of the file which in this case is
stored in the variable F name so I'll
type in dollar F name here right on line
12 I'm going to create a variable called
count and I'll give it the value 1
we want to start displaying our line
numbers 1 onwards but this variable is
not going to be my control variable for
the loop and since we are not listening
from the keyboard we're gonna have the
while statement in a slightly different
format this time so I'll type in the
keyword while on line 14 and then I'll
type in the keyword read and we're doing
this because we want to read the
contents from the text file right so
Reed goes there and then I'll type in my
control variable which can be anything
but I'm gonna call it line so while we
line that's going to be the syntax for
the while statement on the next line
I'll type in the keyword du which is a
part of the wire syntax and on line 16
I'm just gonna echo out the variable of
the control variable for the loop which
is a line so equatorial line and on line
17 I'll type in the keyword done now how
do we display the line numbers here we
do that by typing in daughter count
immediately before the control variable
which is line and I separate the two by
putting a full stop in between and since
we don't want to display one or all the
lines we want to increment our line
numbers I'll have to use the xpr to
increment the value of the variable
count I do that by typing in expr space
dollar count plus 1 and remember to put
a space immediately before and after the
addition symbol there and I'll assign
this value this new value or each
iteration to the variable count again
and since exp are dollar countless one
is is a mathematical expression I'll
have to put the entire thing within
reverse codes right and after we are
finished displaying the contents of the
text file and along with the line
numbers what we would have to do is we
would have to restore the original
settings back again and you do that by
typing in exe C again and then you'll
have to type in the name of
you're variable in which you stored the
settings before changing your input
stream and remember in the script I
stored the settings in the variable
terminal and I did that by typing in the
score at 9/8 so exact space tolerant
terminal would do the job for me and I
save this file now bring up my terminal
here and let me first clear the screen
and the name of the script file is read
so in order to execute it I'll have to
type in a set space read with the
extension SH and when I press the Enter
key the shell asks me to provide a file
name and text is a file that I created
in the home folder so I'm just going to
provide that as the input but before I
do that let me just show you guys what
happens when I don't type in any
character and I just press the Enter key
nothing happens
my dollar prompt is returned back to me
and that means that the exit statement
in the if block got executed right
because the length of the character
input that I provided was 0 but if I
provide valid input such as text which
is the name of a file in my home folder
and press the Enter key then you see
that this time I see line numbers and I
also see the content so line one has I'm
feeling dizzy tonight line three has the
dog in my Street seems to be at rest
night line 4 has the final sentence so
this is one method of displaying line
numbers on screen but this script or
this tutorial wasn't to discuss how to
display line numbers for text files on
screen but it was to discuss the
significance of the exact command and
how you can switch the default input
stream from the keyboard to a text file
and how you can restore the settings
back by the command that you see on line
20 in this script so I hope you guys had
fun watching this tutorial
in the next tutorial we're gonna discuss
something interesting and important
again thank you so much for watching
this one I'll see you in the next
tutorial please subscribe to my channel