Keyboard is considered as a file in most OS
<STDIN> is the file handle used to read from the keyboard
when we use STDIN in <>(angular) brackets it will return the file pointer or file handler using which we can read the contents from the Keyboard
A very simple example
eg : print "Enter your name";
$name = <STDIN>; # Read from the keyboard
print "Good Morning, $name..!! \n";
When you execute the program you will notice that it will print two new lines
//$name will also contain the new line character
Chop function
---------------
This will remove the last character of whatever is given to it.
So we can rewrite the above program as
print "Enter your name";
chop($name = <STDIN>);
print "Good Morning, $name..!! \n";
This time the program will only print one new line char which is given in the program
But remenber chopping sometimes might not be good, becuse it blindly removes the last char irrespective of what it is
Chomp Function
----------------
The chomp functions similar to the chop, witha difference that it chops of the last char only if it is a new line char
print "Enter your name";
chomp($name = <STDIN>);
print "Good Morning, $name..!! \n";
Opening a File in Perl
-----------------------
The open command opens the file and return the file handle
For standard input we had a predefined file handle <STDIN>
eg : $file = "/home/bin/report.txt";
open XYZ, $file;
while(<XYZ>) {
print "Line number $. is $_";
}
Difference between XYZ and <XYZ> is , XYZ represents the file and <XYZ> represents the contents of the file
$. is the line number
$_ is the present line which is being read
Sometimes file name may be wrong, or file may not exist, to handle that we use the following construct
$file = "/home/bin/report.txt";
open XYZ, $file or die "Error in open: $!"
while(<XYZ>) {
print "Line number $. is $_";
}
$! returns the error code or message
Reading from the file in Perl
-------------------------------
The <> is the line input operator, all the data read goes into $_
Writing into a file in Perl
----------------------------
Eg : $out = "/home/bin/out.txt";
open XYZ, ">$out" or die "Error in Write : $!" ;
for $i (1..20) {
print XYZ "$i :: Hello the time is scalar(localtime) \n";
}
Here ">$out" open the file given in the path
print XYZ, will write the output to the file and not on the screen
scalar(localtime) is the function in perl which returns the system time.
Appending the file
-------------------
Eg : $out = "/home/bin/out.txt";
open XYZ, ">>$out" or die "Error in Write : $!" ;
for $i (1..20) {
print XYZ "$i :: Hello the time is scalar(localtime) \n";
}
Here ">>$out" will open the file for appending
Print the file in Perl
------------------------
Print the contents of Perl is very easy
eg : $out = "/home/bin/out.txt";
open IN $input or die "Error in opening : $!";
while(<IN>){
print; //By defaults print $_
}
close IN;
Command line args in Perl
--------------------------
Perl uses special array called @ARGV
List of arguments passed in the command line along with the scipt name will be stored in @ARGV
eg : If ou invoke perl as
perl test.pl red blue green
then @ARGV wil be red blue green
printing the command line arguments
foreach(@ARGV) {
print "$_";
}
Difference between @ARGV and <ARGV>
--------------------------------------
@ARGV will contain the text after the scrpt nam efrom thr command line
<ARGV> will assume that each element after the script name is a name of the file
If no file name is given and <ARGV is used then by default it starts reading from the default input it keyboard
Consider an example
This program will print the contents of the file you are passing
eg : $lineno = 1;
while(<>) {
print $lineno++;
print ":$_";
}
perl list_lines.pl file1.txt //contents of the file file1.txt will be printed
perl list_lines.pl a.txt b.txt c.txt //contents of all the three files
