Reading a file line by line in Perl - perl

Question

I want to read a file by one line, but it's reading just the first line. How to read all lines?
My code:
open(file_E, $file_E);
while ( <file_E> ) {
/([^\n]*)/;
print $line1;
}
close($file_E);

Answer 1

Let's start by looking at your code.
open(file_E, $file_E);
while ( <file_E> ) {
/([^\n]*)/;
print $line1;
}
close($file_E);
On the first line you open a file named in $file_E using the bareword filehandle file_E. This should work so long as the file successfully opens. It would be better to also check the success of this operation one of two ways: Either put use autodie; at the top of your script (but then risk applying its semantics in places where your code is incompatible with this level of error handling), or change your open to look like this:
open(file_E, $file_E) or die "Failed to open $file_E: $!\n";
Now if you fail to open the file you will get an error message that will help track down the problem.
Next lets look at the while loop, because it's here where you have an issue that is causing the bug you are experiencing. On the first line of the while loop you have this:
while ( <file_E> ) {
By consulting perldoc perlsyn you will see that line is special-cased to actually do this:
while (defined($_ = <file_E>)) {
So your code is implicitly assigning each line to $_ on successive iterations. Also by consulting perldoc perlop you'll find that when the match operator (/.../ or m/.../) is invoked without binding the match explicitly using =~, the match will bind against $_. Still then, so far so good. However, you are not actually doing anything useful with the match. The match operator will return Boolean truth / falsehood for whether or not the match succeeded. And because your pattern contains capturing parenthesis, it will capture something into the capture variable $1. But you are never testing for match success, nor are you ever referring to $1 again.
On the line that follows, you do this: print $line1. Where, in your code, is $line1 being assigned a value? Because it is never being assigned a value in what you've shown us.
I can only guess that your intent is to iterate over the lines of the file, capture the line but without the trailing newline, and then print it. It seems that you wish to print it without any newlines, so that all of the input file is printed as a single line of output.
open my $input_fh_e, '<', $file_E or die "Failed to open $file_E: $!\n";
while(my $line = <$input_fh_e>) {
chomp $line;
print $line;
}
close $input_fh_e or die "Failed to close $file_E: $!\n";
No need to capture anything -- if all that the capture is doing is just grabbing everything up to the newline, you can simply strip off the newline with chomp to begin with.
In my example I used a lexical filehandle (a file handle that is lexically scoped, declared with my). This is generally a better practice in modern Perl as it avoids using a bareword, avoids possible namespace collisions, and assures that the handle will get closed as soon as the lexical scope closes.
I also used the 'three arg' version of open, which is safer because it eliminates the potential for $file_E to be used to open a pipe or do some other nefarious or simply unintended shell manipulation.
I suggest also starting your script with use strict;, because had you done so, you would have gotten an error message at compiletime telling you that $line1 was never declared. Also start your script with use warnings, so that you would get a warning when you try to print $line1 before assigning a value to it.
Most of the issues in your code will be discussed in perldoc perlintro, which you can arrive at from your command line simply by typing perldoc perlintro, assuming you have Perl installed. It typically takes 20-40 minutes to read through perlintro. If ever there were a document that should constitute required reading before getting started writing Perl code, that reading would probably include perlintro.

Answer 2

Another alternative, note that $_ will include newline so you will need to chomp it if you don't want the newline in $line:
open(file_E, $file_E);
while ( <file_E> ) {
my $line = $_;
print $line;
}
close($file_E);