Question

Please note – I am not looking for the ‘right’ way to open/read a file, or the way I should open/read a file every single time. I am just interested to find out what way most people use, and maybe learn a few new methods at the same time :)*

A very common block of code in my Perl programs is opening a file and reading or writing to it. I have seen so many ways of doing this, and my style on performing this task has changed over the years a few times. I’m just wondering what the best (if there is a best way) method is to do this?

I used to open a file like this:

my $input_file = '/path/to/my/file'; open INPUT_FILE, '<$input_file'  || die 'Can't open $input_file: $!\n'; 

But I think that has problems with error trapping.

Adding a parenthesis seems to fix the error trapping:

open (INPUT_FILE, '<$input_file')  || die 'Can't open $input_file: $!\n'; 

I know you can also assign a filehandle to a variable, so instead of using ‘INPUT_FILE’ like I did above, I could have used $input_filehandle – is that way better?

For reading a file, if it is small, is there anything wrong with globbing, like this?

my @array = <INPUT_FILE>; 

or

my $file_contents = join( '\n', <INPUT_FILE> ); 

or should you always loop through, like this:

my @array; while (<INPUT_FILE>) {   push(@array, $_); } 

I know there are so many ways to accomplish things in perl, I’m just wondering if there are preferred/standard methods of opening and reading in a file?

Answer 1

There are no universal standards, but there are reasons to prefer one or another. My preferred form is this:

open( my $input_fh, '<', $input_file ) || die 'Can't open $input_file: $!'; 

The reasons are:

• You report errors immediately. (Replace ‘die’ with ‘warn’ if that’s what you want.)
• Your filehandle is now reference-counted, so once you’re not using it it will be automatically closed. If you use the global name INPUT_FILEHANDLE, then you have to close the file manually or it will stay open until the program exits.
• The read-mode indicator ‘<‘ is separated from the $input_file, increasing readability.

The following is great if the file is small and you know you want all lines:

my @lines = <$input_fh>; 

You can even do this, if you need to process all lines as a single string:

my $text = join('', <$input_fh>); 

For long files you will want to iterate over lines with while, or use read.