Cogs and Levers A blog full of technical stuff

Basic file IO in Perl

03 Mar 2017

One of the most basic, yet most useful operations you can perform in Perl is working with files. In today’s post, I’ll show you through a few basic patterns to get started with file IO in Perl.

open

The cornerstone to working with a file, is the open function. It takes the following forms:

  • open FILEHANDLE,EXPR
  • open FILEHANDLE,MODE,EXPR
  • open FILEHANDLE,MODE,EXPR,LIST
  • open FILEHANDLE,MODE,REFERENCE
  • open FILEHANDLE

FILEHANDLE being the local variable that you’ll use to reference the file.

MODE determines the type of file access you’re requesting over the file

Mode Description
< File is opened for reading
> File is opened for writing
>> File is opened for appending
+< File is opened for reading and writing
+> File is opened for reading and writing, but clobbered first
|- File is interpreted as a command and piped out
-| File is interpreted as a command and piped in
<:encoding(UTF-8) File is opened for reading and interpreted as UTF-8

Throwing on failure

use strict;
use warnings;
 
my $filename = 'data.txt';
open(my $fh, '<:encoding(UTF-8)', $filename)
  or die "Could not open file '$filename' $!";

# TODO: work with the file here

Warning on failure

use strict;
use warnings;
 
my $filename = 'data.txt';
if (open(my $fh, '<:encoding(UTF-8)', $filename)) {
  # TODO: work with the file here
} else {
  warn "Could not open file '$filename' $!";
}

Diamond operator <>

The diamond-operator is normally used in while loops and used to iterate through files:

# File is opened here into $fh

while (my $row = <$fh>) {
  chomp $row;
  print "$row\n";
}

Writing with print

Sending information into file is done so with print.

# File is opened here into $fh (using >)

print $fh, "This is a line of text for the file\n";

Finishing up with close

When you’re finished with your files, you’ll use close

# File is opened here into $fh 
# File work --happens--

close $fh or die "Can't close file: $!";

These are just the simple operations for working with files in Perl.

Related Posts