File handling

• Reading files
• Declaring file encoding and using autodie
• Writing to files
• Nested file handling

Reading files

#!/usr/bin/perl
use strict;
use warnings;

my $filename = "hello_world.pl";
my $hello_file = undef;
open($hello_file, "< :encoding(ASCII)", $filename) || die "cannot open $filename for reading";

# read contents of file line by line
while(<$hello_file>)
{
    # $_ gets the line by default
    print $_;
}
close($hello_file) || die "cannot close $filename";

• In this tutorial, we'll handle only ASCII file types. We indicate that by :encoding(ASCII)
  • for other encoding types and for more discussion on open in general, refer to links given later in this section
• undef is a way of telling that the variable doesn't have any value yet
• < symbol in open indicates that we wish to only read the file
• in case file cannot be opened for reading (for ex: file does not exist) die will cause the program to exit, along with error message
• while is the most frequently used construct to read file contents line by line. It automatically takes care of stopping the loop at end of file

$ ./file_reading.pl
#!/usr/bin/perl

print "Hello World\n";

Declaring file encoding and using autodie

#!/usr/bin/perl
use strict;
use warnings;

# automatically handle open/close errors
use autodie;
# declare file encoding for all open functions in this program
use open qw< :encoding(ASCII) >;

my $filename = "file_doesnt_exist.txt";
my $handle = undef;
open($handle, "<", $filename);

# one can also declare another variable instead of using $_
while(my $line = <$handle>)
{
    print $line;
}
close($handle);

• this way, code looks cleaner and avoids issues due to missing out on die constructs
• autodie works on other functions too, refer to link given later in the section

$ ./file_reading_error.pl
Can't open 'file_doesnt_exist.txt' for reading: 'No such file or directory' at ./file_reading_error.pl line 12

Writing to files

#!/usr/bin/perl
use strict;
use warnings;

use autodie;
use open qw< :encoding(ASCII) >;

my $filename = "new_file.txt";
my $out_file = undef;
open($out_file, ">", $filename);

print $out_file "This is a sample line of text\n";
print $out_file "Yet another line\n";

close($out_file);

# sanity check
system("cat new_file.txt");

• > symbol in open indicates that we wish to write to a new file
  • if the file already exists, its contents will get overwritten
  • use >> to append to an existing file
• all print statements we have seen prior to this example uses the filehandle STDOUT by default and thus we see the print result on terminal
• to print error messages instead of normal output, use the STDERR filehandle. We'll see examples in next section
• by using print $out_file, printing is redirected to that filehandle instead of default STDOUT

$ ./file_writing.pl
This is a sample line of text
Yet another line

Nested file handling

Often, one needs to read one file to get list of filenames for further processing. Nesting while loop with filehandles may result in unintended consequences
In such cases, one can read entire file contents into an array and iterate using foreach for one loop and use while for the other

# assign entire contents of file to array variable, each item contains single line from file
my @file_contents = <$file_handle>;

This method also allows an easier way to process file lines relative to current line by using for loop to iterate using explicit indices

Further Reading

• open
• why opening files in old way is bad
• autodie