Filehandles and File Tests

This lecture notes is based on Prof Doreen De Leon's lectures.

Reading

Learning Perl, Chapter 11

Filehandles

Name for an I/O connection between Perl process and the outside world
Naming of filehandles: all uppercase letters
Perl provides six filehandles: STDIN,STDOUT,STDERR,DATA,ARGV,ARGVOUT

Opening and Closing a Filehandle

To open a file for reading:open SCORES, "midterm"; or open SCORES, "<midterm";
To open a file for writing:open RESULT, ">grade.txt"
To open a file for appending:open RESULT, ">>grade.txt"
All forms of open return true for success, false for failure.

open SCORES, "midterm"; 
$firstscore = <SCORES>;
chomp($firstscore);
print $firstscore, "\n";

If the following file midterm exists

The output is

If the file midterm doesn't exists

readline() on closed filehandle SCORES at test.pl line 2.
Use of uninitialized value in scalar chomp at test.pl line 3.
Use of uninitialized value in print at test.pl line 4.

Remove the file midterm and run the following program

$success = open SCORES, "midterm"; 
if($success) {
   $firstscore = <SCORES>;
   chomp($firstscore);
   print $firstscore, "\n";
} else {
    print "Fail to open the file midterm.\n";
}

Here is the output

Fail to open the file midterm.

put the midterm file back

open SCORES, "midterm"; 
$sum=0;
$count=0;
while(defined($score = <SCORES>)) {
   chomp($score);
   $sum+=$score;
   $count++;
} 
print "Average: ", ($sum/$count);

For the following program, if file square doesn't exist, the computer will create one. If it exists, the computer will overwrite the content of the file.

open OUT, ">squres"; 
for($i = 1; $i <= 10; $i++) { 
    print OUT "$i ", ($i*$i), "\n";
}

Run the above program several times and run the following program several times

open OUT, ">>squres"; 
for($i = 1; $i <= 10; $i++) { 
    print OUT "$i ", ($i*$i), "\n";
}

close

Used to close a filehandle.
Syntax: close FH;
Reopening a filehandle automatically closes the previously opened filehandle so many simple programs don't do this.
Exiting a program automatically closes any open filehandles.
Should close each filehandle soon after finishing with it. Why?

open SCORES, "midterm"; 
$sum=0;
$count=0;
while(defined($score = <SCORES>)) {
   chomp($score);
   $sum+=$score;
   $count++;
} 
print "Average: ", ($sum/$count);
close SCORES;

open OUT, ">squres"; 
for($i = 1; $i <= 10; $i++) { 
    print OUT "$i ", ($i*$i), "\n";
}
close OUT;

die

If you try to read from a bad filehandle (i.e., you can't open a file), you will get the end-of-file. Trying to write to a bad filehandle results in the data being discarded.
But open always returns false if you try to use a bad filehandle.
If open fails, usually want to print an error message and terminate the program - use die to do this.
Use or not || since or has lower precedence.
To get the error string describing the most recent operating system error, use the $! variable (only good after a failed system request).

Remove the file midterm and test the next program.

open SCORES, "midterm" or die "Cannot open file midterm $!";
$firstscore = <SCORES>;
chomp($firstscore);
print $firstscore, "\n";

Output

Cannot open file midterm: No such file or directory at test.pl line 1.

$0

$0 holds the name of the file containing the Perl script being executed.

The following program print out the file name of the perl script.

print $0;

Using Filehandles

Once a filehandle is open for reading, it can be used like <STDIN>.
If a filehandle is open for writing or appending, to print to the file, put the filehandle after the print keyword and before other arguments. No comma goes between the filehandle and the rest of the arguments.

$scores_file = "scores.txt";
$result = "result.txt";
open SCORES, $scores_file or die "Can't open file $scores_file";
open RESULT, ">$result" or die "Can't open file $result"; 
while(<SCORES>) {
    chomp;
    @array = split(",", $_);
    print RESULT 0.3*$array[0] + 0.25*$array[1] + 0.45*$array[2];
    print RESULT "\n";
}

The file scores.txt is

55,22,33
11,33,44
39,21,11

The output result.txt is

36.85
31.35
21.9

$scores_file = "scores.txt";
$result = "result.txt";
open SCORES, $scores_file or die "Can't open file $scores_file";
open RESULT, ">$result" or die "Can't open file $result"; 
select RESULT; # change the default of output
while(<SCORES>) {
    chomp;
    @array = split(",", $_);
    print 0.3*$array[0] + 0.25*$array[1] + 0.45*$array[2];
    print "\n";
}

File Tests

Suppose you want to make sure that a file does not exist so you don't accidentally overwrite it: use the -e file test.
Suppose you want to check permissions of a file: -r,-w,-x
To check if a file exists and has non-zero size, use -s.

$filename = "file";
print "$filename exists" if -e $filename;