Lecture#6
Chapter 11 - Filehandles and File Tests

A couple you should already know about:

STDIN - Default input filehandle (keyboard/redirection/pipe)
STDOUT - Default output filehandle (screen/pipe/file)
STDERR - Default error filehandle (screen/file) ./myprogram 2>errlog

In our programs, we'll be concerned with reading and writing to files.

open FH, "filename"         -   opens filename for reading
open FH, "<filename"       -   opens filename for reading
open FH, ">filename"       -   opens filename for writing (overwrite/create)
open FH, ">>filename"     -   opens filename for append

you can also use variables here...

$filename = "myfile";
open FH, "> $filename"; #use the space to avoid crazy appends if $filename starts with >

Filehandles are automatically closed when they are reused or when the program ends. You can also explicitly close them:
close FH;

This flushes the buffer immediately instead of waiting.

When opening a file, you should always make sure the open worked....there's a few ways to do this...

$error = open FH, "<logfile.txt";
if (!$error) {
   print "Unable to open that file!";
}

open FH, "<logfile.txt" or die "Can't open that file!\n"; #Prints msg and ends program

You could get more info on this failure by using one of Perl's special variables:

open FH, "<logfile.txt" or die "Can't open that file: $!\n"; #Prints out some internal info when a system call fails

open FH, "<logfile.txt" or warn "Can't open that file!\n"; #same as before, but execution continues

#!/usr/local/bin/perl
open (IN, "<infile.txt") || die "Can't open the file\n";
open (OUT,">outfile.txt") || die "Can't create output file\n";
while ($input = <IN>) {
   if ($input =~ /txt/) {
      print OUT $input;
   }
}
close OUT;
close IN;

select FH;
print "Hello"; #prints to FH

Flushing the buffer

$|=1; #Now things aren't kept in the buffer

if -e $filename {
   print "That file exists...";
}

Filetests are useful if you need to know something about a file before manipulating it. The list of tests are on p. 159 in the book. The 2 I want you to know are:

-e    File/directory exists
-d    File is a directory

($sec, $min, $hour, $day, $mon, $year, $wday, $yday, $isdst) = localtime;

Gives seconds, minutes, hour, day of month, year-1900, day of week (0-Sun), month (0-Jan), days since Jan 1, is it daylight savings time?.

Similar to other languages....

Operator	Meaning	Example	Explanation
&	Bitwise AND	7 & 2 = 2	0111 AND 0010 = 0010 = 2
|	Bitwise OR	7 | 2 = 7	0111 OR 0010 = 0111 = 7
^	Bitwise XOR	7 ^ 2=5	0111 XOR 0010 = 0101 = 5
<<	Bitwise left shift	7 << 2=28	0111 shifted 2 bits left= 011100=28
>>	Bitwise right shift	7 >> 2=1	0111 shifted 2 bits right = 0001 = 1
~	Bitwise NOT	None good	Depends on how numbers are stored

You can use bitstrings as well, example: "10" | "01" = "11"

Stat and Lstat
Gives info about files...read this section if interested.

Underscore Filehandle
Stores information received from last call of stat or lstat