In order to interact with a file, Perl makes you specify a thing called a "filehandle". This is basically a variable but, of course, Perl has a special way of talking about variables that reference files.
Normal variables in Perl typically begin with a symbol, such as $, @, or %. Filehandles, by convention, are specified in all caps, like this: MYINFILE, and have no type of symbol preceding them.
You can open files for reading, writing/creating/truncating, and appending.
To open a file for reading you would type:
open(FILEHANDLE, "/name/o/me/file.here"); ## or you could use open(FILEHANDLE, "<infile.txt"); # the "<" means "read-only mode"
And that's it - your file is now open for reading.
If you wanted to open a file for writing you would use:
open(MYWRITEFILE, ">out.txt"); # the ">" makes the file open for writing/creating and truncates # (erases) the existing contents before any writting takes place.
There are several "modes" you can open a file in:
< Reads only
> Creates, writes and truncates
>> Writes, appends and creates
+< Reads and writes
+> Reads, writes, creates and trucates
+>> Reads, writes, appends and creates
Appending to a file opens the file and places the file pointer at the very end of the file. When you begin writing to the file everything gets added to the end of the existing content.
Any time you open a file, you must also close it when you are finished. To close a file you use:
close(FILEHANDLENAME);
Once a file is open for reading you can parse through it line by line like so:
...
while(<FILEHAND>){
# do stuff here
}
...
To print to a file you could use the following syntax:
print FILEHANDLE "the stuff I want to print and the contents of this variable"; print FILEHANDLE "$myVariable";
Here is a small sample program that reads in the contents of a file, modifies them and writes them back out to a new file. It will also append a note to a log file recording the modification. Finally, it closes the files and exits. If you want to try it, just create a text file named "in.txt" in the same directory as the program and fill it with some text.
#!/usr/bin/perl -w
use strict;
open(READFILE, "in.txt") or die print "Can't open the read file: $!\n";
open(WRITEFILE, ">out.txt") or die print "Can't open the write file: $!\n";
while(<READFILE>){
my $line = $_; # store the contents of the line in $line
$line = uc($line); # convert everything to uppercase
print WRITEFILE "$line"; # write the converted contents to out.txt
}
open(LOGFILE, ">>log.txt");
my $time = time();
print LOGFILE "Just wrote all caps to out.txt at $time\n";
close(READFILE);
close(WRITEFILE);
close(LOGFILE);
If you'll notice, we use the "die" command when we opened the files for interaction. "die" halts the programs execution when perl fails to open the file for whatever reason. You can optionally tell it to do stuff after it dies, such as print a message. In the previous example, "$!" prints the error message that normally would appear on the command prompt.
If you don't want to halt the program, you can use the "warn" command instead of "die", which will allow you to print a message stating that the open failed, but will let the program proceed with execution.
And that's a sample of file IO in Perl.
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