Beginning PERL: Part II
July 31, 2000
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This manuscript is an abridged version of a chapter from the Wrox Press book
Beginning PERL.
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Writing to Files
We've been using the print operator to print a list to standard output. We'll also use a different form of the print operator to print to a file. However, as we mentioned above, files are usually open either for reading or for writing — not both. We've been opening files and reading from them, but how do we open them for writing?
Opening a File for Writing
We actually use a syntax that's used by the shell for writing to files. In Windows and UNIX, if we want to put standard output into a file, we add the operator > and the file name to the end of the command. For example, saying something like this:
> perl summary.plx chapter6 > summary6
will create a file called summary6, which contains the following text:
1:We're starting to write real programs now, and real programs
2:What we want to do in this chapter is extend these techniques
3:First though, let's do some groundwork. When we're dealing
4:We've actually already seen a filehandle: the STDIN of <STDIN>.
5:As a counterpart to standard input, there's also standard
6:Every program has these three filehandles available, at least
Now, to open a file for writing, we do this:
open FH, "> $filename" or die $!;
This will either create a new file or completely wipe out the contents of an already existing file and let us start writing from the beginning. Don't use this on files you want to keep! If we want to add things to the end of an existing file, use two arrows, like this:
open FH, ">> $filename" or die $!;
There's no easy way of adding or changing text at the beginning or middle of a file. The typical way to do this is to read in the original and write the changed data to another file. We'll see shortly how this is done.
Similarly, you can redirect data to a program's standard input by using the left arrow, like this:
>perl summary.plx < chapter6.txt
As you've probably guessed, this means you can open files for input by saying:
open FH, "< $filename";
This is exactly the same as open FH, $filename; as we've used previously; it's just a little more explicit.
Writing on a Filehandle
We're now ready to write the file, which we'll do by using a special form of the print operator. Normally, to print things out from the screen, we say this:
print list;
When we want to write to a file, we'll use this instead:
print FH list;
So, for instance, here's one way of copying a file:
Try It Out: File Copying
We'll read in a file one line at a time, writing each line onto the new file:
#!/usr/bin/perl
# copy.plx
use warnings;
use strict;
my $source = shift @ARGV;
my $destination = shift @ARGV;
open IN, $source or die "Can't read source file $source: $!\n";
open OUT, ">$destination" or die "Can't write on file $destination: $!\n";
print "Copying $source to $destination\n";
while (<IN>) {
print OUT $ _;
}
Now there isn't much to see in this program, but let's run it anyway:
> perl copy.plx gettysburg.txt speech.txt
Copying gettysburg.txt to speech.txt
>
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