How to read and write files in Perl
Perl is an ideal language for
working with files. It has the
basic capability of any shell
script, and some very advanced
tools, like regular expressions,
that make it infinitely more
useful. In order to work with
files, we first need to learn
how to read and write to them.
Reading a file is done in Perl
by opening a filehandle to a
specific resource - in this case
a file on our system.
#!/usr/local/bin/perl
open (MYFILE, 'data.txt');
while (<MYFILE>) {
chomp;
print "$_\n";
}
close (MYFILE);
In order to work with this
example, you'll need a file for
our Perl script to read. Create a
new text document called data.txt and
place it in the same directory as
the above Perl program. In
the file itself, just type in a
few names - one per line:
Larry
Curly
Moe
When you run the script, the
output should be the same as
the file itself. The script
is simply opening the
specified file, and looping
through it line by line,
printing each line as it
goes. First we create a
filehandle called MYFILE,
open it, and point it at our data.txt file.
open (MYFILE, 'data.txt');
Then we use a simple while
loop to automatically read
each line of the data file
one at a time - this places
the value of each line in
the temporary variable $_
for one loop.
while (<MYFILE>) {
Inside the loop, we use the chomp function
to clear off the newlines
from the end of each line,
then we print the value of
$_ to show that it was read.
chomp;
print "$_\n";
Finally we close the
filehandle to finish out the
program.
close (MYFILE);
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Writing to a file in Perl
Let's take the same data file
we worked with while learning to
read a file in Perl, and we'll
write to it this time. In order to
write to a file in Perl, you must
open a filehandle and point it at
the file you're writing. If you're
on a unix, linux, or mac you might
also need to double check your
file permissions to see if your
Perl script is allowed to write to
the data file.
#!/usr/local/bin/perl
open (MYFILE, '>>data.txt');
print MYFILE "Bob\n";
close (MYFILE);
If you run this program, then
run the program from the previous
example on reading a file in perl,
you'll see that it's added one
more name to the list.
Larry
Curly
Moe
In fact, every time you run the
program it will add another Bob to
the end of the file.
This is happening because
we opened the file in append mode.
To open a file in append
more, just prefix the
filename with the >> symbol.
This tells the open
function that you want to
write to the file by
tacking more onto the end
of it. If instead you want
to overwrite the existing
file with a new one, you
can use the > single
greater than symbol to
tell the open function
that you want a fresh file
each time. Try replacing
the >> with a > and you'll
see that your data.txt
file is cut down to a
single name - Bob - each
time you run the program.
open (MYFILE, '>>data.txt');
Next we use the print
function to print our new
name to the file. You
print to a filehandle
simply by following the
print statement with the
filehandle.
print MYFILE "Bob\n";
Finally we close the
filehandle to finish out
the program.
close (MYFILE);
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