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Perl File Open: Creating, Reading and Writing to Filesin
Perl
Dealing with files in Perl is very easy once you get used to the
slightly odd syntax. Hereare some examples to get you started.
Open a File in Perl
To open a file in Perl, just the open() subroutine.
Heres an example of a program that opens a file, reads the file
one line at a time andprints each line to the terminal.
use strict;use warnings;
# Put the file name in a string variable# so we can use it both
to open the file# and to refer to in an error message# if needed.my
$file = "temp.txt";
# Use the open() function to open the file.unless(open FILE,
$file) {
# Die with error message# if we can't open it.die "\nUnable to
open $file\n";
}
# Read the file one line at a time.while(my $line = ) {
# We'll just print the line for now.print $line;
}
# close the file.close FILE;
If you want to read binary files in Perl, you need to set the
binmode on the file handle.
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The following example also illustrates how you can read an
entire file in one go. Of courseif the file is large and your
memory limited, this might be a bad idea. Usually youprobably want
to read a fi le in chunks, writing out the chunks to another file
as you go.
use strict;use warnings;
# Put the file name in a string variable# so we can use it both
to open the file# and to refer to in an error message# if needed.my
$file = "temp.bin";
# Use the open() function to open the file.unless(open FILE,
$file) {
# Die with error message# if we can't open it.die "\nUnable to
open $file\n";
}
# Tell Perl we want to read data as binary.binmode(FILE);
# Get rid of the line separator.# This allows us to read
everything# in one go.undef $/;
# Read the entire file. If you don't want# to read all of it at
once, you need the# read() subroutine.my $contents = ;
print "Read " . length($contents) . " bytes\n";
# close the file.close FILE;
Read 520 bytes
If you are dealing with a text file, its often useful to set the
file record separator $/ (setto the newline character by default)
to some other value, such as a particular closing XMLtag.
Undefining it altogether, as in the above example, causes the
entire file to be readat once.
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Creating Files In Perl
To create a file in Perl, you also use open(). The difference is
that you need to prefix thefile name with a > character to make
Perl open the file for writing. Any existing file withthe name you
supply to open() will be overwritten, unless you specify >>
instead, whichopens a file for appending.
This program creates a file called temp.txt and writes two lines
to it.
use strict;use warnings;
# Put the file name in a string variable# so we can use it both
to open the file# and to refer to in an error message# if needed.my
$file = "temp.txt";
# Use the open() function to create the file.unless(open FILE,
'>'.$file) {
# Die with error message# if we can't open it.
die "\nUnable to create $file\n";}
# Write some text to the file.
print FILE "Hello there\n";print FILE "How are you?\n";
# close the file.close FILE;
Open A Text File, Process It and Write to Another File
Heres a complete example of a program that opens a text file in
Perl, reads it line byline, carries out processing on each line and
writes the results to another file.
# Always define these two at the tops of# your scripts.use
strict;use warnings;
# This turns off output buffering.# Useful for real-time display
of# progress or error messages.$|=1;
# This is for getting command-line options.# Could alternatively
use @ARGV instead.use Getopt::Std;
# Return a usage message. Usually we'd indent the# contents of a
subroutine, but here we can't since# that would affect the usage
message.sub usage {return q|
usage:process.pl -i -o
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Reads all lines in , changes all occurences of'dog' to 'cat' and
writes the results to .
itself is not changed.
|;}
sub main {
# Collect the -i and -o options from the command line.my
%opts;
# The colons specify that the preceeding flags take# arguments
(file names in this case)getopts('i:o:', \%opts);
# Check we got the flags and arguments we need.unless($opts{'i'}
and $opts{'o'}) {
die usage();}
# Now we've got the input and output file names.my $input =
$opts{'i'};my $output = $opts{'o'};
# Try to open the input file.unless(open INPUT, $input) {
die "\nUnable to open '$input'\n";}
# Try to create the output file# (open it for
writing)unless(open OUTPUT, '>'.$output) {
die "\nUnable to create '$output'\n";}
# Read one line at a time from the input file# till we've read
them all.while(my $line = ) {
# Change dog to cat$line =~ s/dog/cat/ig;
# Write the line to the output file.print OUTPUT $line;
# Print a progress indicator.print '.';
}
# Close the files.close INPUT;close OUTPUT;
}
main();
All pages on this site are copyright 2013 John W. Purcell
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