File Processing
Writing To Files
In this lesson the student will learn to:
- Write to files.
- Know how to append to a file or to overwrite a file.
By the end of this lesson the student will be able to:
Write a short Perl script which saves
information to a file.
WRITING TO FILES:
The following example will take each line from gern.xml that contains the
pattern "company" and print it to a file called output.txt.
<![CDATA[
#!/usr/bin/perl -w
open (FILE, "gern.xml") || die;
@ray = <FILE>;
open (FILE2, ">output.txt") || die;
foreach $line (@ray){
if( $line =~ /company/){
print FILE2 $line;
}
}
exit;
]]>
ADDING OUTPUT TO FILE:
The following script will add lines of output to a file.
<![CDATA[
#!/usr/bin/perl -w
open (FILE, ">>output.txt") || die;
while(1){
print "INPUT: ";
$line = <STDIN>;
chomp($line);
if($line eq "q"){ last; }
print FILE "$line\n";
}
exit;
]]>
What's the difference between these two lines?
<![CDATA[
open (FILE, ">output.txt") || die;
open (FILE, ">>output.txt") || die;
]]>
There is just one character added to the second line. It has >>
instead of just >. The second line appends (or adds to a file). The first
line over-writes the file which means that any thing that was in the file
before running the script will be lost after the script is run.
FILE TEST OPERATORS
<![CDATA[
-b Is name block device?
-c Is name a character device?
-d Is name a directory?
-e Does name exist?
-f Is name an ordinary file?
-l Is name a symbolic link?
-o Is named owned by user?
-r Is name a readable file?
-s Is name a non-empty file?
-w Is name a writable file?
-x Is name an executable file?
-z Is name an empty file?
-A How long since name accessed?
-M How long since name modified?
-T Is name a text file?
]]>
A PROGRAM THAT PRINTS THE SIZE OF A FILE IN BYTES:
<![CDATA[
#!/usr/bin/perl -w
print ("ENTER FILE NAME: ");
$fn = <STDIN>;
chomp($fn);
if(!(-e $fn)){
print ("File $fn does not exist.\n");
} else {
$size = -s $fn;
print ("File $fn contains $size bytes.\n");
}
]]>
ECHOES CONTENTS OF ONE OR MORE FILES:
<![CDATA[
#!/usr/bin/perl -w
while($input = <>){
print $input;
}
exit;
TO RUN TYPE: test.pl file1.txt file2.txt
]]>
PROGRAM TO CREATE RANDOM DNA SEQUENCE AND SAVE TO FILE:
<![CDATA[
#!/usr/bin/perl -w
my @DNA = ("A", "G", "T", "C");
srand(time);
unless(open(OUTFILE, ">DNAseq.txt")){
die("Cannot open output file\n");
}
foreach my $n (0..240){
my $i = int rand(4);
print OUTFILE "$DNA[$i]";
if( ($n+1)%40 == 0) {
print OUTFILE"\n";
}
}
close(OUTFILE);
exit;
]]>
ASSIGNMENT:
Write the following two short scripts:
- LINE COUNT: Write a script that prompts the user for a file name and
reports, if the file exists, how many lines it contains.
- RANDOM DNA: Write a script which generates a file that contains a DNA
sequence that is 40 lines long with lines which contain 60 nucleotides
each.