Extracting the first two characters from a file in perl into another file - bash

Question

I'm having a little bit of trouble with my code below -- I'm trying to figure out how to open up all these text files (.csv files that end in DIS that all have one line in them) and get the first two characters (these are all numbers) from them and print them into another file of the same name, with a ".number" suffix. Some of these .DIS files don't have anything in them, in which case I want to print "0".
Lastly, I would like to go through each original .DIS file and delete the first 3 characters -- I did this through bash.
my #DIS = <*.DIS>;
foreach my $file (#DIS){
my $name = $file;
my $output = "$name.number";
open(INHANDLE, "< $file") || die("Could not open file");
while(<INHANDLE>){
open(OUT_FILE,">$output") || die;
my $line = $_;
chomp ($line);
my $string = $line;
if ($string eq ""){
print "0";
} else {
print substr($string,0,2);
}
}
system("sed -i 's/\(.\{3\}\)//' $file");
}
When I run this code, I get a list of numbers are concatenated together and empty .DIS.number files. I'm rather new to Perl, so any help would be appreciated!

Answer 1

When I run this code, I get a list of numbers are concatenated together and empty .DIS.number files.
This is because of this line.
print substr($string,0,2);
print defaults to printing to STDOUT (ie. the screen). You need to give it the filehandle to print to.
print OUT_FILE substr($string,0,2);
They're being concatenated because print just prints what you tell it to, it won't put newlines in for you (there are some global variables which can change this, don't mess with them). You have to add the newline yourself.
print OUT_FILE substr($string,0,2), "\n";
As a final note, when working with files in Perl I would suggest using lexical filehandles, Path::Tiny, and autodie. They will avoid a great number of classic problems working with files in Perl.

Answer 2

I suggest you do it like this
Each *.dis file is opened and the contents read into $text. Then a regex substitution is used to remove the first three characters from the string and capture the first two in $1
If the substitution succeeded then the contents of $1 are written to the number file, otherwise the original file is empty (or shorter than two characters) and a zero is written instead. The remaining contents of $text are then written back to the *.dis file
use strict;
use warnings;
use v5.10.1;
use autodie;
for my $dis_file ( glob '*.DIS' ) {
my $text = do {
open my $fh, '<', $dis_file;
<$fh>;
};
my $num_file = "$dis_file.number";
open my $dis_fh, '>', $dis_file;
open my $num_fh, '>', $num_file;
if ( defined $text and $text =~ s/^(..).?// ) {
print $num_fh "$1\n";
print $dis_fh $text;
}
else {
print $num_fh "0\n";
print $dis_fh "-\n";
}
}

Answer 3

this awk script extract the first two chars of each file to it's own file. Empty files expected to have one empty line based on the spec.
awk 'FNR==1{pre=substr($0,1,2);pre=length(pre)==2?pre:0; print pre > FILENAME".number"}' *.DIS
This will remove the first 3 chars
cut -c 4-
Bash for loop will be better to do both, which we'll need to modify the awk script little bit
for f in *.DIS;
do awk 'NR==1{pre=substr($0,1,2);$0=length(pre)==2?pre:0; print}' $f > $f.number;
cut -c 4- $f > $f.cut;
done
explanation: loop through all files in *.DTS, for the first line of each file, try to get first two chars (1,2) of the line ($0) assign to pre. If the length of pre is not two (either the line is empty or with 1 char only) set the line to 0 or else use pre; print the line, output file name will be input file appended with .number suffix. The $0 assignment is a trick to save couple keystrokes since print without arguments prints $0, otherwise you can provide the argument.
Ideally you should quote "$f" since it may contain space in file name...