#!/usr/bin/perl -w

#############################################
# This document explains Perl facilities for
# file testing, directories etc...
# AAK: Last Modification
# Tue Sep 11 22:07:39 PDT 2012
# ###########################################

use 5.010;

$filename = "mydata.dat";

if (-e $filename) {
   warn "file $filename exists!";
   $mdpopt=0;
}
else
{
   open OUTPUT1, ">mydata.dat";
   $mdpopt=1;
}

#flag -e checks if file or directory exists:
#other useful flags are:
# -r -R file readable by effective/real user
# -w -W file writable """"""""""""""""""""""
# -x -X """" exectutable """""""""""""""""""
# -o -O """" owened """"""""""""""""""""""""
# -z file exists with zero size
# -s file exists with nonzero size (actually returns file size)
# -d entry is a directory
# -f entry is a plain file
# -M returns modification age (in days)
if (-M $filename > 20) {say "pretty old file!";}
# -A access age (in days)
# -T file looks like a text file
# -B file looks like a binary file

#you can combine several attributes using and:

if ( -r $filename and -w $filename ) { print "both readable and writable\n";}

#or you can stack them (need perl v 5.010:)

use 5.010;
if (-w -r $filename) { say" stacking example";}

#to get pretty much everything about a file, use stat
#which returns almost everything that stat command returns in Unix:

@fileinfo = stat($filename);

say "@fileinfo";

my $timestamp = 0;
my $date = localtime $timestamp;

say "the beginning of time for computer is: $date";

$timestamp = 40*3.14E+7;
my @date = localtime $timestamp;

say $date[0], $date[1];
say "@date";

#chdir changes the "working directory" to the desired location:
# "ENV" is a hash that stores the enviroument variables
# See hash section of these files for more detail
say "your home is: $ENV{HOME}";
chdir "$ENV{HOME}" or die "cannot chdir to $ENV{HOME}: $!";

#globbing is done by use of glob operator:

#following will return all of the files in current working directory into
#the array all_files:
my @all_files = glob "*";
say "@all_files";

if ($frt = glob "*.f") {say "you got some fortran file here!";}
else { say "no .f file found";}

#directory handles are the same as file handles, you open a directory 
#and work with it:

my $dir_name = "/etc";
opendir ETC, $dir_name or die "cannot open $dir_name: $!";
foreach $file (readdir ETC) {
   print "file $file is in $dir_name\n";
}
closedir ETC;

#using unlink you can delete files in Perl:

$nfd = unlink "filetmp29382", "filetmp3827321", "filetmp3827172";
#the return of unlink is number of files successfully deleted

say "I deleted $nfd files";

#renaming a file is as following:

rename "filetmp28321", "filetmpds23721" or warn "unable to rename: $!";

#link and symlink are to create hard and symbolic links to a file:
# link "linkname", "targetfile";
# symlink "linkname". "targetfile";

#using mkdir you can create directory:

chdir $ENV{HOME};
mkdir "newdirtmp123", 0755 or warn "cannot create directory: $!";

# 0755 is the initial permission of the newly created directory (unix standards)

#and change of permission is as following:

#chmod 0755, "file1", "file2";

#to run other programs in perl one way is to use system followed by unix command:

system "date";
system "ls -l $ENV{HOME}";

#you can also send a program to background and let the script keep running:
#using standard & at the end:

# system "some_program_that_takes_log &";

#you can also run other scripts:

system "somescript";

#you can also use the backquotes similar to unix standard backquote
#to capture the output of a program as argument:
#the only difference is that perl's `` will also return \n at the end
#so it is better to chomp it:

chomp (my $time = `date`);
say $time;

#if the returning variable is an array, then you can have various lines:

@processes =`ps`;

foreach $i (@processes) {
   print $i;
}

#and you can use I/O of a command as filehandle:

open DT, "date|" or die "cannot pipe from date :$!";
open MP, "somescript|" or warn "cannot pipe from somescript :$!";
open AP, "|anotherscript" or warn "cannot pipe to anotherscirpt: $!";
$x=`somescript`;
print $x;

#and you can now read and write from/to the file handles:

print AP "this will be sent to anotherscript if exist" or warn "anotherscript is not available";
my $now = <DT>;
print $now;