Why would you want to convert timestamps to Unix epoch time?

Unix epoch time is an ever-growing counter which increments every second and shows the number of seconds elapsed since 00:00:00 UTC on January 1, 1970. Such numbers are pretty useless in their raw form for the tasks of confirming current time and date, but they're perfect for measuring between two points in time.

Current epoch time in Perl

First things first. If you want to confirm the current Unix epoch time in Perl, here's how you do it: just use the time() subroutine.

#!/bin/perl
print "Current (epoch) time: " . time() . "\n";

Converting regular date and time into epoch time

It's equally easy to convert time into Unix epoch representation, for this you should use timegm() or localtime() subroutine from the Time::Local module.

timegm() has the following syntax:

$current = timegm($sec,$min,$hours,$day,$month,$year);

Important: While most of the parameters are self-explanatory, it is probably worth reminding that $year should be specified as a number like 109 and not 2009.
Also, the $month is not the number of a month, but a number of months since January. So, 0 will mean January itself, 1 will mean February, and 11 means December.

Here's a fully working example, showing the countdown in seconds until midnight on the New Year's Eve 2009:

#!/usr/bin/perl
#
use Time::Local;
#
$sec=59;
$min=59;
$hours=23;
$day=31;
$month=11;
$year=109;
#
$current = time();
print "Current (epoch) time: $current\n";
#
$newyear = timegm($sec,$min,$hours,$day,$month,$year);
print "New Year's Eve 2009: $newyear\n";
#
print "Only " .($newyear-$current) . " seconds to go\n";

When you save this script as epoch.pl and make the file executable, here's what you'll get if you run it:

# ./epoch.pl
Current (epoch) time: 1234911622
New Year's Eve 2009: 1262303999
Only 27392377 seconds to go

That's it for today! Hope this little post will save you time when tacking next scripting challenge with Perl.

See also:

• Time and date in Unix scripts
• Date calculations with GNU date command
• Glossary of Unix terms
  

How to check if a file exists in Perl

First of all, we need to create one file for our experiments:

ubuntu$ touch /tmp/file.try

Now, create a new file with our script: /tmp/file-try.pl (don't forget to chmod a+x /tmp/file-try.pl afterwards to make it executable):

#!/usr/bin/perl
#
$FILE1="/tmp/file.try";
$FILE2="/tmp/file2.try";
#
if (-e $FILE1) {
        print "File $FILE1 exists!\n";} else {
        print "File $FILE1 is not found!\n";
}
if (-e $FILE2) {
        print "File $FILE2 exists!\n";
} else {
        print "File $FILE2 is not found!\n";
}

Now, as you can see from the example, the actual condition checking for the existence of a file is this:

if (-e $FILE)

where $FILE is the string containing the filename.

In my example, there's two filenames ($FILE1 and $FILE2) and two conditions, so that you can see for yourself how one of them confirms the existence of a file and another one proves that there's no such file found.

To see the script work, simply run it without any parameters:

ubuntu$ /tmp/file-try.pl
File /tmp/file.try exists!
File /tmp/file2.try is not found!

That's all for today! Enjoy your Perl scripting!

See also:

• How to find out file permissions in Perl
• Walking a directory tree in Perl
• Unix glossary
  

lstat and stat functions, return, among other things, the file mode value. While it looks confusing initially, it is in fact quite simply a combination field, which includes both the file type and all the permissions for it. If you print this field as a single decimal number, you will not recognize it, but if you simply convert it to octal, you will immediately start seeing the pattern:

Mysterious mode 33261 from the example below becomes 100755 when converted into octal, and you can easily see then the permission part of it: 0755.

In order to find out a file type in Perl, you need to be able to separate the type part from the permissions part of the file mode value. You can do this manually, but applying the bitmasks, but the easier way is to use the Fcntl ':mode' Perl module.

Fcntl':mode' module contains all the constants and functions you need to extract the file type and permissions without complicating the task too much.

S_IMODE is the function which helps you extract the permissions:

        $permissions = S_IMODE($mode);

and S_IFMT is the function which extracts the file type:

        $filetype = S_IFMT($mode);

Now, obviously these extracted values are just the raw numbers, that's why you'll likely want to convert the permissions into octal, and to somehow connect the file type with the common symbolic notation, that's exactly what I'm doing in today's code example.

Here's how it looks (I have the empty lines commented out just so that the code can be correctly shown in WordPress):

#!/usr/bin/perl
use Fcntl ':mode';
#
if ($ARGV[0] ne "") {
        $filename = $ARGV[0];
} else {
        print "Please specify a file!\n";
        exit;
}
#
if (($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,$atime,$mtime,$ctime,$blksize,$blocks) = lstat($filename)) {
        $user = getpwuid($uid);
        $group = getgrgid($gid);
#
        $ftypes[S_IFDIR] = "d";
        $ftypes[S_IFCHR] = "c";
        $ftypes[S_IFBLK] = "b";
        $ftypes[S_IFREG] = "-";
        $ftypes[S_IFIFO] = "p";
        $ftypes[S_IFLNK] = "l";
        $ftypes[S_IFSOCK] = "s";
#
        $permissions = sprintf "%04o", S_IMODE($mode);
        $filetype = S_IFMT($mode);
#
        $ftype = $ftypes[$filetype];
#
        print "File: $filename\n";
	printf "File mode: %o (%d)\n", $mode, $mode;
        printf "File type: %s (%o)\n", $ftype, $filetype;
        print "File permissions: $permissions\n";
        print "File size: $size\n";
        print "File owner user: $user\n";
        print "File group: $group\n";
} else {
        print "Please specify an EXISTING file!\n";
}

And this is how it works:

root@ubuntu:/usr/include# /tmp/stat.pl /tmp/stat.pl
File: /tmp/stat.pl
File mode: 100755 (33261)
File type: - (100000)
File permissions: 0755
File size: 918
File owner user: greys
File group: greys

See also:

• Directory tree in Perl
• stat function in Perl