How to list the n'th youngest file (without parsing ls!)

Perlishly:

#!/usr/bin/env perl

use strict;
use warnings;

my %files_by_mtime;

foreach my $file ( glob ( "./*" ) ) {
   my ( $mtime ) = (stat($file))[9];
   $files_by_mtime{$file} = $mtime; 
}

foreach my $f (  sort { $files_by_mtime{$b} <=> $files_by_mtime{$a} } keys %files_by_mtime ) {
   print "$f $files_by_mtime{$f}\n";
}

Reducing to:

perl -e '%f = map { $_ => (stat)[9] } glob("./*");print "" . ( sort { $f{$b} <=> $f{$a} } keys %f )[4];'

Where the "N"th file is 4, because perl arrays start from zero.

Assuming you have access to GNU tools (you do if you're running Linux), I would use stat instead. For example:

$ ls -l
total 0
-rw-r--r-- 1 terdon terdon 0 Sep 15 16:49 file1
-rw-r--r-- 1 terdon terdon 0 Sep 15 16:39 file2
-rw-r--r-- 1 terdon terdon 0 Sep 15 16:29 file3
-rw-r--r-- 1 terdon terdon 0 Sep 15 16:19 file4
-rw-r--r-- 1 terdon terdon 0 Sep 15 16:09 file5
-rw-r--r-- 1 terdon terdon 0 Sep 15 15:59 file6
-rw-r--r-- 1 terdon terdon 0 Sep 15 15:49 file7

So, the 5th newest file is file5. To print just that, you can do:

$ stat --printf '%Y %n\0' * | sort -zrnk1 | 
    awk -vRS='\0' 'NR==5{sub(/^[^ ]* /,"",$0); print}'

You could then easily make this into a shell function that can take N (5 in your example) as an argument. Just add these lines to your ~/.bashrc or equivalent:

nthfile() {
  stat --printf '%Y %n\0' * | sort -zrnk1 |
     awk -vRS='\0' -vn="$1" 'NR==n{sub(/^[^ ]* /,"",$0); print}'
}

Note that this will also show you directories. If you need it to match hidden files as well run (assuming you're using bash) shopt -s dotglob before the above command.

Explanation

• stat --printf '%Y %n\0' * : for each file or directory in the current folder, print the modification date in seconds since the epoch (%s) and the file name (%n) and end each line with \0 instead of \n. This lets us deal correctly with file names containing newline characters.
• sort -zrnk1 : sort the output in reverse sort order (-r), from the newest to the eldest. The -z tells sort to expect null-terminated input lines. The -n tells it to sort numerically and the -k1 to only consider the first field when sorting.
• awk:
  • -vRS='\0' : set the input record (line) separator to \0;
  • vn=$1 : set the variable n to whatever was given as input to the function;
  • NR==n{} : run this only on line n (5 in the first example);
  • sub(/^[^ ]* /,"",$0); print : substitute all non-space characters from the beginning of the line (^[^ ]*) up to the 1st space and print the result

One way to do this (assuming no newlines in your file names) would be

ls -t ... | head -n5 | tail -n1

With zsh:

print -r -- *(.Dom[5])

give you the 5th file in current directory sorted by modification time. Change m to a to sort by access time. . and D include only regular and hidden files.