Common path

Given a list of absolute Linux file paths, determine the deepest path to the directory that contains all of them.

Examples

Example
Input:
    /a/b/c/1/x.pl
    /a/b/c/d/e/2/x.pl
    /a/b/c/d/3/x.pl
    /a/b/c/4/x.pl
    /a/b/c/d/5/x.pl
Output:
    /a/b/c

Analysis

Let's start with the first item in the list - say /a/b/c/d/xx. We can immediately see that the longest possible answer is /a/b/c/d/, because any longer one would not be common to the first item.

So let's go down the list looking to see if they all start with /a/b/c/d/. As soon as we find one that doesn't, lets start again, stripping the deepest directory from our initial guess, which in our case leaves us with /a/b/c/.

Eventually we'll find a path that is common to all the items in the list, though that may simply be /.

Amended 09.06.2026.

Story telling narrative of weekly task is irresistable. Thanks for everything.

This review may cover either or both challenges for this week.

I am sorry that the 'Try it' feature is currently working very slowly or not at all owing to some issue with my web hosting provider.

Try it 

Try running the script with any input:

Your @list =

example: /a/b/c/d, /a/b/x/d, /a/b/x/y

⤡

#!/usr/bin/perl

# Peter Campbell Smith - 2022-09-15
# PWC 182 task 2

use v5.28;
use utf8;
use warnings;

common_folder(
 qw[/a/b/c/1/x.pl
    /a/b/c/d/e/2/x.pl
    /a/b/c/d/3/x.pl
    /a/b/c/4/x.pl
    /a/b/c/d/5/x.pl]);
    
common_folder(
 qw[/a/b/c/x.pl 
    /d/e/f/x.pl]);
    
common_folder(
 qw[/m/n/o/p/xx
    /m/n/o/p/xx]);

sub common_folder {
    
    my (@paths, $guess, $try, $path, $good);
    @paths = @_;
    
    # show input
    say qq[\nInput:\n    ] . join(qq[\n    ], @paths);
    
    # get the path to the first one
    $paths[0] =~ m|(.*/)|;
    $guess = $1;

    # work backwards, stripping off the last folder
    TRY: for $try (1 .. 99) {
        
        # loop over the rest to see if they match
        $good = 1;
        for $path (@paths) {
            unless ($path =~ m|^$guess|) {
                $good = 0;
                last;
            }
        }
        # found the answer
        last if $good;
        
        # remove the last folder and try again
        $guess =~ s|/[^/]*/$|/|;
    }
    $guess .= ' ' if $guess eq '/';
    say qq[Output:\n    $guess];
}   

    

16 lines of code

Output from script

Input:
    /a/b/c/1/x.pl
    /a/b/c/d/e/2/x.pl
    /a/b/c/d/3/x.pl
    /a/b/c/4/x.pl
    /a/b/c/d/5/x.pl
Output:
    /a/b/c/

Input:
    /a/b/c/x.pl
    /d/e/f/x.pl
Output:
    /

Input:
    /m/n/o/p/xx
    /m/n/o/p/xx
Output:
    /m/n/o/p/