Recursively walk up a directory tree

[1nickt]

Greetings all,

I need to walk up the directory tree from where I am until I find a directory containing a certain marker file. (This is to locate the base directory of an installation, which is then used to add the project's /lib to @INC.)

Is there anything more elegant than this?

use strict; use warnings;
use Path::Tiny;

sub find_base { -f "$_[0]/.marker" ? $_[0] : find_base($_[0]->parent) 
+}
my $base = find_base( path(__FILE__)->realpath->parent );
unshift @INC, "$base/lib";

use Some::Class;
...

(I am aware that this does not handle the file not being found.)

Thanks for any suggestions!

[tybalt89]

Elegant can be very subjective...

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1208898

use strict;
use warnings;
use Cwd;

cwd =~ m#^(.*)(?:/|$)(??{! -f "$1/.marker"})# or die ".marker not foun
+d";
print "found in $1\n";

Update: Small fix for the problem mentioned by AnomalousMonk in Re^3: Recursively walk up a directory tree

cwd =~ m#^(.*)(?=/|$)(??{! -f "$1/.marker"})# or die ".marker not foun
+d";

[1nickt]

Thanks for the suggestion! I see the (??{ ... }) code-as-match, but can't see where it recurses or loops. Could you please explain?

[tybalt89]

The looping is done by the regex engine, trying to find the longest match, and if it fails, the next longest, etc.

Try adding a print statement inside the code block and you can see each directory before the test for .marker.

Like so:

cwd =~ m#^(.*)(?:/|$)(??{print "testing $1\n"; ! -f "$1/.marker"})# or
+ die "    .marker not found";

[AnomalousMonk]

The  cwd() function gives the full path of the... well, CWD. The match looks for a file at the end (rightmost part) of the path. If the file is not found, the regex engine effectively backtracks up a level in the directory.

The  ! -f "$1/.marker" code in the  (??{ CODE }) construct evaluates to  '' (empty string) if the file exists at a given level, and this is interpolated into the regex pattern and always matches, hence the file is "found" at that level.

If the file is not found, the negated file test evaluates to  '1' and this will probably not be found at the beginning of a directory name. However, this can fail (debug print added to show progress):

use strict;
use warnings;
use Cwd;

my $marker = '.marker';

cwd =~ m#^(.*)(?:/|$)(??{ print "at level '$1' \n";  ! -f "$1/$marker"
+})# or die "'$marker' not found";
print "found '$marker' in '$1' \n";

Output (with no  .marker file present anywhere):

c:\@Work\Perl\monks\1nickt\two\one\zero>perl find_marker_1.pl
at level 'c:/@Work/Perl/monks/1nickt/two/one/zero'
at level 'c:/@Work/Perl/monks/1nickt/two/one'
at level 'c:/@Work/Perl/monks/1nickt/two'
at level 'c:/@Work/Perl/monks/1nickt'
at level 'c:/@Work/Perl/monks'
found '.marker' in 'c:/@Work/Perl/monks'

If you have Perl version 5.10+, there's a variation that avoids this problem:

use 5.010;  # needs (?(?{ CODE })yes-pattern) regex extension

use strict;
use warnings;

use Cwd;

my $marker = '.marker';

cwd =~ m{
    \A (.*) (?: / | \z)
    # (??{ print "at level '$1' \n";  ! -f "$1/$marker"})
    (?(?{ print "at level '$1' \n";  ! -f "$1/$marker" }) (*FAIL))
    }xms
    or die "'$marker' not found";

print "found '$marker' in '$1' \n";

Output (still no  .marker file):

c:\@Work\Perl\monks\1nickt\two\one\zero>perl find_marker_2.pl
at level 'c:/@Work/Perl/monks/1nickt/two/one/zero'
at level 'c:/@Work/Perl/monks/1nickt/two/one'
at level 'c:/@Work/Perl/monks/1nickt/two'
at level 'c:/@Work/Perl/monks/1nickt'
at level 'c:/@Work/Perl/monks'
at level 'c:/@Work/Perl'
at level 'c:/@Work'
at level 'c:'
'.marker' not found at find_marker_2.pl line 62.

Both versions of code behave the same if a  .marker is present at some level, although the first version will fail if the  .marker file is present above the level of, say, the  1nickt directory.

[choroba]

No need for recursion, a loop is just fine:

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use Path::Tiny;

my $path = 'Path::Tiny'->cwd;
$path = path("$path/..")->realpath
    until -e "$path/.marker" || '/' eq $path;
say -e "$path/.marker" ? 'F' : 'Not f', 'ound';

[1nickt]

Thanks choroba, I knew there must be a way with until! Maybe even more elegant as:

    my $path = Path::Tiny->cwd;
    $path = $path->parent until -f "$path/.marker" or $path->is_rootdi
+r;

[Your Mother]

Isn't relying on cwd less robust than using __FILE__? I guess I'm having trouble coming up with what might chdir/cd on/under the script, so maybe not.

[Anonymous Monk]

Its called a module

[stevieb]

I believe you actually meant "distribution" if I've understood what you meant correctly. Semantics, I know, but an important distinction.

[Anonymous Monk]

I believe you actually meant "distribution" if I've understood what you meant correctly. Semantics, I know, but an important distinction.

Hehe,

OP posts some code and asks Is there anything more elegant than this?

Yeah, its called a module.

Instead of doing ... 20 lines to modify global variable ... do use HideTwentyLines; and

elegance achieved