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How do I change .. to the real filename?

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Contributed by dcardamo on Jan 22, 2001 at 08:09 UTC
Q&A  > regular expressions


Description:

I have a filename with the entire path '/home/projects/..' The '..' means to go back a directory, so its real path is '/home' Can this be done with a regex or do I have to split this?

Answer: How do I change .. to the real filename?
contributed by merlyn

use File::Spec; print File::Spec->rel2abs("/home/projects/..");
Answer: How do I change .. to the real filename?
contributed by AltBlue

use Cwd 'abs_path'; print abs_path('/home/projects/..');
Answer: How do I change .. to the real filename?
contributed by matthewm

To answer your specifc example..

$file =~ s%/[^/]+/\.\.$%%
... will eat the last directory name that's up'ed by the trailing ..
To give a more general/complete example that can handle multiple .'s and ..'s at any point in the path.
# colapse /./ and /.$ 1 while ($file =~ s%/\.(/|$)%$1%g); # colapse /dir/../ and /dir/..$ # ignore /../../ and /../..$ 1 while ($file =~ s%/[^/]+/(?<!/\.\./)\.\.(/|$)%$1%g); # remove ^dir/../ and ^dir/../ # ignore ^../../ and ^../.. 1 while ($file =~ s%^[^/]+/(?<!^\.\./)\.\.(/|$)%%); # colapse ^/../ to / 1 while ($file =~ s%^/\.\./%/%); # expand null to . $file =~ s%^$%.%;
We need the itterations of each loop as each substitution may reveal additional matchs.
We need multiple expressions to get the order of susbistion right. That is /../../ should colapse the two proceding directoires and not itself.
I'm sure it can be reduced more than that but it is as simple as I can see it can be and still catch all the special cases.
I use something very similar to this at the beginning of my scripts to get the conical path for $0 and not the called path. Although spliting the path into its components and working backwards is probably easier/clearer than using a regex.
Answer: How do I change .. to the real filename?
contributed by dcardamo

I tried the first solution, and it didn't work properly. I haven't tried the second, but here is a non-regex routine that I wrote to do that:

sub relpath($) { my $file = shift; my $newfile = ""; $file =~ s/\n//g; # remove new lines my @peices = split /\//, $file; for (my $i = 0; $i <= $#peices; $i++) { if ($peices[$i + 1] eq "..") { last; } if ($peices[$i] ne ".") { $newfile .= "/$peices[$i]"; } } $newfile =~ s/\/\//\//g; return $newfile; }
Answer: How do I change .. to the real filename?
contributed by Shakya

You can even try this -

1 while ($string =~ s!(/(?:[^/.]+/)*)((?:[^/.]+/)+\.\./?)(.*)$!$1$3!g) +;

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