Question

I need to get Perl to remove relative path components from a Linux path. I’ve found a couple of functions that almost do what I want, but:

File::Spec->rel2abs does too little. It does not resolve “..” into a directory properly.

Cwd::realpath does too much. It resolves all symbolic links in the path, which I do not want.

Perhaps the best way to illustrate how I want this function to behave is to post a bash log where FixPath is a hypothetical command that gives the desired output:

'/tmp/test'$ mkdir -p a/b/c1 a/b/c2
'/tmp/test'$ cd a
'/tmp/test/a'$ ln -s b link
'/tmp/test/a'$ ls
b  link
'/tmp/test/a'$ cd b
'/tmp/test/a/b'$ ls
c1  c2
'/tmp/test/a/b'$ FixPath . # rel2abs works here
===> /tmp/test/a/b
'/tmp/test/a/b'$ FixPath .. # realpath works here
===> /tmp/test/a
'/tmp/test/a/b'$ FixPath c1 # rel2abs works here
===> /tmp/test/a/b/c1
'/tmp/test/a/b'$ FixPath ../b # realpath works here
===> /tmp/test/a/b
'/tmp/test/a/b'$ FixPath ../link/c1 # neither one works here
===> /tmp/test/a/link/c1
'/tmp/test/a/b'$ FixPath missing # should work for nonexistent files
===> /tmp/test/a/b/missing

Answer 1

Alright, here is what I came up with:

sub mangle_path {
  # NOT PORTABLE
  # Attempt to remove relative components from a path - can return
  # incorrect results for paths like ../some_symlink/.. etc.

  my $path = shift;
  $path = getcwd . "/$path" if '/' ne substr $path, 0, 1;

  my @dirs = ();
  for(split '/', $path) {
    pop @dirs, next if $_ eq '..';
    push @dirs, $_ unless $_ eq '.' or $_ eq '';
  }
  return '/' . join '/', @dirs;
}

I know this is possibly insecure and invalid, but any input to this routine will come from me on the command line, and it solves a couple of tricky use cases for me.