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Globbing is a term used to refer to a pattern matching language, usually used in UNIX environments to search for files with a concise syntax. They do not possess the same flexibility as regular expressions, but are often good enough to get the job done.

They’re also really confusing if you try to use them in Ruby.

A Quick Recap

As a reminder, here are a basic set of glob patterns for a typical UNIX shell:

  • *: match any number of any characters
  • ?: match any single character
  • [AB]: match any of the characters in the set (A or B, in this case)

Unfortunately, as was already alluded to above, different languages and platforms have slightly different implementations/definitions for globs. This can start to make your life frustrating when trying to use them.

Oh My Stars

I recently ran into an issue where Ruby’s implementation of glob patterns wasn’t giving me the results I expected when using a double asterisk (**). From the Dir.glob documentation, ** is supposed to match directories recursively, leading one to assume you can use it to represent “zero or more directories”.

Consider the following directory structure:

cat.js
dog.rb
directory/
mouse.js
fox.rb

If you were to run Dir.glob('**/*.rb') from an irb console in this directory, you’d get the following output:

=> ["directory/fox.rb", "dog.rb"]

In this case, the behaviour of ** matches our expectation given the documentation, in that it finds all files ending in .rb, regardless of how many directories they are nested within.

Ruby also has File.fnmatch?, which allows you to perform glob matching against strings. This saves you from having to scan the file system like in the case of Dir.glob.

What happens if we find all files that match the glob using File.fnmatch?? Note we’re using Dir.glob just to get all the files in the directory so we can compare that list against what would be matched if we used File.fnmatch?.

all_files = Dir.glob('**/*')

all_files.select { |f| File.fnmatch?('**/*.rb', f) }
=> ["directory/fox.rb"]

While we would expect the output to be identical, File.fnmatch? didn’t include files from the root directory! It seemed to only find files that were nested within at least one directory. Why?

It turns out that there are flags that Dir.glob and File.fnmatch? accept which change the behaviour of the glob matching, and for some reason they don’t have the same defaults (this is true in Ruby 2.1.2, the latest version at time of writing). Flags of note are:

  • FNM_DOTMATCH: Allow * to match filenames starting with . (i.e. dotfiles)
  • FNM_EXTGLOB: Enable globbing via {a,b,c} (match a, b, or c, etc.)
  • FNM_PATHNAME: Prevent wildcards from matching path separators

For the example above, we can get File.fnmatch? to produce the same output by enabling the FNM_PATHNAME flag. If you want your globs in Ruby to act like globs in a typical UNIX shell, you’ll want to enable the FNM_EXTGLOB flag as well.

Ruby’s Globbing in a Nutshell

The ** shenanigans described above is really just a Ruby nuance, as most glob implementations get the job done with *. Ruby complicated the issue by providing a way to change the behavior of wildcards such that matching a path separator could be toggled.

Furthermore, not having the default behavior for both Dir.glob and File.fnmatch? match each other is a significant source of confusion. Unfortunately, this area of Ruby’s documentation and code isn’t very well documented, so it’s difficult to tell what the original intention was.

At the end of the day, you can work around the problem by always explicitly passing in the same flags to both Dir.glob and File.fnmatch?.

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Shane da Silva


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Shane da  Silva

Coding by the woods

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