consolidate shebang logic

lib/linguist/shebang.rb

@@ -0,0 +1,49 @@
+module Linguist
+  # Check if there's a shebang line and use that as authoritative
+  class Shebang
+    def self.call(blob, _)
+      Language.find_by_interpreter(new(blob.data).interpreter)
+    end
+
+    attr_reader :data
+
+    def initialize(data)
+      @data = data
+    end
+
+    def interpreter
+      lines = data.lines.to_a
+
+      if lines.any? && (match = lines[0].match(/(.+)\n?/)) && (bang = match[0]) =~ /^#!/
+        bang.sub!(/^#! /, '#!')
+        tokens = bang.split(' ')
+        pieces = tokens.first.split('/')
+
+        if pieces.size > 1
+          script = pieces.last
+        else
+          script = pieces.first.sub('#!', '')
+        end
+
+        script = script == 'env' ? tokens[1] : script
+
+        # If script has an invalid shebang, we might get here
+        return unless script
+
+        # "python2.6" -> "python2"
+        script.sub! $1, '' if script =~ /(\.\d+)$/
+
+        # Check for multiline shebang hacks that call `exec`
+        if script == 'sh' &&
+          lines[0...5].any? { |l| l.match(/exec (\w+).+\$0.+\$@/) }
+          script = $1
+        end
+
+        File.basename(script)
+      else
+        nil
+      end
+
+    end
+  end
+end