# File.yaml?(fname) **URL:** \ **Category:** Ruby\ **Created:** [December 9, 2006, 6:29pm UTC](https://www.ruby-forum.com/t/file-yaml-fname/78873 "2006-12-09T18:29:45Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Trans](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/dfb087/32.png) [@Trans](https://www.ruby-forum.com/u/Trans)\ **Post date:** [December 9, 2006, 6:29pm UTC](https://www.ruby-forum.com/t/file-yaml-fname/78873/1 "2006-12-09T18:29:45Z")
what’s the best way to determine if a file is yaml? thanks, t. ---
**Author:** ![Trans](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/dfb087/32.png) [@Trans](https://www.ruby-forum.com/u/Trans)\ **Post date:** [December 9, 2006, 6:45pm UTC](https://www.ruby-forum.com/t/file-yaml-fname/78873/2 "2006-12-09T18:45:48Z")
Trans wrote: > what’s the best way to determine if a file is yaml? > Naive answer: def File.yaml?(fname) YAML.load(IO.read(fname)) true rescue ArgumentError false end Though, open up irb -ryaml and keep running this line: YAML.load Array.new(60){rand 256}.pack(‘c\*’) I’m not sure that’s what you’re after. 🙂 And I’m guessing you didn’t mean: def File.yaml?(fname) extname(fname) =~ /^ya?ml$/ end Devin ---
**Author:** ![Trans](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/dfb087/32.png) [@Trans](https://www.ruby-forum.com/u/Trans)\ **Post date:** [December 9, 2006, 6:55pm UTC](https://www.ruby-forum.com/t/file-yaml-fname/78873/3 "2006-12-09T18:55:49Z")
Trans wrote: > what’s the best way to determine if a file is yaml? Process the file using a parser meant to process YAML. If the parse fails, it means: 1. The file isn’t YAML. 2. The chosen parser is not robust enough to process this specific, valid YAML file. 3. The YAML file, although more or less valid YAML, has syntax errors not consistent with the formal YAML specification. 4. The YAML specification contains ambiguities that allow a valid parser to fail on valid YAML syntax. 5. Other. In other words, you cannot really say, absolutely and unambiguously, that a particular file is a YAML file. ---
**Author:** ![Trans](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/dfb087/32.png) [@Trans](https://www.ruby-forum.com/u/Trans)\ **Post date:** [December 9, 2006, 9:06pm UTC](https://www.ruby-forum.com/t/file-yaml-fname/78873/4 "2006-12-09T21:06:01Z")
Trans wrote: > what’s the best way to determine if a file is yaml? In light of the other responses, which show how hard it is to do this in general, what about a pragmatic approach that might work in most of the cases you are interested in? Look at the first N lines. If any line has _any_ non-printing characters, it’s not correct YAML and wasn’t generated by YAML#dump.[1] If any are longer than M chars or other binary file heuristics apply[2], it’s probably not a manually written YAML file. If it passes at least _one_ of these two checks, then check to see if 80% of the (first N) lines match the following: /^\s\*(-|?|[\w\s]\*:)\s/ Maybe add some logic to skip blocks of text like this (so they don’t count against the 80%): a: | skip me Also, check for \> in place of |. And also skip blanks and comments /^\s\*(#|$)/. And then finally load it and rescue any ArgumentError. There are probably a lot of corner cases that kill this approach if you cannot tolerate false negatives (i.e., legit yaml that gets rejected by the above). * * * [1] The YAML spec, [http://yaml.org/spec/current.html](http://yaml.org/spec/current.html), says nonprinting chars are encoded (see 4.1.1. Character Set), and it seems to be true, at least in the dump output: ## irb(main):023:0\> puts({“a”=\>“\002”}.to\_yaml) a: !binary | Ag== However, YAML can load unescaped binary data, as Devin showed: irb(main):025:0\> YAML.load “a: \002” =\> {“a”=\>“\002”} [2] For example, [http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/52548](http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/52548) ---
**Author:** ![Trans](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/dfb087/32.png) [@Trans](https://www.ruby-forum.com/u/Trans)\ **Post date:** [December 10, 2006, 12:02am UTC](https://www.ruby-forum.com/t/file-yaml-fname/78873/5 "2006-12-10T00:02:18Z")
Joel VanderWerf wrote: > wasn’t generated by YAML#dump.[1] > count against the 80%): > > There are probably a lot of corner cases that kill this approach if you > cannot tolerate false negatives (i.e., legit yaml that gets rejected by > the above). yikes! if that’s what it takes then i must run away! 🙂 i need something snappy. actually it just occured to me that as of YAML 1.1 the document declaration is mandetory. I had forgotten about that. So checking for an initial line starting with %YAML would do the trick as long as docs where 1.1 compliant --at least in this regard. Unfortuantely Syck itself isn’t 1.1 compliant in this respect whatsoever ☹ In the mean time I’m just going to go with ara’s suggestion. the use of an initial ‘—’ is an acceptable requirment for my needs. t. ---
**Author:** ![Trans](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/dfb087/32.png) [@Trans](https://www.ruby-forum.com/u/Trans)\ **Post date:** [December 9, 2006, 9:08pm UTC](https://www.ruby-forum.com/t/file-yaml-fname/78873/6 "2006-12-09T21:08:01Z")
On Sun, 10 Dec 2006, Trans wrote: > what’s the best way to determine if a file is yaml? > > thanks, > t. in ruby queue i detect whether stdin input is a normal list or yaml in this way: if first\_non\_blank\_line =~ %r/^\s\*—\s\*$/ load\_yaml\_from\_stdin else process\_line first\_non\_blank\_line while((line = next\_line[stdin])) process\_line line end end not perfect, but’s it worked well enough so far cheers. also, from the command line i’ve taken to this approach list\_input\_on\_stdin = ARGV.delete ‘-’ yaml\_input\_on\_stdin = ARGV.delete ‘—’ for, for example cat.rb - # dump stdin cat.rb — # load the yaml doc on stdin and dump that note that ‘–’ is used to indicate the end of options so it is not a good flag. regards. -a