41

I have several .htm files which open in Gedit without any warning/error, but when I open these same files in Jedit, it warns me of invalid UTF-8 encoding...

The HTML meta tag states "charset=ISO-8859-1". Jedit allows a List of fallback encodings and a List of encoding auto-detectors (currently "BOM XML-PI"), so my immediate problem has been resolved. But this got me thinking about: What if the meta data wasn't there?

When the encoding information is just not available, is there a CLI program which can make a "best-guess" of which encodings may apply?

And, although it is a slightly different issue; is there a CLI program which tests the validity of a known encoding?

53

The file command makes "best-guesses" about the encoding. Use the -i parameter to force file to print information about the encoding.

Demonstration:

$ file -i *
umlaut-iso88591.txt: text/plain; charset=iso-8859-1
umlaut-utf16.txt:    text/plain; charset=utf-16le
umlaut-utf8.txt:     text/plain; charset=utf-8

Here is how I created the files:

$ echo ä > umlaut-utf8.txt 

Nowadays everything is utf-8. But convince yourself:

$ hexdump -C umlaut-utf8.txt 
00000000  c3 a4 0a                                          |...|
00000003

Compare with https://en.wikipedia.org/wiki/Ä#Computer_encoding

Convert to the other encodings:

$ iconv -f utf8 -t iso88591 umlaut-utf8.txt > umlaut-iso88591.txt 
$ iconv -f utf8 -t utf16 umlaut-utf8.txt > umlaut-utf16.txt 

Check the hex dump:

$ hexdump -C umlaut-iso88591.txt 
00000000  e4 0a                                             |..|
00000002
$ hexdump -C umlaut-utf16.txt 
00000000  ff fe e4 00 0a 00                                 |......|
00000006

Create something "invalid" by mixing all three:

$ cat umlaut-iso88591.txt umlaut-utf8.txt umlaut-utf16.txt > umlaut-mixed.txt 

What file says:

$ file -i *
umlaut-iso88591.txt: text/plain; charset=iso-8859-1
umlaut-mixed.txt:    application/octet-stream; charset=binary
umlaut-utf16.txt:    text/plain; charset=utf-16le
umlaut-utf8.txt:     text/plain; charset=utf-8

without -i:

$ file *
umlaut-iso88591.txt: ISO-8859 text
umlaut-mixed.txt:    data
umlaut-utf16.txt:    Little-endian UTF-16 Unicode text, with no line terminators
umlaut-utf8.txt:     UTF-8 Unicode text

The file command has no idea of "valid" or "invalid". It just sees some bytes and tries to guess what the encoding might be. As humans we might be able to recognize that a file is a text file with some umlauts in a "wrong" encoding. But as a computer it would need some sort of artificial intelligence.

One might argue that the heuristics of file is some sort of artificial intelligence. Yet, even if it is, it is a very limited one.

Here is more information about the file command: http://www.linfo.org/file_command.html

  • Thanks, that worked... I had tried 'file, but without any option :( ... I've now also tried a mixof UTF-16 and UTF-8 and ISO-8859-1. file -i` reported unknown-8bit. So, this also seems to be the answer to: "How to detect an invalid/unknown encoding" – Peter.O Apr 19 '11 at 9:21
20

It isn't always possible to find out for sure what the encoding of a text file is. For example, the byte sequence \303\275 (c3 bd in hexadecimal) could be ý in UTF-8, or ý in latin1, or Ă˝ in latin2, or in BIG-5, and so on.

Some encodings have invalid byte sequences, so it's possible to rule them out for sure. This is true in particular of UTF-8; most texts in most 8-bit encodings are not valid UTF-8. You can test for valid UTF-8 with isutf8 from moreutils or with iconv -f utf-8 -t utf-8 >/dev/null, amongst others.

There are tools that try to guess the encoding of a text file. They can make mistakes, but they often work in practice as long as you don't deliberately try to fool them.

  • file
  • Perl Encode::Guess (part of the standard distribution) tries successive encodings on a byte string and returns the first encoding in which the string is valid text.
  • Enca is an encoding guesser and converter. You can give it a language name and text that you presume is in that language (the supported languages are mostly East European languages), and it tries to guess the encoding.

If there is metadata (HTML/XML charset=, TeX \inputenc, emacs -*-coding-*-, …) in the file, advanced editors like Emacs or Vim are often able to parse that metadata. That's not easy to automate from the command line though.

  • Thanks for the good overview... Yes, "best-guess" can be the only option when the encoding is not known... Using iconv, I just ran all 1168 encodings (including aliases) listed by iconv -l against one of my .htm files... There were 683 encodings which passed muster.. The file's actual charset=ISO-8859-1 ..made up of all bar one ASCII-range values.. The non-ASCII char was \xA9 . – Peter.O Apr 19 '11 at 23:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.