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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?

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

It isn't always possible to find out for sure what the encoding of a text file is. For example, the byte sequence 303275 (c3 bd in hexadecimal) could be ý in UTF-8, or ý in latin1, or Ă˝ in latin2, or 羸 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.