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