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Comments on Why does the file command fail to recognize non-text files as such?

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Why does the file command fail to recognize non-text files as such?

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POSIX defines

  • Text file as

    A file that contains characters organized into zero or more lines. The lines do not contain NUL characters and none can exceed {LINE_MAX} bytes in length, including the <newline> character.

  • Line as

    A sequence of zero or more non- <newline> characters plus a terminating <newline> character.

  • Character as

    A sequence of one or more bytes representing a single graphic symbol or control code.

Consider then six files, each with two bytes, created with these Printf commands (using octals):

printf "\101\012" > file1 #A<newline>
printf "\010\012" > file2 #<backspace><newline>
printf "\101\101" > file3 #AA
printf "\200\012" > file4
printf "\200\200" > file5
printf "\000\012" > file6 #<null><newline>

Now, in the UTF-8 encoding, the octal 012 (0x0A) is the newline character, 101 (0x41) is the graphic symbol A, 010 (0x08) is the backspace control character and 200 (0x80) is a continuation byte that never occurs as the first byte of a multi-byte sequence, so it does not form a valid character.

Hence, I would regard files 1 and 2 as text files, but the remaining as non-text files, because file 3 is not newline terminated, files 4 and 5 have an invalid character and file 6 contains a null byte.

However, the file command does not seem to completely agree with me; it lists files 3, 4 and 5 as text files,

$ file --mime-type file*
file1: text/plain
file2: text/plain
file3: text/plain
file4: text/plain
file5: text/plain
file6: application/octet-stream

Why does the file command fail to identify files 3, 4 and 5 as non-text files (I'm assuming it can't possibly be a bug) even though I use en_US.UTF-8 as my locale, or else what did I incorrectly understand?

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General comments (6 comments)
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https://theasciicode.com.ar/extended-ascii-code/majuscule-c-cedilla-uppercase-ascii-code-128.html

Octal 101 is 65, which is ASCII/UTF-8 for A - a valid character.

Octal 200 is 128, which is ASCII/UTF-8 or whatever for Ç - which is also a valid character.

A file that contains characters organized into zero or more lines.

Lines are newline characters, which are octal 012. The above rule means newlines can appear, but don't have to.

Really, as long as the file contains no zero bytes (which are NUL), it's a text file.

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1 comment thread

If it contains a non-line, it is not a text file. See for example a more extended answer in [What con... (3 comments)
If it contains a non-line, it is not a text file. See for example a more extended answer in [What con...
Quasímodo‭ wrote almost 3 years ago

If it contains a non-line, it is not a text file. See for example a more extended answer in What conditions must be met for a file to be a text file as defined by POSIX?. Also ASCII does not encode octal 200, it's a 7-bit encoding.

DavidCary‭ wrote over 2 years ago

LawrenceC‭ LawrenceC‭, You're probably thinking of the common "code page 850", where 0x80 represents a Capital C with cedilla. UTF-8 encodes the Capital C with cedilla as two bytes 0xC3 0x87, equivalent to Unicode U+00C7.

DavidCary‭ wrote over 2 years ago

or possibly "code page 437", used by the original IBM PC and ANSI art, where 0x80 represents a Capital C with cedilla. There are many extended ASCII character encodings.