Why does the file command fail to recognize non-text files as such?
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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.
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Line as
A sequence of zero or more non- <newline> characters plus a terminating <newline> character.
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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?
2 answers
You might be enlightened by reading the man page for file(1).
A brief quotation:
This manual page documents version 5.35 of the file command. file tests each argument in an attempt to classify it. There are three sets of tests, performed in this order: filesystem tests, magic tests, and language tests. The first test that succeeds causes the file type to be printed. The type printed will usually contain one of the words text (the file contains only printing characters and a few common control characters and is probably safe to read on an ASCII terminal), executable (the file contains the result of compiling a program in a form understandable to some UNIX kernel or another), or data meaning anything else (data is usually “binary” or non-printable).
Then we skip a bit:
If a file does not match any of the entries in the magic file, it is examined to see if it seems to be a text file. ASCII, ISO-8859-x, non-ISO 8-bit extended-ASCII character sets (such as those used on Macintosh and IBM PC systems), UTF-8-encoded Unicode, UTF-16-encoded Unicode, and EBCDIC character sets can be distinguished by the different ranges and sequences of bytes that constitute printable text in each set. If a file passes any of these tests, its character set is reported. ASCII, ISO-8859-x, UTF-8, and extended-ASCII files are identified as “text” because they will be mostly readable on nearly any terminal; UTF-16 and EBCDIC are only “character data” because, while they contain text, it is text that will require translation before it can be read. In addition, file will attempt to determine other characteristics of text-type files. If the lines of a file are terminated by CR, CRLF, or NEL, instead of the Unix-standard LF, this will be reported. Files that contain embedded escape sequences or overstriking will also be identified.
So: you have very small UTF-8 files. file(1) does as specified by its man page, and announces that pretty much all of them are plausibly text.
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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