file 命令显然返回错误的 MIME 类型

Question

为什么以下不返回text/csv？

$ echo 'foo,bar\nbaz,quux' > temp.csv;file -b --mime temp.csv
text/plain; charset=us-ascii

我使用这个例子是为了更加清晰，但我也遇到了其他 CSV 文件的问题。

$ file -b --mime '/Users/jasonswett/projects/client_work/gd/spec/test_files/wtf.csv'
text/plain; charset=us-ascii

为什么它不认为 CSV 是 CSV？我可以对 CSV 做些什么来file返回“正确”的东西吗？

Answer 1

mimetypes 由 unix manpages 称为“magic numbers”的内容决定。在每个文件中都有一个确定文件类型和文件格式的幻数。下面的摘录来自文件命令手册页

The magic number tests are used to check for files with data in partic-
       ular fixed formats.  The canonical example of this  is  a  binary  exe-
       cutable  (compiled  program)  a.out  file,  whose  format is defined in
       a.out.h and possibly exec.h in the standard include  directory.   These
       files  have  a  'magic  number'  stored  in a particular place near the
       beginning of the file that tells the UNIX  operating  system  that  the
       file  is  a binary executable, and which of several types thereof.  The
       concept of 'magic number' has been applied by extension to data  files.
       Any  file  with  some invariant identifier at a small fixed offset into
       the file can usually be described in this way.  The information identi-
       fying   these   files   is   read   from   the   compiled   magic  file
       /usr/share/file/magic.mgc , or  /usr/share/file/magic  if  the  compile
       file  does  not exist. In addition file will look in $HOME/.magic.mgc ,
       or $HOME/.magic for magic entries.

unix 手册页还提到，如果文件与幻数不匹配，则文本文件将被视为 ASCII/ISO-8859-x/非 ISO 8 位扩展 ASCII（最适合的格式）

 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  Macin-
       tosh  and  IBM  PC systems), UTF-8-encoded Unicode, UTF-16-encoded Uni-
       code, 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 iden-
       tified as ''text'' because they will be mostly readable on  nearly  any
       terminal

建议

使用mimetype命令而不是文件命令

mimetype temp.csv

用于进一步挖掘的网络链接

http://unixhelp.ed.ac.uk/CGI/man-cgi?file

Answer 2

不幸的是，您可能无法使文件产生正确的输出。

该file命令根据幻数数据库测试文件的前几个字节。这很容易在二进制文件（如图像或可执行文件）中检查，这些文件在文件开头有一些特定的标识符。

如果文件不是二进制文件，它会检查编码并查找文件中的某些特定单词以确定类型，但仅限于有限数量的文件类型（大多数是编程语言）。