Why doesn't the following return text/csv?

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

I used this example for extra clarity but I'm also experiencing the problem with other CSV files.

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

Why doesn't it think the CSV is a CSV? Is there anything I can do to the CSV to make file return the "right" thing?

2 Answers 2


The mimetypes are determined by what the unix manpages called 'magic numbers'. In every file there is a magic number that determine the file type and file format. The extract below is from the file command man pages

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.

The unix man pages also mentioned that if the file does not match a magic number, the text file is considered ASCII/ISO-8859-x/non-ISO 8-bit extended-ASCII (best suited format)

 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


use the mimetype command instead of the file command

mimetype temp.csv

web link for further digging

  • I can’t find a command called ”mimetype”. Could you link to it?
    – Smar
    Commented Nov 13, 2018 at 9:11

Unfortunately, there is probably nothing you can do to make file produce the correct output.

The file command tests the first few bytes of a file against a database of magic numbers. That is easy to check for in binary files (like images or executables) which have some specific identifiers at the beginning of the file.

If the file is not a binary file, it will check the encoding as well as look for some specific words in the file to determine the type, but only for a limited number of file types (most of which are programming languages).

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