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Newtron Malayalam
Asked: 2020-04-09 08:39:17 +0800 CST

How "file --mime-type path" works for a file wich has no extension

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To find a files mime type which we don't know we will use file command with --mime-type argument. We know it is showing results based on given file paths extension(such like text/x-python for *.py). The list about the relation between extension and mime type would be saved somewhere such as /etc/mime.type or in /usr/share/mime/glob. But what about files which doesn't have any extensions? How could we find mime type? Explain, please. Can we do the same thing using python?

command-line python files mime-type

1 Answers

  1. Best Answer

    Extensions are mostly irrelevant in Linux systems. Unlike Windows, the vast majority of Linux tools don't rely on the extension at all. For example:

    $ cat script.py
#!/usr/bin/python
print("Works!")
$ chmod 777 script.py 
$ ./script.py 
Works!

    Now, with a different extension:

    $ mv script.py script.foo
$ ./script.foo 
Works!

    So, forget about extensions. They're very rarely relevant and file doesn't use them. Have a look at man file (please read man file, this is only part of the information there):

    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 filesystem tests are based on examining the return from a stat(2) system call. The program checks to see if the file is empty, or if it's some sort of special file. Any known file types appropriate to the system you are running on (sockets, symbolic links, or named pipes (FIFOs) on those systems that implement them) are intuited if they are defined in the system header file .

    The magic tests are used to check for files with data in particular fixed formats. The canonical example of this is a binary executable (compiled program) a.out file, whose format is defined in , and possibly in the standard include directory. These files have a “magic number” stored in a particular place near the beginning of the

    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.

    Once file has determined the character set used in a text-type file, it will attempt to determine in what language the file is written. The lan‐ guage tests look for particular strings (cf. ) that can appear anywhere in the first few blocks of a file. For example, the keyword .br indicates that the file is most likely a troff(1) input file, just as the keyword struct indicates a C program. These tests are less reliable than the previous two groups, so they are performed last. The language test routines also test for some miscellany (such as tar(1) archives, JSON files).

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