How do I distinguish between 'binary' and 'text' files?

Informally, most of us understand that there are 'binary' files (object files, images, movies, executables, proprietary document formats, etc) and 'text' files (source code, XML files, HTML files, email, etc).

In general, you need to know the contents of a file to be able to do anything useful with it, and form that point of view if the encoding is 'binary' or 'text', it doesn't really matter. And of course files just store bytes of data so they are all 'binary' and 'text' doesn't mean anything without knowing the encoding. And yet, it is still useful to talk about 'binary' and 'text' files, but to avoid offending anyone with this imprecise definition, I will continue to use 'scare' quotes.

However, there are various tools that work on a wide range of files, and in practical terms, you want to do something different based on whether the file is 'text' or 'binary'. An example of this is any tool that outputs data on the console. Plain 'text' will look fine, and is useful. 'binary' data messes up your terminal, and is generally not useful to look at. GNU grep at least uses this distinction when determining if it should output matches to the console.

So, the question is, how do you tell if a file is 'text' or 'binary'? And to restrict is further, how do you tell on a Linux like file-system? I am not aware of any filesystem meta-data that indicates the 'type' of a file, so the question further becomes, by inspecting the content of a file, how do I tell if it is 'text' or 'binary'? And for simplicity, lets restrict 'text' to mean characters which are printable on the user's console. And in particular how would you implement this? (I thought this was implied on this site, but I guess it is helpful, in general, to be pointed at existing code that does this, I should have specified), I'm not really after what existing programs can I use to do this.

You can use the file command. It does a bunch of tests on the file (man file) to decide if it's binary or text. You can look at/borrow its source code if you need to do that from C.

file README
README: ASCII English text, with very long lines

file /bin/bash
/bin/bash: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), for GNU/Linux 2.2.5, dynamically linked (uses shared libs), stripped

You can determine the MIME type of the file with

file --mime FILENAME

The shorthand is file -i on Linux and file -I (capital i) on macOS (see comments).

If it starts with text/, it's text, otherwise binary. The only exception are XML applications. You can match those by looking for +xml at the end of the file type.

The spreadsheet software my company makes reads a number of binary file formats as well as text files.

We first look at the first few bytes for a http://en.wikipedia.org/wiki/Magic_number_(programming)">magic number which we recognize. If we do not recognize the magic number of any of the binary types we read, then we look at up to the first 2K bytes of the file to see whether it appears to be a http://en.wikipedia.org/wiki/UTF-8">UTF-8</a>;, http://en.wikipedia.org/wiki/UTF-8">UTF-16</a> or a text file encoded in the current http://en.wikipedia.org/wiki/Code_page">code page of the host operating system. If it passes none of these tests, we assume that it is not a file we can deal with and throw an appropriate exception.

• To list text file names in current dir/subdirs:

  grep -rIl ''
  

• Binaries:

  grep -rIL ''
  

• To check for a particular file:

  grep -qI '' FILE
  

  then, exit status '0' would mean the file is a text; '1' - binary. To check:

  echo $?
  

Key option is this: > -I Process a binary file as if it did not contain matching data;

Other options: > -r, --recursive > Read all files under each directory, recursively; > -l, --files-with-matches > Suppress normal output; instead print the name of each input file from which output would normally have been printed. > -L, --files-without-match > Suppress normal output; instead print the name of each input file from which no output would normally have been printed. > -q, --quiet, --silent > Quiet; do not write anything to standard output. Exit immediately with zero status if any match is found, even if an error was detected.

Perl has a decent heuristic. Use the -B operator to test for binary (and its opposite, -T to test for text). Here's shell a one-liner to list text files:

$ find . -type f -print0 | perl -0nE 'say if -f and -s _ and -T _'

(Note that those underscores without a preceding dollar are correct (RTFM).)

Well, if you are just inspecting the entire file, see if every character is printable with isprint(c). It gets a little more complicated for Unicode.

To distinguish a unicode text file, MSDN offers some great advice as to what to do.

The gist of it is to first inspect up to the first four bytes:

EF BB BF     UTF-8 
FF FE        UTF-16, little endian 
FE FF        UTF-16, big endian 
FF FE 00 00  UTF-32, little endian 
00 00 FE FF  UTF-32, big-endian 

That will tell you the encoding. Then, you'd want to use iswprint(c) for the rest of the characters in the text file. For UTF-8 and UTF-16, you need to parse the data manually since a single character can be represented by a variable number of bytes. Also, if you're really anal, you'll want to use the locale variant of iswprint if that's available on your platform.

Its an old topic, but maybe someone will find this useful. If you have to decide in a script if something is a file then you can simply do like this :

if file -i $1 | grep -q text;
then 
.
.
fi

This will get the file type, and with a silent grep you can decide if its a text.

You can use libmagic which is a library version of the Unix file command line.

There are wrappers for many languages:

• Python
• .NET
• Nodejs
• Ruby
• Go

Most programs that try to tell the difference use a heuristic, such as examining the first n bytes of the file and seeing if those bytes all qualify as 'text' or not (i.e., do they all fall within the range of printable ASCII charcters). For finer distiction there's always the 'file' command on UNIX-like systems.

One simple check is if it has \0 characters. Text files don't have them.

As previously stated *nix operating systems have this ability within the file command. This command uses a configuration file that defines magic numbers contained within many popular file structures.

This file, called magic was historically stored in /etc, although this may be in /usr/share on some distributions. The magic file defines offsets of values known to exist within the file and can then examine these locations to determine the type of the file.

The structure and description of the magic file can be found by consulting the relevant manual page (man magic)

As for an implementation, well that can be found within file.c itself, however the relevant portion of the file command that determines whether it is readable text or not is the following

/* Make sure we are dealing with ascii text before looking for tokens */
	for (i = 0; i < nbytes - 1; i++) {
		if (!isascii(buf[i]) ||
		    (iscntrl(buf[i]) && !isspace(buf[i]) &&
		     buf[i] != '\b' && buf[i] != '\032' && buf[i] != '\033'
		    )
		   )
			return 0;	/* not all ASCII */
	}