unicode - What's the difference between UTF-8 and UTF-8 without BOM?

It'a an old question with many good answers but one thing should be added.

All answers are very general. What I'd like to add are examples of the BOM usage that actually cause real problems and yet many people don't know about it.

BOM breaks scripts

Shell scripts, Perl scripts, Python scripts, Ruby scripts, Node.js scripts or any other executable that needs to be run by an interpreter - all start with a shebang line which looks like one of those:

#!/bin/sh
#!/usr/bin/python
#!/usr/local/bin/perl
#!/usr/bin/env node

It tells the system which interpreter needs to be run when invoking such a script. If the script is encoded in UTF-8, one may be tempted to include a BOM at the beginning. But actually the "#!" characters are not just characters. They are in fact a magic number that happens to be composed out of two ASCII characters. If you put something (like a BOM) before those characters, then the file will look like it had a different magic number and that can lead to problems.

See Wikipedia, article: Shebang, section: Magic number:

The shebang characters are represented by the same two bytes in
extended ASCII encodings, including UTF-8, which is commonly used for
scripts and other text files on current Unix-like systems. However,
UTF-8 files may begin with the optional byte order mark (BOM); if the
"exec" function specifically detects the bytes 0x23 and 0x21, then the
presence of the BOM (0xEF 0xBB 0xBF) before the shebang will prevent
the script interpreter from being executed. Some authorities recommend
against using the byte order mark in POSIX (Unix-like) scripts,[14]
for this reason and for wider interoperability and philosophical

concerns. Additionally, a byte order mark is not necessary in UTF-8,
as that encoding does not have endianness issues; it serves only to
identify the encoding as UTF-8. [emphasis added]

BOM is illegal in JSON

See RFC 7159, Section 8.1:

Implementations MUST NOT add a byte order mark to the beginning of a JSON text.

BOM is redundant in JSON

Not only it is illegal in JSON, it is also not needed to determine the character encoding because there are more reliable ways to unambiguously determine both the character encoding and endianness used in any JSON stream (see this answer for details).

BOM breaks JSON parsers

Not only it is illegal in JSON and not needed, it actually breaks all software that determine the encoding using the method presented in RFC 4627:

Determining the encoding and endianness of JSON, examining the first 4 bytes for the NUL byte:

00 00 00 xx - UTF-32BE
00 xx 00 xx - UTF-16BE
xx 00 00 00 - UTF-32LE
xx 00 xx 00 - UTF-16LE
xx xx xx xx - UTF-8

Now, if the file starts with BOM it will look like this:

00 00 FE FF - UTF-32BE
FE FF 00 xx - UTF-16BE
FF FE 00 00 - UTF-32LE
FF FE xx 00 - UTF-16LE
EF BB BF xx - UTF-8

Note that:

1. UTF-32BE doesn't start with three NULs so it won't be recognized


2. UTF-32LE the first byte is not followed by 3 NULs so it won't be recognized


3. UTF-16BE has only 1 NUL in the first 4 bytes so it won't be recognized


4. UTF-16LE has only 1 NUL in the first 4 bytes so it won't be recognized

Depending on the implementation, all of those may be interpreted incorrectly as UTF-8 and then misinterpreted or rejected as invalid UTF-8, or not recognized at all.

Additionally if the implementation tests for valid JSON as I recommend, it will reject even the input that is indeed encoded as UTF-8 because it doesn't start with an ASCII character < 128 as it should according to the RFC.

Other data formats

BOM in JSON is not needed, is illegal and breaks software that works correctly according to the RFC. It should be a nobrainer to just not use it then and yet, there are always people who insist on breaking JSON by using BOMs, comments, different quoting rules or different data types. Of course anyone is free to use things like BOMs or anything else if you need it - just don't call it JSON then.

For other data formats than JSON, take a look how it really looks like. If the only encodings are UTF-* and the first character must be an ASCII character lower than 128 then you already have all the information needed to determine both the encoding and the endianness of your data. Adding BOMs even as an optional feature would only make it more complicated and error prone.

Other uses of BOM

As for the uses outside of JSON or scripts, I think there are already very good answers here. I wanted to add more detailed info specifically about scripting and serialization because it is an example of BOM characters causing real problems.