RE error: illegal byte sequence on Mac OS X

^{A sample command that exhibits the symptom: sed 's/./@/' <<<$'\xfc' fails, because byte 0xfc is not a valid UTF-8 char.
Note that, by contrast, GNU sed (Linux, but also installable on macOS) simply passes the invalid byte through, without reporting an error.}

Using the formerly accepted answer is an option if you don't mind losing support for your true locale (if you're on a US system and you never need to deal with foreign characters, that may be fine.)

However, the same effect can be had ad-hoc for a single command only:

LC_ALL=C sed -i "" 's|"iphoneos-cross","llvm-gcc:-O3|"iphoneos-cross","clang:-Os|g' Configure

Note: What matters is an effective LC_CTYPE setting of C, so LC_CTYPE=C sed ... would normally also work, but if LC_ALL happens to be set (to something other than C), it will override individual LC_*-category variables such as LC_CTYPE. Thus, the most robust approach is to set LC_ALL.

However, (effectively) setting LC_CTYPE to C treats strings as if each byte were its own character (no interpretation based on encoding rules is performed), with no regard for the - multibyte-on-demand - UTF-8 encoding that OS X employs by default, where foreign characters have multibyte encodings.

In a nutshell: setting LC_CTYPE to C causes the shell and utilities to only recognize basic English letters as letters (the ones in the 7-bit ASCII range), so that foreign chars. will not be treated as letters, causing, for instance, upper-/lowercase conversions to fail.

Again, this may be fine if you needn't match multibyte-encoded characters such as é, and simply want to pass such characters through.

If this is insufficient and/or you want to understand the cause of the original error (including determining what input bytes caused the problem) and perform encoding conversions on demand, read on below.

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The problem is that the input file's encoding does not match the shell's.
More specifically, the input file contains characters encoded in a way that is not valid in UTF-8 (as @Klas Lindbäck stated in a comment) - that's what the sed error message is trying to say by invalid byte sequence.

Most likely, your input file uses a single-byte 8-bit encoding such as ISO-8859-1, frequently used to encode "Western European" languages.

Example:

The accented letter à has Unicode codepoint 0xE0 (224) - the same as in ISO-8859-1. However, due to the nature of UTF-8 encoding, this single codepoint is represented as 2 bytes - 0xC3 0xA0, whereas trying to pass the single byte 0xE0 is invalid under UTF-8.

Here's a demonstration of the problem using the string voilà encoded as ISO-8859-1, with the à represented as one byte (via an ANSI-C-quoted bash string ($'...') that uses \x{e0} to create the byte):

Note that the sed command is effectively a no-op that simply passes the input through, but we need it to provoke the error:

  # -> 'illegal byte sequence': byte 0xE0 is not a valid char.
sed 's/.*/&/' <<<$'voil\x{e0}'

To simply ignore the problem, the above LCTYPE=C approach can be used:

  # No error, bytes are passed through ('á' will render as '?', though).
LC_CTYPE=C sed 's/.*/&/' <<<$'voil\x{e0}'

If you want to determine which parts of the input cause the problem, try the following:

  # Convert bytes in the 8-bit range (high bit set) to hex. representation.
  # -> 'voil\x{e0}'
iconv -f ASCII --byte-subst='\x{%02x}' <<<$'voil\x{e0}'

The output will show you all bytes that have the high bit set (bytes that exceed the 7-bit ASCII range) in hexadecimal form. (Note, however, that that also includes correctly encoded UTF-8 multibyte sequences - a more sophisticated approach would be needed to specifically identify invalid-in-UTF-8 bytes.)

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Performing encoding conversions on demand:

Standard utility iconv can be used to convert to (-t) and/or from (-f) encodings; iconv -l lists all supported ones.

Examples:

Convert FROM ISO-8859-1 to the encoding in effect in the shell (based on LC_CTYPE, which is UTF-8-based by default), building on the above example:

  # Converts to UTF-8; output renders correctly as 'voilà'
sed 's/.*/&/' <<<"$(iconv -f ISO-8859-1 <<<$'voil\x{e0}')"

Note that this conversion allows you to properly match foreign characters:

  # Correctly matches 'à' and replaces it with 'ü': -> 'voilü'
sed 's/à/ü/' <<<"$(iconv -f ISO-8859-1 <<<$'voil\x{e0}')"

To convert the input BACK to ISO-8859-1 after processing, simply pipe the result to another iconv command:

sed 's/à/ü/' <<<"$(iconv -f ISO-8859-1 <<<$'voil\x{e0}')" | iconv -t ISO-8859-1

Add the following lines to your ~/.bash_profile or ~/.zshrc file(s).

export LC_CTYPE=C 
export LANG=C

My workaround had been using Perl:

find . -type f -print0 | xargs -0 perl -pi -e 's/was/now/g'