What does "LC_ALL=C" do?

LC_ALL is the environment variable that overrides all the other localisation settings (except $LANGUAGE under some circumstances).

Different aspects of localisations (like the thousand separator or decimal point character, character set, sorting order, month, day names, language or application messages like error messages, currency symbol) can be set using a few environment variables.

You'll typically set $LANG to your preference with a value that identifies your region (like fr_CH.UTF-8 if you're in French speaking Switzerland, using UTF-8). The individual LC_xxx variables override a certain aspect. LC_ALL overrides them all. The locale command, when called without argument gives a summary of the current settings.

For instance, on a GNU system, I get:

$ locale
LANG=en_GB.UTF-8
LANGUAGE=
LC_CTYPE="en_GB.UTF-8"
LC_NUMERIC="en_GB.UTF-8"
LC_TIME="en_GB.UTF-8"
LC_COLLATE="en_GB.UTF-8"
LC_MONETARY="en_GB.UTF-8"
LC_MESSAGES="en_GB.UTF-8"
LC_PAPER="en_GB.UTF-8"
LC_NAME="en_GB.UTF-8"
LC_ADDRESS="en_GB.UTF-8"
LC_TELEPHONE="en_GB.UTF-8"
LC_MEASUREMENT="en_GB.UTF-8"
LC_IDENTIFICATION="en_GB.UTF-8"
LC_ALL=

I can override an individual setting with for instance:

$ LC_TIME=fr_FR.UTF-8 date
jeudi 22 août 2013, 10:41:30 (UTC+0100)

Or:

$ LC_MONETARY=fr_FR.UTF-8 locale currency_symbol
€

Or override everything with LC_ALL.

$ LC_ALL=C LANG=fr_FR.UTF-8 LC_MESSAGES=fr_FR.UTF-8 cat /
cat: /: Is a directory

In a script, if you want to force a specific setting, as you don't know what settings the user has forced (possibly LC_ALL as well), your best, safest and generally only option is to force LC_ALL.

The C locale is a special locale that is meant to be the simplest locale. You could also say that while the other locales are for humans, the C locale is for computers. In the C locale, characters are single bytes, the charset is ASCII (well, is not required to, but in practice will be in the systems most of us will ever get to use), the sorting order is based on the byte values¹, the language is usually US English (though for application messages (as opposed to things like month or day names or messages by system libraries), it's at the discretion of the application author) and things like currency symbols are not defined.

On some systems, there's a difference with the POSIX locale where for instance the sort order for non-ASCII characters is not defined.

You generally run a command with LC_ALL=C to avoid the user's settings to interfere with your script. For instance, if you want [a-z] to match the 26 ASCII characters from a to z, you have to set LC_ALL=C.

On GNU systems, LC_ALL=C and LC_ALL=POSIX (or LC_MESSAGES=C|POSIX) override $LANGUAGE, while LC_ALL=anything-else wouldn't.

A few cases where you typically need to set LC_ALL=C:

• sort -u or sort ... | uniq.... In many locales other than C, on some systems (notably GNU ones), some characters have the same sorting order. sort -u doesn't report unique lines, but one of each group of lines that have equal sorting order. So if you do want unique lines, you need a locale where characters are byte and all characters have different sorting order (which the C locale guarantees).

• the same applies to the = operator of POSIX compliant expr or == operator of POSIX compliant awks (mawk and gawk are not POSIX in that regard), that don't check whether two strings are identical but whether they sort the same.

• Character ranges like in grep. If you mean to match a letter in the user's language, use grep '[[:alpha:]]' and don't modify LC_ALL. But if you want to match the a-zA-Z ASCII characters, you need either LC_ALL=C grep '[[:alpha:]]' or LC_ALL=C grep '[a-zA-Z]'². [a-z] matches the characters that sort after a and before z (though with many APIs it's more complicated than that). In other locales, you generally don't know what those are. For instance some locales ignore case for sorting so [a-z] in some APIs like bash patterns, could include [B-Z] or [A-Y]. In many UTF-8 locales (including en_US.UTF-8 on most systems), [a-z] will include the latin letters from a to y with diacritics but not those of z (since z sorts before them) which I can't imagine would be what you want (why would you want to include é and not ź?).

• floating point arithmetic in ksh93. ksh93 honours the decimal_point setting in LC_NUMERIC. If you write a script that contains a=$((1.2/7)), it will stop working when run by a user whose locale has comma as the decimal separator:

   $ ksh93 -c 'echo $((1.1/2))'
   0.55
   $ LANG=fr_FR.UTF-8  ksh93 -c 'echo $((1.1/2))'
   ksh93: 1.1/2: arithmetic syntax error
  

Then you need things like:

    #! /bin/ksh93 -
    float input="$1" # get it as input from the user in his locale
    float output
    arith() { typeset LC_ALL=C; (($@)); }
    arith output=input/1.2 # use the dot here as it will be interpreted
                           # under LC_ALL=C
    echo "$output" # output in the user's locale

As a side note: the , decimal separator conflicts with the , arithmetic operator which can cause even more confusion.

• When you need characters to be bytes. Nowadays, most locales are UTF-8 based which means characters can take up from 1 to 6 bytes³. When dealing with data that is meant to be bytes, with text utilities, you'll want to set LC_ALL=C. It will also improve performance significantly because parsing UTF-8 data has a cost.

• a corollary of the previous point: when processing text where you don't know what character set the input is written in, but can assume it's compatible with ASCII (as virtually all charsets are). For instance grep '<.*>' to look for lines containing a <, > pair will no work if you're in a UTF-8 locale and the input is encoded in a single-byte 8-bit character set like iso8859-15. That's because . only matches characters and non-ASCII characters in iso8859-15 are likely not to form a valid character in UTF-8. On the other hand, LC_ALL=C grep '<.*>' will work because any byte value forms a valid character in the C locale.

• Any time where you process input data or output data that is not intended from/for a human. If you're talking to a user, you may want to use their convention and language, but for instance, if you generate some numbers to feed some other application that expects English style decimal points, or English month names, you'll want to set LC_ALL=C:

   $ printf '%g\n' 1e-2
   0,01
   $ LC_ALL=C printf '%g\n' 1e-2
   0.01
   $ date +%b
   août
   $ LC_ALL=C date +%b
   Aug
  

That also applies to things like case insensitive comparison (like in grep -i) and case conversion (awk's toupper(), dd conv=ucase...). For instance:

    grep -i i

is not guaranteed to match on I in the user's locale. In some Turkish locales for instance, it doesn't as upper-case i is İ (note the dot) there and lower-case I is ı (note the missing dot).

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Notes

¹ again, only on ASCII based systems (the immense majority of systems). POSIX requires the collation order for the C locale to be that of the order of characters in the ASCII charset, even on EBCDIC systems which are not allowed to do the strcoll() === strcmp() optimisation in the C locale.

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² Depending on the encoding of the text, that's not necessarily the right thing to do though. That's valid for UTF-8 or single-byte character sets (like iso-8859-1), but not necessarily non-UTF-8 multibyte character sets.

For instance, if you're in a zh_HK.big5hkscs locale (Hong Kong, using the Hong Kong variant of the BIG5 Chinese character encoding), and you want to look for English letters in a file encoded in that charsets, doing either:

LC_ALL=C grep '[[:alpha:]]'

or

LC_ALL=C grep '[a-zA-Z]'

would be wrong, because in that charset (and many others, but hardly used since UTF-8 came out), a lot of characters contain bytes that correspond to the ASCII encoding of A-Za-z characters. For instance, all of A䨝䰲丕乙乜你再劀劈呸哻唥唧噀噦嚳坽 (and many more) contain the encoding of A. 䨝 is 0x96 0x41, and A is 0x41 like in ASCII. So our LC_ALL=C grep '[a-zA-Z]' would match on those lines that contain those characters as it would misinterpret those sequences of bytes.

LC_COLLATE=C grep '[A-Za-z]'

would work, but only if LC_ALL is not otherwise set (which would override LC_COLLATE). So you may end up having to do:

grep '[ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz]'

if you wanted to look for English letters in a file encoded in the locale's encoding.

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³ some would argue it's rather 1 to 4 bytes these days now that Unicode code points (and the libraries that encode/decode UTF-8 data) have been arbitrarily restricted to code points U+0000 to U+10FFFF (0xD800 to 0xDFFF excluded) down from U+7FFFFFFF to accommodate the UTF-16 encoding, but some applications will still happily encode/decode 6-byte UTF-8 sequences (including the ones that fall in the 0xD800 .. 0xDFFF range).

It forces applications to use the default language for output:

$ LC_ALL=es_ES man
¿Qué página de manual desea?

$ LC_ALL=C man
What manual page do you want?

and forces sorting to be byte-wise:

$ LC_ALL=en_US sort <<< $'a\nb\nA\nB'
a
A
b
B

$ LC_ALL=C sort <<< $'a\nb\nA\nB'
A
B
a
b

C is the default locale,"POSIX" is the alias of "C". I guess "C" is derived from ANSI-C. Maybe ANSI-C define the "POSIX" locale.