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Notes on Unicode in Shell

Philosophy

Oils is UTF-8 centric, unlike bash and other shells.

That is, its Unicode support is like Go, Rust, Julia, and Swift, as opposed to JavaScript, and Python (despite its Python heritage). The former languages use UTF-8, and the latter have the notion of "multibyte characters".

A Mental Model

Program Encoding

Shell programs should be encoded in UTF-8 (or its ASCII subset). Unicode characters can be encoded directly in the source:

echo 'μ'

or denoted in ASCII with C-escaped strings:

echo $'[\u03bc]'

(Such strings are preferred over echo -e because they're statically parsed.)

Data Encoding

Strings in OSH are arbitrary sequences of bytes, which may be valid UTF-8. Details:

• When passed to external programs, strings are truncated at the first NUL ('\0') byte. This is a consequence of how Unix and C work.
• Some operations like length ${#s} and slicing ${s:1:3} require the string to be valid UTF-8. Decoding errors are fatal if shopt -s strict_word_eval is on.

List of Unicode-Aware Operations in OSH / bash

Length / Slicing

• ${#s} -- length in code points (buggy in bash)
  • Note: len(s) counts bytes.
• ${s:1:2} -- offsets in code points

Globs

Globs have character classes [^a] and ?.

This is a glob() call:

echo my?glob

These glob patterns are fnmatch() calls:

case $x in ?) echo 'one char' ;; esac
[[ $x == ? ]]
${s#?}  # remove one character suffix, quadratic loop for globs

This uses our glob to ERE translator for position info:

echo ${s/?/x}

Regexes (ERE)

Regexes have character classes [^a] and ..

• [[ $x =~ $pat ]] where pat='.'

More bash operations

• [[ a < b ]] and [ a '<' b ] for sorting
• ${foo,} and ${foo^} for lowercase / uppercase
• printf '%d' \'c where c is an arbitrary character. This is an obscure syntax for ord(), i.e. getting an integer from an encoded character.

Local-aware operations:

• Prompt string has time, which is locale-specific.
• In bash, printf also has time.

Other:

• The prompt width is calculated with wcswidth(), which doesn't just count code points. It calculates the display width of characters, which is different in general.

YSH-Specific

• Eggex matching depends on ERE semantics.
  • mystr ~ / [ \xff ] /
  • case (x) { / dot / }
• for offset, rune in (mystr) decodes UTF-8, like Go
• Str.{trim,trimLeft,trimRight} respect unicode space, like JavaScript does
• Str.{upper,lower} also need unicode case folding
• split() respects unicode space?

Data Languages

• Decoding JSON/J8 needs to validate UTF-8
• Encoding JSON/J8 needs to decode/validate UTF-8
  • Decoding to print \u{123456} in j"" strings

Tips

• The GNU iconv program converts text from one encoding to another.

Implementation Notes

Unlike bash and CPython, Oils doesn't call setlocale(). (Although GNU readline may call it.)

It's expected that your locale will respect UTF-8. This is true on most distros. If not, then some string operations will support UTF-8 and some won't.

For example:

• String length like ${#s} is implemented in Oils code, not libc, so it will always respect UTF-8.
• [[ s =~ $pat ]] is implemented with libc, so it is affected by the locale settings. Same with Oils (x ~ pat).

TODO: Oils should support LANG=C for some operations, but not LANG=X for other X.