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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, and not like Python or JavaScript. The former languages internally represent strings as UTF-8, while the latter use arrays of code points or UTF-16 code units.

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'   # bash style

echo u'\u{3bc}'  # YSH style

(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 Features That Respect Unicode

OSH / bash

These operations are currently implemented in Python, in osh/string_ops.py:

• ${#s} -- length in code points (buggy in bash)
  • Note: YSH len(s) returns a number of bytes, not code points.
• ${s:1:2} -- index and length are a number of code points
• ${x#glob?} and ${x##glob?} (see below)

More:

• ${foo,} and ${foo^} for lowercase / uppercase
• [[ a < b ]] and [ a '<' b ] for sorting
  • these can use libc strcoll()?
• 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.

Globs

Globs have character classes [^a] and ?.

This pattern results in a glob() call:

echo my?glob

These patterns result in 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 .:

pat='.'  # single "character"
[[ $x =~ $pat ]]

Locale-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

• Eggex matching depends on ERE semantics.
  • mystr ~ / [ \xff ] /
  • case (x) { / dot / }
• for offset, rune in (runes(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?

Not unicode aware:

• strcmp() does byte-wise and UTF-8 wise comparisons?

Data Languages

• Decoding JSON/J8 validates UTF-8
• Encoding JSON/J8 decodes and validates UTF-8
  • So we can distinguish valid UTF-8 and invalid bytes like \yff

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.

List of Low-Level UTF-8 Operations

libc:

• glob() and fnmatch()
• regexec()
• strcoll() respects LC_COLLATE, which bash probably does

Our own:

• Decode next rune from a position, or previous rune
  • trimLeft() and ${s#prefix} need this
• Decode UTF-8
  • J8 encoding and decoding need this
  • for r in (runes(x)) needs this
  • respecting surrogate half
    • JSON needs this
• Encode integer rune to UTF-8 sequence
  • J8 needs this, for \u{3bc} (currently in data_lang/j8.py Utf8Encode())

Not sure:

• Case folding
  • both OSH and YSH have uppercase and lowercase

Tips

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