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Notes on Unicode in Shell

Philosophy

Oil's unicode support is unlike that of other shells because it's UTF-8-centric.

In other words, it's like newer languages like Go, Rust, Julia, and Swift, as opposed , JavaScript, and Python (despite its Python heritage). The latter languages use the notion of "multibyte characters".

In particular, Oil doesn't have global variables like LANG for libc or a notion of "default encoding". In my experience, these types of globals cause correctness problems.

List of Unicode-Aware Operations in Shell

• ${#s} -- length in code points
• ${s:1:2} -- offsets in code points
• ${x#?} and family (not yet implemented)

Where bash respects it:

• [[ a < b ]] and [ a '<' b ] for sorting
• ${foo,} and ${foo^} for lowercase / uppercase

This is a list of operations that SHOULD be aware of Unicode characters. OSH doesn't implement all of them yet, e.g. the globbing stuff.

• Length operator counts code points: ${#s}
  • TODO: provide an option to count bytes.
• String slicing counts code points: ${s:0:1}
• Any operation that uses glob, because it has ? for a single character, character classes like [[:alpha:]], etc.
  • case $x in ?) echo 'one char' ;; esac
  • [[ $x == ? ]]
  • ${s#?} (remove one character)
  • ${s/?/x} (note: this uses our glob to ERE translator for position)
• 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.

List of operations that depend on the locale (not implemented):

• String ordering: [[ $a < $b ]] -- should use current locale? TODO: compare with sort command.
• Lowercase and uppercase operators: ${s^} and ${s,}
• 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.