SC2018 – ShellCheck Wiki

Use `[:lower:]` to support accents and foreign alphabets.

Problematic code:

PLATFORM="$(uname -s | tr 'A-Z' 'a-z')"

Correct code:

PLATFORM="$(uname -s | tr '[:upper:]' '[:lower:]')"

Rationale:

A-Z and a-z are commonly intended to mean "all uppercase" and "all lowercase letters" respectively. This ignores accented characters in English, and foreign characters in other languages:

$ tr 'a-z' 'A-Z' <<< "My fiancée ordered a piña colada."
MY FIANCéE ORDERED A PIñA COLADA.

Instead, you can use [:lower:] and [:upper:] to explicitly specify case:

$ tr '[:lower:]' '[:upper:]' <<< "My fiancée ordered a piña colada."
MY FIANCÉE ORDERED A PIÑA COLADA.

Exceptions:

If you don't want a-z to match é or A-Z to match Ñ, you can ignore this message.

As of 2019-09-08, BusyBox tr does not support character classes, so you would have to ignore this message.

Note that the examples used here are multibyte characters in UTF-8. Many implementations (including GNU) fails to deal with them.