(or "In dash, ... is not supported." when using dash)

#!/bin/sh
x='test'
echo "${x^^[t]}"

The easiest solution is to switch to a shell that does support name matching prefixes, like bash:

#!/bin/bash
x='test'
echo "${x^^[t]}"

Alternatively, use external tools like tr for case conversion

#!/bin/sh
x='test'
echo "$x" | tr 't' 'T'

Case modification in parameter expansion (e.g., ${x^^}, ${x,,}, ${x^}, ${x,[pattern]}) is a bash and ksh extension. dash and POSIX sh do not support it.

If you only intend to target shells that supports this feature, you can change the shebang to a shell that guarantees support, or ignore this warning.

You can use # shellcheck disable=SC3000-SC4000 to ignore all such compatibility warnings.

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