Modifying the case of alphabetic characters

$ v="hello"
# Just the first character
$ printf '%s\n' "${v^}"
Hello
# All characters
$ printf '%s\n' "${v^^}"
HELLO
# Alternative
$ v="hello world"
$ declare -u string="$v"
$ echo "$string"
HELLO WORLD

$ v="BYE"
# Just the first character
$ printf '%s\n' "${v,}"
bYE
# All characters
$ printf '%s\n' "${v,,}"
bye
# Alternative
$ v="HELLO WORLD"
$ declare -l string="$v"
$ echo "$string"
hello world

$ v="Hello World"
# All chars
$ echo "${v~~}"
hELLO wORLD
$ echo "${v~}"
# Just the first char
hello World