Braces are required for positionals over 9 SH-1037

For legacy reasons, $10 is interpreted as the variable $1 followed by the literal string 0. Curly braces are needed to tell the shell that both digits are part of the parameter expansion.

Problematic code:

echo "Ninth parameter: $9"
echo "Tenth parameter: $10"

Correct code:

echo "Ninth parameter: $9"
echo "Tenth parameter: ${10}"

Exceptions:

If you wanted the trailing digits to be literal, ${1}0 will make this clear to both humans and shellcheck. In dash, $10 is (wrongly) interpreted as ${10}, so some 'reversed' care should also be taken:

bash -c 'set a b c d e f g h i j; echo $10 ${1}0' # POSIX: a0 a0
dash -c 'set a b c d e f g h i j; echo $10 ${1}0' # WRONG: j a0