Bash has the ability to also flag use of an undefined variable an error, it is just not on by default. set -u Man page quote: "Treat unset variables and param

Bash has the ability to also flag use of an undefined variable an error, it is just not on by default. set -u Man page quote: "Treat unset variables and parameters other than the special parameters "@" and "*" as an error when performing parameter expansion. If expansion is attempted on an unset variable or parameter, the shell prints an error message, and, if not interactive, exits with a non-zero status."