How the various bash operators work

bash-var-operators.sh

#!/bin/bash

cat <<!
+--------------+-----------------+-----------------+-----------------+
|              |       var       |     var         |       var       |
|   Statement  | SetNotNull      |   SetButNull    |      Unset      |
+--------------+-----------------+-----------------+-----------------+
| \${var:-word} | substitute var  | substitute word | substitute word |
| \${var-word}  | substitute var  | substitute null | substitute word |
| \${var:=word} | substitute var  | assign word     | assign word     |
| \${var=word}  | substitute var  | substitute null | assign word     |
| \${var:+word} | substitute word | substitute null | substitute null |
| \${var+word}  | substitute word | substitute word | substitute null |
| \${var:?word} | substitute var  | error, exit     | error, exit     |
| \${var?word}  | substitute var  | substitute null | error, exit     |
+--------------+-----------------+-----------------+-----------------+
!

set +u

{
    printf '@ Statement @ SetNotNull/Assigned @ SetButNull/Assigned @ Unset/Assigned @\n'
    for op in ':-' '-' ':=' '=' ':+' '+' ':?' '?'; do

        stmt="\${var${op}word}"
        printf "@ %s" "${stmt}"

        var='data'
        printf ' @ '
        ( eval printf -- "%s/" "${stmt}" 2>/dev/null; printf "%s" ${var}) || printf "ERR"

        var=''
        printf ' @ '
        ( eval printf -- "%s/" "${stmt}" 2>/dev/null; printf "%s" ${var}) || printf "ERR"

        unset var
        printf ' @ '
        ( eval printf -- "%s/" "${stmt}" 2>/dev/null; printf "%s" ${var}) || printf "ERR"

        echo ' @'
    done
} | column -n -s@ -t