Ternary operator (?:) in Bash
ternary operator ? : is just short form of if/else
case "$b" in 5) a=$c ;; *) a=$d ;;esacOr
[[ $b = 5 ]] && a="$c" || a="$d"
If the condition is merely checking if a variable is set, there's even a shorter form:
a=${VAR:-20}will assign to a the value of VAR if VAR is set, otherwise it will assign it the default value 20 -- this can also be a result of an expression.
This approach is technically called "Parameter Expansion".