Ternary operator (?:) in Bash

ternary operator ? : is just short form of if/else

case "$b" in
 5) a=$c ;;
 *) a=$d ;;
esac

Or

 [[ $b = 5 ]] && a="$c" || a="$d"

Code:

a=$([ "$b" == 5 ] && echo "$c" || echo "$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".

if [ "$b" -eq 5 ]; then a="$c"; else a="$d"; fi

The cond && op1 || op2 expression suggested in other answers has an inherent bug: if op1 has a nonzero exit status, op2 silently becomes the result; the error will also not be caught in -e mode. So, that expression is only safe to use if op1 can never fail (e.g., :, true if a builtin, or variable assignment without any operations that can fail (like division and OS calls)).

Note the "" quotes. The first pair will prevent a syntax error if $b is blank or has whitespace. Others will prevent translation of all whitespace into single spaces.

(( a = b==5 ? c : d )) # string + numeric