How to combine Bash array offset with default value? - bash

Question

What I'm after is to have the most compact expression that expands the special parameter # with an offset of 2 or else to a default value of foobar if the subscript expands to the empty string or null. I tried the following notations but without luck:
"$#:2:-foobar"
"${#:2:-foobar}"
"${#:2: -foobar}"
Is there such a compact notation? Alternatively what would be a similar solution; ideally without temporary variables?

Answer 1

You may combine the expansion for the second parameter or its default value, followed by the expansion from the next offset.
Assuming your array is the program or function's argument array $# then,
#!/bin/bash
echo A "${#:2}"
echo B "${#:2:}" # your attempt #1
echo C "${#:2-foobar}" # your attempt #2
echo D "${#:2: -foobar}" # your attempt #3
echo E "${2:-foobar}"
echo F "$1" "${2:-foobar}" ${#:3}
G=("$1" "${2:-foobar}" ${#:3})
echo G "${G[#]}"
Will yield the desired result for line F and G (G uses a temp variable though).
Ex:
$ bash expand.sh 1
A
B
C
D
E foobar
F 1 foobar
G 1 foobar
$ bash expand.sh 1 2 3 4
A 2 3 4
B
C 2 3 4
D
E 2
F 1 2 3 4
G 1 2 3 4
If you're trying to do this with a different array than "$#", say H=(1 2 3), providing defaults to index expansions ("${H[2]:-foobar}") doesn't seem to work. Your best bet in this case, assuming you don't want to introduce temporary variables is to use a function or eval. But at that point you might be better off just adding a conditional e.g.,
# assuming that H wasn't sparse. redefine H based on its values
H=(
"${H[0]}"
$([[ -n "${H[1]}" ]] && echo "${H[1]}" || echo "foobar")
${H[#]:2}
)
But, readability will suffer.