How can I join elements of an array in Bash?

Question

If I have an array like this in Bash:

FOO=( a b c )

How do I join the elements with commas? For example, producing a,b,c.

Answer 1

x=${"${arr[*]}"// /,}

This is the shortest way to do it.

Example,

arr=(1 2 3 4 5)
x=${"${arr[*]}"// /,}
echo $x  # output: 1,2,3,4,5