How to join elements of an array in Bash

Let's imagine we have an array defined in Bash

arr = ( 1 a 2 b c d e 3 )

echo ${arr[@]}
1 a 2 b c d e 3

We would like to join elements of that array using a custom separator e.g. , so it could be returned as a single string.

Here is the function that will do the job.

function join { local IFS="$1"; shift; echo "$*"; }

We can display it directly to STDOUT

join , ${arr[@]}
1,a,2,b,c,d,e,3

or, we can assign the return value to a variable

result=$(join , ${arr[@]})
echo $result