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

Many, if not most, of these solutions rely on arcane syntax, brain-busting regex tricks, or calls to external executables. I would like to propose a simple, bash-only solution that is very easy to understand, and only slightly sub-optimal, performance-wise.

join_by () {
    # Argument #1 is the separator. It can be multi-character.
    # Argument #2, 3, and so on, are the elements to be joined.
    # Usage: join_by ", " "${array[@]}"
    local SEPARATOR="$1"
    shift

    local F=0
    for x in "$@"
    do
        if [[ F -eq 1 ]]
        then
            echo -n "$SEPARATOR"
        else
            F=1
        fi
        echo -n "$x"
    done
    echo
}

Example:

$ a=( 1 "2 2" 3 )
$ join_by ", " "${a[@]}"
1, 2 2, 3
$ 

I'd like to point out that any solution that uses /usr/bin/[ or /usr/bin/printf is inherently slower than my solution, since I use 100% pure bash. As an example of its performance, Here's a demo where I create an array with 1,000,000 random integers, then join them all with a comma, and time it.

$ eval $(echo -n "a=("; x=0 ; while [[ x -lt 1000000 ]]; do echo -n " $RANDOM" ; x=$((x+1)); done; echo " )")
$ time join_by , ${a[@]} >/dev/null
real    0m8.590s
user    0m8.591s
sys     0m0.000s
$