Passing an array name to a function in bash

Question

I have a script that has a couple of arrays in it. I didn't want to repeat counting the array size so I tried to create a function that does it for me. I'm passing the function the name of the array whose size I want. It runs through the function until it finds an empty space and then returns the count. This is my function:

array_size ()
{
    # Calculate the size of your array
    count=0
    while [ ${$1[$count]} != "" ]
    do
         count=$(( $count + 1 ))
    done

    #Subtracts 1 from count to reset array position
    count=$(( $count - 1 ))

    return $count

}

I get the following error when I call this: ${$1[$count]}: bad substitution. How can I pass the array name to this function?

Why are you not using the capabilities built into bash (i.e. ${#array[@]})? — Lewis M
– Lewis M, Commented Sep 24, 2018 at 15:00
Additionally, the array ('' 1 2 3) would be considered to have length zero by your function. — Benjamin W.
– Benjamin W., Commented Sep 24, 2018 at 15:11
It also wouldn't be able to deal with arrays with non-contiguous indices: arr[1]=x has no index 0. — Benjamin W.
– Benjamin W., Commented Sep 24, 2018 at 15:13

glenn jackman · Accepted Answer · 2018-09-24 15:35:49Z

(Note, in these examples, I haven't implemented your counting logic.)

With a recent bash (starting from version 4.3 I think) you can use a nameref:

$ declare -a main
$ main[4]=a main[6]=b main[11]=c
$ declare -p main
declare -a main='([4]="a" [6]="b" [11]="c")'
$ aryfunc () { 
    local -n ary=$1
    for key in "${!ary[@]}"; do
        printf "%d => %s\n" "$key" "${ary[$key]}"
    done
}
$ aryfunc main
4 => a
6 => b
11 => c

Otherwise, you can use indirect variables, but you lose the indices:

$ aryfunc2() {
    local tmp="${1}[@]"
    local -a localary=( "${!tmp}" )
    for key in "${!localary[@]}"; do
        printf "%d => %s\n" "$key" "${localary[$key]}"
    done
}
$ aryfunc2 main
0 => a
1 => b
2 => c

Or if you want to get really gross, pass the definition of the array:

$ aryfunc3() {
    local declaration="$1"
    eval local -a localary="${declaration#*=}"
    for key in "${!localary[@]}"; do
         printf "%d => %s\n" "$key" "${localary[$key]}"
    done
}
$ aryfunc3 "$(declare -p main)"
4 => a
6 => b
11 => c

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