Returning array from a Bash function

Question

I am making a bash script and I have encountered a problem. So let's say I got this

function create_some_array(){
  for i in 0 1 2 3 .. 10
  do
    a[i]=$i
  done
}

create_some_array
echo ${a[*]}

Is there any way I can make this work? I have searched quite a lot and nothing I found worked. I think making the a[] a global variable should work but I can't find something that actually works in my code. Is there any way to return the array from the function to main program?

Thanks in advance

Answer 1

This won't work as expected when there are whitespaces in the arrays:

function create_some_array() {
    local -a a=()
    for i in $(seq $1 $2); do
        a[i]="$i $[$i*$i]"
    done
    echo ${a[@]}
}

and worse: if you try to get array indices from the outside "a", it turns out to be a scalar:

echo ${!a[@]}

even assignment as an array wont help, as possible quoting is naturally removed by the echo line and evaluation order cannot be manipulated to escape quoting: try

function create_some_array() {
...
    echo "${a[@]}"
}

a=($(create_some_array 0 10))
echo ${!a[@]}

Still, printf seems not to help either:

function create_some_array() {
...
    printf " \"%s\"" "${a[@]}"
}

seems to produce correct output on one hand:

$ create_some_array 0 3; echo
 "0 0" "1 1" "2 4" "3 9"

but assignment doesn't work on the other:

$ b=($(create_some_array 0 3))
$ echo ${!b[@]}
0 1 2 3 4 5 6 7

So my last trick was to do assignment as follows:

$ eval b=("$(create_some_array 0 3)")
$ echo -e "${!b[@]}\n${b[3]}"
0 1 2 3
3 9

Tataaa!

P.S.: printf "%q " "${a[@]}" also works fine...

Answer 2

This works fine as described. The most likely reason it doesn't work in your actual code is because you happen to run it in a subshell:

cat textfile | create_some_array
echo ${a[*]}

would not work, because each element in a pipeline runs in a subshell, and

myvalue=$(create_some_array)
echo ${a[*]}

would not work, since command expansion happens in a subshell.

Answer 3

You can make an array local to a function, and then return it:

function create_some_array(){
    local -a a=()
    for i in $(seq $1 $2); do
        a[i]=$i
    done
    echo ${a[@]}
}

declare -a a=()

a=$(create_some_array 0 10)

for i in ${a[@]}; do
   echo "i = " $i
done