Passing multiple distinct arrays to a shell function

Question

Because shells other than ksh do not support pass-by-reference, how can multiple arrays be passed into a function in bash without using global variables, and in a way which allows any legal variable content to be included as an array element (no reserved sigils)?

Answer 1

Since bash 4.3

As of 2016, modern bash supports pass-by-reference (a.k.a nameref attribute) as:

demo_multiple_arrays() {
  local -n _array_one=$1
  local -n _array_two=$2
  printf '1: %q\n' "${_array_one[@]}"
  printf '2: %q\n' "${_array_two[@]}"
}

array_one=( "one argument" "another argument" )
array_two=( "array two part one" "array two part two" )

demo_multiple_arrays array_one array_two

See also declare -n in the man page.

---

Before bash 4.3

This can be done safely by using a calling convention which puts number-of-arguments before each array, as such:

demo_multiple_arrays() {
  declare -i num_args array_num;
  declare -a curr_args;
  while (( $# )) ; do
    curr_args=( )
    num_args=$1; shift
    while (( num_args-- > 0 )) ; do
      curr_args+=( "$1" ); shift
    done
    printf "$((++array_num)): %q\n" "${curr_args[@]}"
  done
}

This can then be called as follows:

array_one=( "one argument" "another argument" )
array_two=( "array two part one" "array two part two" )
demo_multiple_arrays \
  "${#array_one[@]}" "${array_one[@]}" \
  "${#array_two[@]}" "${array_two[@]}"

Answer 2

I found a concise way to pass multiple arrays to a function. It isn't a perfect solution (because it doesn't work if any of the elements in your array contains a space), but I thought it might be helpful to share for anyone still using a Bash version older than 4.3 like me.

Declaring function:

myFunction()
{
  IFS=' ' read -ra array1 <<< "${1}"
  IFS=' ' read -ra array2 <<< "${2}"
  
  echo "first array (length=${#array1[@]}): ${array1[@]}"
  echo "second array (length=${#array2[@]}): ${array2[@]}"
}

Passing two distinct arrays to function (as space-separated strings):

declare -a firstArray=(1 2 3 4 5)
declare -a secondArray=(one two three)

myFunction "${firstArray[*]}" "${secondArray[*]}"

Output:

first array (length=5): 1 2 3 4 5
second array (length=3): one two three

Although this solution isn't perfect, it made it easier for me to pass additional non-array arguments to my function. (I found that using nested while-loops to sort out multiple arrays in addition to other non-array arguments got messy really fast.)

Answer 3

Can also be done using eval:

declare -a a=( "aa bb" 123 '$ $ $' )
declare -a b=( "bb cc" 456 '###' )

printf "\n%s\n" 'a before sub:'
printf "'%s'\n" "${a[@]}"
printf "\n%s\n" 'b after sub:'
printf "'%s'\n" "${b[@]}"


sub ()
{
  eval a0=\${$1[0]}                     # get value a[0]
  eval b1=\${$2[1]}                     # get value b[1]
  echo "a[0] = '$a0'"
  echo "b[1] = '$b1'"

  eval $1[0]='a---a'                    # set value a[0]
  eval $2[1]=999                        # set value b[1]

} # ----------  end of function sub  ----------

sub a b     # call function sub

printf "\n%s\n" 'a after sub:'
printf "'%s'\n" "${a[@]}"
printf "\n%s\n" 'b after sub:'
printf "'%s'\n" "${b[@]}"

The output:

a before sub:
'aa bb'
'123'
'$ $ $'

b after sub:
'bb cc'
'456'
'###'
a[0] = 'aa bb'
b[1] = '456'

a after sub:
'a---a'
'123'
'$ $ $'

b after sub:
'bb cc'
'999'
'###'