How to Pass an Array as Function Argument?

If you want to pass one or more arguments AND an array, I propose this change to the script of @A.B.
Array should be the last argument and only one array can be passed

#!/bin/bash
function copyFiles() {
   local msg="$1"   # Save first argument in a variable
   shift            # Shift all arguments to the left (original $1 gets lost)
   local arr=("$@") # Rebuild the array with rest of arguments
   for i in "${arr[@]}";
      do
          echo "$msg $i"
      done
}

array=("one" "two" "three")

copyFiles "Copying" "${array[@]}"

Output:

$ ./foo   
Copying one
Copying two
Copying three

You could also pass the array as a reference. i.e.:

#!/bin/bash

function copyFiles {
   local -n arr=$1

   for i in "${arr[@]}"
   do
      echo "$i"
   done
}

array=("one" "two" "three")

copyFiles array

but note that any modifications to arr will be made to array.

There are couple of problems. Here is the working form :

#!/bin/bash
function copyFiles {
   arr=( "$@" )
   for i in "${arr[@]}";
      do
          echo "$i"
      done

}

array=("one" "two" "three")
copyFiles "${array[@]}"

• There need to be at least a space between function declaration and {

• You can not use $array, as array is an array not a variable. If you want to get all the values of an array use "${array[@]}"

• In you main function declaration you need arr="$@" as "${array[@]}" will expand to the indexed values separated by spaces, if you use $1 you would get only the first value. To get all the values use arr="${arr[@]}".

Here follows a slightly larger example. For explanation, see the comments in the code.

#!/bin/bash -u
# ==============================================================================
# Description
# -----------
# Show the content of an array by displaying each element separated by a
# vertical bar (|).
#
# Arg Description
# --- -----------
# 1   The array
# ==============================================================================
show_array()
{
    declare -a arr=("${@}")
    declare -i len=${#arr[@]}
    # Show passed array
    for ((n = 0; n < len; n++))
    do
        echo -en "|${arr[$n]}"
    done
    echo "|"
}

# ==============================================================================
# Description
# -----------
# This function takes two arrays as arguments together with their sizes and a
# name of an array which should be created and returned from this function.
#
# Arg Description
# --- -----------
# 1   Length of first array
# 2   First array
# 3   Length of second array
# 4   Second array
# 5   Name of returned array
# ==============================================================================
array_demo()
{
    declare -a argv=("${@}")                           # All arguments in one big array
    declare -i len_1=${argv[0]}                        # Length of first array passad
    declare -a arr_1=("${argv[@]:1:$len_1}")           # First array
    declare -i len_2=${argv[(len_1 + 1)]}              # Length of second array passad
    declare -a arr_2=("${argv[@]:(len_1 + 2):$len_2}") # Second array
    declare -i totlen=${#argv[@]}                      # Length of argv array (len_1+len_2+2)
    declare __ret_array_name=${argv[(totlen - 1)]}     # Name of array to be returned

    # Show passed arrays
    echo -en "Array 1: "; show_array "${arr_1[@]}"
    echo -en "Array 2: "; show_array "${arr_2[@]}"

    # Create array to be returned with given name (by concatenating passed arrays in opposite order)
    eval ${__ret_array_name}='("${arr_2[@]}" "${arr_1[@]}")'
}

########################
##### Demo program #####
########################
declare -a array_1=(Only 1 word @ the time)                                       # 6 elements
declare -a array_2=("Space separated words," sometimes using "string paretheses") # 4 elements
declare -a my_out # Will contain output from array_demo()

# A: Length of array_1
# B: First array, not necessary with string parentheses here
# C: Length of array_2
# D: Second array, necessary with string parentheses here
# E: Name of array that should be returned from function.
#          A              B             C              D               E
array_demo ${#array_1[@]} ${array_1[@]} ${#array_2[@]} "${array_2[@]}" my_out

# Show that array_demo really returned specified array in my_out:
echo -en "Returns: "; show_array "${my_out[@]}"

The best way is to pass as position arguments. Nothing else. You may pass as string, but this way may cause some troubles. Example:

array=(one two three four five)

function show_passed_array(){
  echo $@
}

or

function show_passed_array(){
  while $# -gt 0;do
    echo $1;shift
  done
}

    show_passed_array ${array[@]}

output:

  one two three four five

You mean if array value has space symbols you must quote elements first before pass for accessing value by index in function use $1 $2 $3 ... position parameters. Where index 0 -> 1, 1 -> 2,... To iterate access it is best to use always $1 and after Shift. Nothing additional is needed. You may pass arguments without any array like this:

show_passed_array one two three four five

bash media automatically builds an array from passed arguments that passed them to function and then you have position arguments. Furthermore when you write ${array[2]} you really write consequent argument one two three four and passed them to the function. So those calls are equivalent.

As ugly as it is, here is a workaround that works as long as you aren't passing an array explicitly, but a variable corresponding to an array:

function passarray()
{
    eval array_internally=("$(echo '${'$1'[@]}')")
    # access array now via array_internally
    echo "${array_internally[@]}"
    #...
}

array=(0 1 2 3 4 5)
passarray array # echo's (0 1 2 3 4 5) as expected

I'm sure someone can come up with a cleaner implementation of the idea, but I've found this to be a better solution than passing an array as "{array[@]"} and then accessing it internally using array_inside=("$@"). This becomes complicated when there are other positional/getopts parameters. In these cases, I've had to first determine and then remove the parameters not associated with the array using some combination of shift and array element removal.

A purist perspective likely views this approach as a violation of the language, but pragmatically speaking, this approach has saved me a whole lot of grief. On a related topic, I also use eval to assign an internally constructed array to a variable named according to a parameter target_varname I pass to the function:

eval $target_varname=$"(${array_inside[@]})"

I propose to avoid any limitation about "passing array(s) args" to a function by... passing strings, and make them array INSIDE the function :

#!/bin/bash

false_array1='1 2 3 4 5'
false_array2='my super fake array to deceive my function'
my_normal_string='John'

function i_love_arrays(){
  local myNumbers=("$1")
  local mySentence=("$2")
  local myName="$3"
  echo "My favorite numbers are, for sure: "
  for number in ${myNumbers[@]}
  do
    echo $number
  done
  echo "Let's make an ugly split of a sentence: "
  for word in ${mySentence[@]}
  do
    echo $word
  done
  echo "Yeah, I know, glad to meet you too. I'm ${myName}."
}

i_love_arrays "${false_array1}" "${false_array2}" "${my_normal_string}"

Using references (easiest way; best and only way for passing associative arrays):

print()
{
    [ "$1" = "arr" ] || { declare -n arr; arr="$1"; }
    # The name of reference mustn't match the input coming to the function.
    # If it does, use the name directly as array instead of reference.
    
    echo "${arr[@]}"
}

print arr

For normal (non-associative) arrays:

Here is a function that copies the passed array in new (local) array for usage. Can be time-taking for long arrays.

copy_fn()
{
    eval local newarr=\(\${$1[@]}\)
    echo ${newarr[@]}
}

Further, like in C++ you can only pass an array pointer, but in python, as a shared variable which you can modify as well.

Here is the shared variables approach.. useful for long arrays.

modify_arr()
{
    eval local ptr=$1        # works as if ptr is a pointer to actual "array-name" (arr)  # the original array is, thus, still same, not a copy
    eval $ptr+=(1)           # modify the array
    echo ${ptr[@]}           # will print name of the array (arr)
    eval echo \$\{$ptr[@]\}  # will print the actual array (arr)
}

modify_arr a
echo ${a[@]}

-Himanshu

@SBF

Your idea sparked some thinking on a situation I needed where two arrays were to be compared.

You can pass multiple arrays using your technique with a minor modification. See below

Thank you!

(works in bash 3.3 on Debian)

#!/bin/bash
#
function TakeTwoArrays() {
    local Var1="$1"
    local Var2="$2"
    local NumArray1="$3"
    local NumArray2="$4"
    shift 4
    local -a AllArray=("$@")
    local -a Array1="${AllArray[@]:0:${NumArray1}}"
    local -a Array2="${AllArray[@]:${NumArray1}}"

    echo "Var1 is ${Var1}"
    echo "Var2 is ${Var2}"
    echo "Array1 has ${NumArray1} entries:"
    printf '%s\n' "${Array1[@]}"
    echo "Array2 has ${NumArray2} entries:"
    printf '%s\n' "${Array2[@]}"
}
FirstArg="So Far"
SecondArg="So Good"
Array1Arg=("But" "the" "real" "question" "is")
NumArray1Arg="${#Array1Arg[@]}"
Array2Arg=("What" "happens" "with" "the" "second" "set" "of" "elements")
NumArray2Arg="${#Array2Arg[@]}"
TakeTwoArrays "${FirstArg}" "${SecondArg}" "${NumArray1Arg}" "${NumArray2Arg}" "${Array1Arg[@]}" "${Array2Arg[@]}"