Passing arrays as parameters in bash

How can I pass an array as parameter to a bash function?

Note: After not finding an answer here on Stack Overflow, I posted my somewhat crude solution myself. It allows for only one array being passed, and it being the last element of the parameter list. Actually, it is not passing the array at all, but a list of its elements, which are re-assembled into an array by called_function(), but it worked for me. If someone knows a better way, feel free to add it here.

You can pass multiple arrays as arguments using something like this:

takes_ary_as_arg()
{
    declare -a argAry1=("${!1}")
    echo "${argAry1[@]}"

    declare -a argAry2=("${!2}")
    echo "${argAry2[@]}"
}
try_with_local_arys()
{
    # array variables could have local scope
    local descTable=(
        "sli4-iread"
        "sli4-iwrite"
        "sli3-iread"
        "sli3-iwrite"
    )
    local optsTable=(
        "--msix  --iread"
        "--msix  --iwrite"
        "--msi   --iread"
        "--msi   --iwrite"
    )
    takes_ary_as_arg descTable[@] optsTable[@]
}
try_with_local_arys

will echo:

sli4-iread sli4-iwrite sli3-iread sli3-iwrite  
--msix  --iread --msix  --iwrite --msi   --iread --msi   --iwrite

Edit/notes: (from comments below)

• descTable and optsTable are passed as names and are expanded in the function. Thus no $ is needed when given as parameters.
• Note that this still works even with descTable etc being defined with local, because locals are visible to the functions they call.
• The ! in ${!1} expands the arg 1 variable.
• declare -a just makes the indexed array explicit, it is not strictly necessary.

Note: This is the somewhat crude solution I posted myself, after not finding an answer here on Stack Overflow. It allows for only one array being passed, and it being the last element of the parameter list. Actually, it is not passing the array at all, but a list of its elements, which are re-assembled into an array by called_function(), but it worked for me. Somewhat later Ken posted his solution, but I kept mine here for "historic" reference.

calling_function()
{
    variable="a"
    array=( "x", "y", "z" )
    called_function "${variable}" "${array[@]}"
}

called_function()
{
    local_variable="${1}"
    shift
    local_array=("${@}")
}

Commenting on Ken Bertelson solution and answering Jan Hettich:

How it works

the takes_ary_as_arg descTable[@] optsTable[@] line in try_with_local_arys() function sends:

1. This is actually creates a copy of the descTable and optsTable arrays which are accessible to the takes_ary_as_arg function.

2. takes_ary_as_arg() function receives descTable[@] and optsTable[@] as strings, that means $1 == descTable[@] and $2 == optsTable[@].

3. in the beginning of takes_ary_as_arg() function it uses ${!parameter} syntax, which is called indirect reference or sometimes double referenced, this means that instead of using $1's value, we use the value of the expanded value of $1, example:

    baba=booba
    variable=baba
    echo ${variable} # baba
    echo ${!variable} # booba
   

likewise for $2.

1. putting this in argAry1=("${!1}") creates argAry1 as an array (the brackets following =) with the expanded descTable[@], just like writing there argAry1=("${descTable[@]}") directly. the declare there is not required.

N.B.: It is worth mentioning that array initialization using this bracket form initializes the new array according to the IFS or Internal Field Separator which is by default tab, newline and space. in that case, since it used [@] notation each element is seen by itself as if he was quoted (contrary to [*]).

My reservation with it

In BASH, local variable scope is the current function and every child function called from it, this translates to the fact that takes_ary_as_arg() function "sees" those descTable[@] and optsTable[@] arrays, thus it is working (see above explanation).

Being that case, why not directly look at those variables themselves? It is just like writing there:

argAry1=("${descTable[@]}")

See above explanation, which just copies descTable[@] array's values according to the current IFS.

In summary

> This is passing, in essence, nothing by value - as usual.

I also want to emphasize Dennis Williamson comment above: sparse arrays (arrays without all the keys defines - with "holes" in them) will not work as expected - we would loose the keys and "condense" the array.

That being said, I do see the value for generalization, functions thus can get the arrays (or copies) without knowing the names:

• for ~"copies": this technique is good enough, just need to keep aware, that the indices (keys) are gone.

• for real copies: we can use an eval for the keys, for example:

    eval local keys=(\${!$1})
  

and then a loop using them to create a copy. Note: here ! is not used it's previous indirect/double evaluation, but rather in array context it returns the array indices (keys).

• and, of course, if we were to pass descTable and optsTable strings (without [@]), we could use the array itself (as in by reference) with eval. for a generic function that accepts arrays.

The basic problem here is that the bash developer(s) that designed/implemented arrays really screwed the pooch. They decided that ${array} was just short hand for ${array[0]}, which was a bad mistake. Especially when you consider that ${array[0]} has no meaning and evaluates to the empty string if the array type is associative.

Assigning an array takes the form array=(value1 ... valueN) where value has the syntax [subscript]=string, thereby assigning a value directly to a particular index in the array. This makes it so there can be two types of arrays, numerically indexed and hash indexed (called associative arrays in bash parlance). It also makes it so that you can create sparse numerically indexed arrays. Leaving off the [subscript]= part is short hand for a numerically indexed array, starting with the ordinal index of 0 and incrementing with each new value in the assignment statement.

Therefore, ${array} should evaluate to the entire array, indexes and all. It should evaluate to the inverse of the assignment statement. Any third year CS major should know that. In that case, this code would work exactly as you might expect it to:

declare -A foo bar
foo=${bar}

Then, passing arrays by value to functions and assigning one array to another would work as the rest of the shell syntax dictates. But because they didn't do this right, the assignment operator = doesn't work for arrays, and arrays can't be passed by value to functions or to subshells or output in general (echo ${array}) without code to chew through it all.

So, if it had been done right, then the following example would show how the usefulness of arrays in bash could be substantially better:

simple=(first=one second=2 third=3)
echo ${simple}

the resulting output should be:

(first=one second=2 third=3)

Then, arrays could use the assignment operator, and be passed by value to functions and even other shell scripts. Easily stored by outputting to a file, and easily loaded from a file into a script.

declare -A foo
read foo <file

Alas, we have been let down by an otherwise superlative bash development team.

As such, to pass an array to a function, there is really only one option, and that is to use the nameref feature:

function funky() {
    local -n ARR

    ARR=$1
    echo "indexes: ${!ARR[@]}"
    echo "values: ${ARR[@]}"
}

declare -A HASH

HASH=([foo]=bar [zoom]=fast)
funky HASH # notice that I'm just passing the word 'HASH' to the function

will result in the following output:

indexes: foo zoom
values: bar fast

Since this is passing by reference, you can also assign to the array in the function. Yes, the array being referenced has to have a global scope, but that shouldn't be too big a deal, considering that this is shell scripting. To pass an associative or sparse indexed array by value to a function requires throwing all the indexes and the values onto the argument list (not too useful if it's a large array) as single strings like this:

funky "${!array[*]}" "${array[*]}"

and then writing a bunch of code inside the function to reassemble the array.

DevSolar's answer has one point I don't understand (maybe he has a specific reason to do so, but I can't think of one): He sets the array from the positional parameters element by element, iterative.

An easier approuch would be

called_function()
{
  ...
  # do everything like shown by DevSolar
  ...

  # now get a copy of the positional parameters
  local_array=("$@")
  ...
}

An easy way to pass several arrays as parameter is to use a character-separated string. You can call your script like this:

./myScript.sh "value1;value2;value3" "somethingElse" "value4;value5" "anotherOne"

Then, you can extract it in your code like this:

myArray=$1
IFS=';' read -a myArray <<< "$myArray"

myOtherArray=$3
IFS=';' read -a myOtherArray <<< "$myOtherArray"

This way, you can actually pass multiple arrays as parameters and it doesn't have to be the last parameters.

function aecho {
  set "$1[$2]"
  echo "${!1}"
}

Example

$ foo=(dog cat bird)

$ aecho foo 1
cat

Modern bash (apparently version 4.3 or later), allows you to pass arrays by reference. I'll show that below. If you'd like to manually serialize and deserialize the arrays instead, see my answer here for bash regular "indexed" arrays, and here for bash associative arrays. Passing arrays by reference, as shown below, is much easier and more-concise, however, so that's what I now recommend.

The code below is also available online in my eRCaGuy_hello_world repo here: array_pass_as_bash_parameter_by_reference.sh. See also this example here: array_pass_as_bash_parameter_2_associative.sh.

Here is a demo for regular bash arrays:

function foo {
    # declare a local **reference variable** (hence `-n`) named `data_ref`
    # which is a reference to the value stored in the first parameter
    # passed in
    local -n data_ref="$1"
    echo "${data_ref[0]}"
    echo "${data_ref[1]}"
}

# declare a regular bash "indexed" array
declare -a data
data+=("Fred Flintstone")
data+=("Barney Rubble")
foo "data"

Sample output: > Fred Flintstone > Barney Rubble

...and here is a demo for associative bash arrays (ie: bash hash tables, "dictionaries", or "unordered maps"):

function foo {
    # declare a local **reference variable** (hence `-n`) named `data_ref`
    # which is a reference to the value stored in the first parameter
    # passed in
    local -n data_ref="$1"
    echo "${data_ref["a"]}"
    echo "${data_ref["b"]}"
}

# declare a bash associative array
declare -A data
data["a"]="Fred Flintstone"
data["b"]="Barney Rubble"
foo "data"

Sample output: > Fred Flintstone > Barney Rubble

References:

1. I modified the above code samples from @Todd Lehman's answer here: How to pass an associative array as argument to a function in Bash?
2. See also my manual serializing/deserializing answer here
3. And see my follow-up Question here: Why do the man bash pages state the declare and local -n attribute "cannot be applied to array variables", and yet it can?

This one works even with spaces:

format="\t%2s - %s\n"

function doAction
{
  local_array=("$@")
  for (( i = 0 ; i < ${#local_array[@]} ; i++ ))
    do
      printf "${format}" $i "${local_array[$i]}"
  done
  echo -n "Choose: "
  option=""
  read -n1 option
  echo ${local_array[option]}
  return
}

#the call:
doAction "${tools[@]}"

With a few tricks you can actually pass named parameters to functions, along with arrays.

The method I developed allows you to access parameters passed to a function like this:

testPassingParams() {

    @var hello
    l=4 @array anArrayWithFourElements
    l=2 @array anotherArrayWithTwo
    @var anotherSingle
    @reference table   # references only work in bash >=4.3
    @params anArrayOfVariedSize

    test "$hello" = "$1" && echo correct
    #
    test "${anArrayWithFourElements[0]}" = "$2" && echo correct
    test "${anArrayWithFourElements[1]}" = "$3" && echo correct
    test "${anArrayWithFourElements[2]}" = "$4" && echo correct
    # etc...
    #
    test "${anotherArrayWithTwo[0]}" = "$6" && echo correct
    test "${anotherArrayWithTwo[1]}" = "$7" && echo correct
    #
    test "$anotherSingle" = "$8" && echo correct
    #
    test "${table[test]}" = "works"
    table[inside]="adding a new value"
    #
    # I'm using * just in this example:
    test "${anArrayOfVariedSize[*]}" = "${*:10}" && echo correct
}

fourElements=( a1 a2 "a3 with spaces" a4 )
twoElements=( b1 b2 )
declare -A assocArray
assocArray[test]="works"

testPassingParams "first" "${fourElements[@]}" "${twoElements[@]}" "single with spaces" assocArray "and more... " "even more..."

test "${assocArray[inside]}" = "adding a new value"

In other words, not only you can call your parameters by their names (which makes up for a more readable core), you can actually pass arrays (and references to variables - this feature works only in bash 4.3 though)! Plus, the mapped variables are all in the local scope, just as $1 (and others).

The code that makes this work is pretty light and works both in bash 3 and bash 4 (these are the only versions I've tested it with). If you're interested in more tricks like this that make developing with bash much nicer and easier, you can take a look at my Bash Infinity Framework, the code below was developed for that purpose.

Function.AssignParamLocally() {
    local commandWithArgs=( $1 )
    local command="${commandWithArgs[0]}"

    shift

    if [[ "$command" == "trap" || "$command" == "l="* || "$command" == "_type="* ]]
    then
        paramNo+=-1
        return 0
    fi

    if [[ "$command" != "local" ]]
    then
        assignNormalCodeStarted=true
    fi

    local varDeclaration="${commandWithArgs[1]}"
    if [[ $varDeclaration == '-n' ]]
    then
        varDeclaration="${commandWithArgs[2]}"
    fi
    local varName="${varDeclaration%%=*}"

    # var value is only important if making an object later on from it
    local varValue="${varDeclaration#*=}"

    if [[ ! -z $assignVarType ]]
    then
        local previousParamNo=$(expr $paramNo - 1)

        if [[ "$assignVarType" == "array" ]]
        then
            # passing array:
            execute="$assignVarName=( \"\${@:$previousParamNo:$assignArrLength}\" )"
            eval "$execute"
            paramNo+=$(expr $assignArrLength - 1)

            unset assignArrLength
        elif [[ "$assignVarType" == "params" ]]
        then
            execute="$assignVarName=( \"\${@:$previousParamNo}\" )"
            eval "$execute"
        elif [[ "$assignVarType" == "reference" ]]
        then
            execute="$assignVarName=\"\$$previousParamNo\""
            eval "$execute"
        elif [[ ! -z "${!previousParamNo}" ]]
        then
            execute="$assignVarName=\"\$$previousParamNo\""
            eval "$execute"
        fi
    fi

    assignVarType="$__capture_type"
    assignVarName="$varName"
    assignArrLength="$__capture_arrLength"
}

Function.CaptureParams() {
    __capture_type="$_type"
    __capture_arrLength="$l"
}

alias @trapAssign='Function.CaptureParams; trap "declare -i \"paramNo+=1\"; Function.AssignParamLocally \"\$BASH_COMMAND\" \"\$@\"; [[ \$assignNormalCodeStarted = true ]] && trap - DEBUG && unset assignVarType && unset assignVarName && unset assignNormalCodeStarted && unset paramNo" DEBUG; '
alias @param='@trapAssign local'
alias @reference='_type=reference @trapAssign local -n'
alias @var='_type=var @param'
alias @params='_type=params @param'
alias @array='_type=array @param'

Just to add to the accepted answer, as I found it doesn't work well if the array contents are someting like:

RUN_COMMANDS=(
  "command1 param1... paramN"
  "command2 param1... paramN"
)

In this case, each member of the array gets split, so the array the function sees is equivalent to:

RUN_COMMANDS=(
    "command1"
    "param1"
     ...
    "command2"
    ...
)

To get this case to work, the way I found is to pass the variable name to the function, then use eval:

function () {
    eval 'COMMANDS=( "${'"$1"'[@]}" )'
    for COMMAND in "${COMMANDS[@]}"; do
        echo $COMMAND
    done
}

function RUN_COMMANDS

Just my 2©

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 clearner 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[@]})"

Hope this helps someone.

My short answer is:

function display_two_array {
    local arr1=$1
    local arr2=$2
    for i in $arr1
    do
       echo "arrary1: $i"
    done
    
    for i in $arr2
    do
       echo "arrary2: $i"
    done
}

test_array=(1 2 3 4 5)
test_array2=(7 8 9 10 11)

display_two_array "${test_array[*]}" "${test_array2[*]}"

It should be noticed that the ${test_array[*]} and ${test_array2[*]} should be surrounded by "", otherwise you'll fail.

The answer below shows you how to pass bash regular "indexed" arrays as parameters to a function essentially by serializing and deserializing them.

1. To see this manual serializing/deserializing for bash associative arrays (hash tables) instead of for regular indexed arrays, see my answer here.
2. For a better way (which requires bash version 4.3 or later, I think), which passes the arrays by reference, see the link just above and my other answer here.
   1. Passing arrays by reference is much easier and more-concise, so that's what I now recommend. That being said, the manual serializing/deserializing techniques I show below are also extremely informative and useful.

Quick summary:
See the 3 separate function definitions below. I go over how to pass:

1. one bash array to a function
2. two or more bash arrays to a function, and
3. two or more bash arrays plus additional arguments (before or after the arrays) to a function.

---

12 years later and I still don't see any answers I really like here and which I would consider to be thorough enough, simple enough, and "canonical" enough for me to just use--answers which I can come back to again and again and copy and paste and expand when needed. So, here is my answer which I do consider to be all of these things.

How to pass bash arrays as parameters to bash functions

You might also call this "variadic argument parsing in bash functions or scripts", especially since the number of elements in each array passed in to the examples below can dynamically vary, and in bash the elements of an array essentially get passed to the function as separate input parameters even when the array is passed in via a single array expansion argument like this "${array1[@]}".

For all example code below, assume you have these two bash arrays for testing:

array1=()
array1+=("one")
array1+=("two")
array1+=("three")

array2=("four" "five" "six" "seven" "eight")

The code above and below is available in my bash/array_pass_as_bash_parameter.sh file in my eRCaGuy_hello_world repo on GitHub.

Example 1: how to pass one bash array to a function

To pass an array to a bash function, you have to pass all of its elements separately. Given bash array array1, the syntax to obtain all elements of this array is "${array1[@]}". Since all incoming parameters to a bash function or executable file get wrapped up in the magic bash input parameter array called @, you can read all members of the input array with the "$@" syntax, as shown below.

Function definition:

# Print all elements of a bash array.
# General form:
#       print_one_array array1
# Example usage:
#       print_one_array "${array1[@]}"
print_one_array() {
    for element in "$@"; do
        printf "    %s\n" "$element"
    done
}

Example usage:

echo "Printing array1"
# This syntax passes all members of array1 as separate input arguments to 
# the function
print_one_array "${array1[@]}"

Example Output:

Printing array1
    one
    two
    three

Example 2: how to pass two or more bash arrays to a function...

(and how to recapture the input arrays as separate bash arrays again)

Here, we need to differentiate which incoming parameters belong to which array. To do this, we need to know the size of each array, meaning the number of elements in each array. This is very similar to passing arrays in C, where we also generally must know the array length passed to any C function. Given bash array array1, the number of elements in it can be obtained with "${#array1[@]}" (notice the usage of the # symbol). In order to know where in the input arguments the array_len length parameter is, we must always pass the array length parameter for each array before passing the individual array elements, as shown below.

In order to parse the arrays, I use array slicing on the input argument array, @.

Here is a reminder on how bash array slicing syntax works (from my answer here). In the slicing syntax :start:length, the 1st number is the zero-based index to start slicing from, and the 2nd number is the number of elements to grab:

# array slicing basic format 1: grab a certain length starting at a certain
# index
echo "${@:2:5}"
#         │ │
#         │ └────> slice length
#         └──────> slice starting index (zero-based)

# array slicing basic format 2: grab all remaining array elements starting at a
# certain index through to the end
echo "${@:2}"
#         │
#         │
#         └──────> slice starting index (zero-based)

Also, in order to force the sliced parameters from the input array to become a new array, I surround them in parenthesis (), like this, for example ("${@:$i:$array1_len}"). Those parenthesis on the outside are important, again, because that's how we make an array in bash.

This example below only accepts two bash arrays, but following the given patterns it can be easily adapted to accept any number of bash arrays as arguments.

Function definition:

# Print all elements of two bash arrays.
# General form (notice length MUST come before the array in order
# to be able to parse the args!):
#       print_two_arrays array1_len array1 array2_len array2
# Example usage:
#       print_two_arrays "${#array1[@]}" "${array1[@]}" \
#       "${#array2[@]}" "${array2[@]}"
print_two_arrays() {
    # For debugging: print all input args
    echo "All args to 'print_two_arrays':"
    print_one_array "$@"

    i=1

    # Read array1_len into a variable
    array1_len="${@:$i:1}"
    ((i++))
    # Read array1 into a new array
    array1=("${@:$i:$array1_len}")
    ((i += $array1_len))

    # Read array2_len into a variable
    array2_len="${@:$i:1}"
    ((i++))
    # Read array2 into a new array
    array2=("${@:$i:$array2_len}")
    ((i += $array2_len))

    # Print the two arrays
    echo "array1:"
    print_one_array "${array1[@]}"
    echo "array2:"
    print_one_array "${array2[@]}"
}

Example usage:

echo "Printing array1 and array2"
print_two_arrays "${#array1[@]}" "${array1[@]}" "${#array2[@]}" "${array2[@]}"

Example Output:

Printing array1 and array2
All args to 'print_two_arrays':
    3
    one
    two
    three
    5
    four
    five
    six
    seven
    eight
array1:
    one
    two
    three
array2:
    four
    five
    six
    seven
    eight

Example 3: pass two bash arrays plus some extra args after that to a function

This is a tiny expansion of the example above. It also uses bash array slicing, just like the example above. Instead of stopping after parsing two full input arrays, however, we continue and parse a couple more arguments at the end. This pattern can be continued indefinitely for any number of bash arrays and any number of additional arguments, accommodating any input argument order, so long as the length of each bash array comes just before the elements of that array.

Function definition:

# Print all elements of two bash arrays, plus two extra args at the end.
# General form (notice length MUST come before the array in order
# to be able to parse the args!):
#       print_two_arrays_plus_extra_args array1_len array1 array2_len array2 \
#       extra_arg1 extra_arg2
# Example usage:
#       print_two_arrays_plus_extra_args "${#array1[@]}" "${array1[@]}" \
#       "${#array2[@]}" "${array2[@]}" "hello" "world"
print_two_arrays_plus_extra_args() {
    i=1

    # Read array1_len into a variable
    array1_len="${@:$i:1}"
    ((i++))
    # Read array1 into a new array
    array1=("${@:$i:$array1_len}")
    ((i += $array1_len))

    # Read array2_len into a variable
    array2_len="${@:$i:1}"
    ((i++))
    # Read array2 into a new array
    array2=("${@:$i:$array2_len}")
    ((i += $array2_len))

    # You can now read the extra arguments all at once and gather them into a
    # new array like this:
    extra_args_array=("${@:$i}")

    # OR you can read the extra arguments individually into their own variables
    # one-by-one like this
    extra_arg1="${@:$i:1}"
    ((i++))
    extra_arg2="${@:$i:1}"
    ((i++))

    # Print the output
    echo "array1:"
    print_one_array "${array1[@]}"
    echo "array2:"
    print_one_array "${array2[@]}"
    echo "extra_arg1 = $extra_arg1"
    echo "extra_arg2 = $extra_arg2"
    echo "extra_args_array:"
    print_one_array "${extra_args_array[@]}"
}

Example usage:

echo "Printing array1 and array2 plus some extra args"
print_two_arrays_plus_extra_args "${#array1[@]}" "${array1[@]}" \
"${#array2[@]}" "${array2[@]}" "hello" "world"

Example Output:

Printing array1 and array2 plus some extra args
array1:
    one
    two
    three
array2:
    four
    five
    six
    seven
    eight
extra_arg1 = hello
extra_arg2 = world
extra_args_array:
    hello
    world

References:

1. I referenced a lot of my own sample code from my eRCaGuy_hello_world repo here:
   1. array_practice.sh
   2. array_slicing_demo.sh
2. [my answer on bash array slicing] Unix & Linux: Bash: slice of positional parameters
3. An answer to my question on "How can I create and use a backup copy of all input args ("$@") in bash?" - very useful for general array manipulation of the input argument array
4. An answer to "How to pass array as an argument to a function in Bash", which confirmed to me this really important concept that: > You cannot pass an array, you can only pass its elements (i.e. the expanded array).

See also:

1. [another answer of mine on this topic] How to pass array as an argument to a function in Bash

Requirement: Function to find a string in an array.
This is a slight simplification of DevSolar's solution in that it uses the arguments passed rather than copying them.

myarray=('foobar' 'foxbat')

function isInArray() {
  local item=$1
  shift
  for one in $@; do
    if [ $one = $item ]; then
      return 0   # found
    fi
  done
  return 1       # not found
}

var='foobar'
if isInArray $var ${myarray[@]}; then
  echo "$var found in array"
else
  echo "$var not found in array"
fi