Passing arrays as parameters in bash

Question

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.

Answer 1

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

Improved by TheBonsai, thanks.

Answer 2

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

Example

$ foo=(dog cat bird)

$ aecho foo 1
cat

Answer 3

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'

Answer 4

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=("$@")
  ...
}

Answer 5

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 

Answer 6

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.

4. 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.