What is the safest way to initialize bash arrays with specified values ​​from function output?

Question

I prefer to program my bash scripts as procedurally as possible. One of the difficulties that I encountered when trying to do this occurs when passing array data between functions, a task that is not supported in bash.

As an example, it is trivial to start an array in bash with several hard-coded quotes, each of which may contain several words:

declare -a LINES=( "Hello there" "loyal user" )
echo "Line 0: '${LINES[0]}'"
echo "Line 1: '${LINES[1]}'"
# Line 0: 'Hello there'
# Line 1: 'Loyal user'

However, replacing such hard-coded values ​​with a function output does not work so well:

getLines() {
    echo "\"Hello there\" \"loyal user\""
}

local LINE_STR=$( getLines )
declare -a LINES=( ${LINE_STR} )
echo "Line 0: '${LINES[0]}'"
echo "Line 1: '${LINES[1]}'"
# Line 0: '"Hello'
# Line 1: 'there"'

I tried almost every permutation of allowed bash statements to overcome this problem. One approach that seems to work well is "eval":

local LINE_STR=$( getLines )
eval declare -a LINES=( ${LINE_STR} )
echo "Line 0: '${LINES[0]}'"
echo "Line 1: '${LINES[1]}'"
# Line 0: 'Hello there'
# Line 1: 'loyal user'

However, this approach is related to security issues, as shown here:

emulateUnsafeInput() {
    echo "\"\`whoami\` just got haxxored\" \"Hahaha!\""
}

local LINE_STR=$( emulateUnsafeInput )
eval declare -a LINES=( "${LINE_STR}" )
echo "Line 0: '${LINES[0]}'"
echo "Line 1: '${LINES[1]}'"
# Line 0: 'root just got haxxored'
# Line 1: 'Hahaha!'

'read -a' , , "" -, , script.

"eval"? script, , :

#!/bin/bash

getLines() {
    echo "\"Hello there\" \"loyal user\""
}

emulateUnsafeInput() {
    echo "\"\`whoami\` just got haxxored\" \"Hahaha!\""
}

execute() {
(
    echo Test 01
    declare -a LINES=( "Hello there" "loyal user" )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: 'Hello there'
    # Line 1: 'loyal user'
);(
    echo Test 02
    local LINE_STR=$( getLines )
    declare -a LINES=( ${LINE_STR} )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: '"Hello'
    # Line 1: 'there"'
);(
    echo Test 03
    local LINE_STR=$( getLines )
    declare -a LINES=( "${LINE_STR}" )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: '"Hello there" "loyal user"'
    # Line 1: ''
);(
    echo Test 04
    local LINE_STR=$( getLines )
    eval declare -a LINES=( ${LINE_STR} )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: 'Hello there'
    # Line 1: 'loyal user'
);(
    echo Test 05
    local LINE_STR=$( getLines )
    eval declare -a LINES=( "${LINE_STR}" )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: 'Hello there'
    # Line 1: 'loyal user'
);(
    echo Test 06
    local LINE_STR=$( getLines )
    declare -a LINES=( $( echo ${LINE_STR} ) )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: '"Hello'
    # Line 1: 'there"'
);(
    echo Test 07
    local LINE_STR=$( getLines )
    declare -a LINES=( $( echo "${LINE_STR}" ) )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: '"Hello'
    # Line 1: 'there"'
);(
    echo Test 08
    local LINE_STR=$( getLines )
    declare -a LINES=( $( eval echo ${LINE_STR} ) )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: 'Hello'
    # Line 1: 'there'
);(
    echo Test 09
    local LINE_STR=$( getLines )
    declare -a LINES=( $( eval echo "${LINE_STR}" ) )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: 'Hello'
    # Line 1: 'there'
);(
    echo Test 10
    local LINE_STR=$( emulateUnsafeInput )
    eval declare -a LINES=( ${LINE_STR} )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: 'root just got haxxored'
    # Line 1: 'Hahaha!'
);(
    echo Test 11
    local LINE_STR=$( emulateUnsafeInput )
    eval declare -a LINES=( "${LINE_STR}" )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: 'root just got haxxored'
    # Line 1: 'Hahaha!'
);(
    echo Test 12
    local LINE_STR=$( emulateUnsafeInput )
    declare -a LINES=( $( eval echo ${LINE_STR} ) )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: 'root'
    # Line 1: 'just'
);(
    echo Test 13
    local LINE_STR=$( emulateUnsafeInput )
    declare -a LINES=( $( eval echo "${LINE_STR}" ) )
    echo "Line 0: '${LINES[0]}'"
    echo "Line 1: '${LINES[1]}'"
    # Line 0: 'root'
    # Line 1: 'just'
)
}

execute

Answer 1

:

#! /bin/bash

# $ ./return_an_array.sh 
# ./return_an_array.sh: line 9: declare: returned_array: not found
# declare -a returned_array='([0]="one" [1]="two three")'

return_an_array()
{
    local -a an_array=( 'one' 'two three' )
    declare -p an_array
}

declare -p returned_array
eval $(return_an_array | sed -e 's/^\(declare -a \)[^=]*=/\1 returned_array=/')
declare -p returned_array

Answer 2

echo -e :

getLines() { echo -e "\"Hello there\"\n\"loyal user\""; }

, :

i=0
while read -r
do
    arr[i++]=$REPLY
done < <(getLines)
# Line 0: '"Hello there"'
# Line 1: '"loyal user"'

.

.

Answer 3

If you are concerned about arbitrary, malicious data from functions, you should just expect and parse the strings using a delimiter \0(aka. NUL). This is the only character that cannot be part of any variables, so there can be no collisions with actual data.

haxxorz() {
    printf '%s\0' whoami
    printf '%s\0' '\`whoami\`'
    printf '%s\0' "\`whoami\`"
    printf '%s\0' "\"\`whoami\` is haxxor-proof"'!'"\""
    printf '%s' 'Trying to poison the $REPLY variable with a missing separator'
}

index=0
while IFS= read -r -d '' || [ -n "$REPLY" ]
do
    array[index++]="$REPLY"
done < <(haxxorz)

for element in "${array[@]}"
do
    echo "$element"
done

echo "${REPLY:-REPLY is empty}"

Result:

whoami
\`whoami\`
`whoami`
"`whoami` is haxxor-proof!"
Trying to poison the $REPLY variable with a missing separator
REPLY is empty

I love it when the plan comes together!