Assign to a bash array variable indirectly, by dynamically constructed variable name

Question

Bash script to create multiple arrays from csv with unknown columns.

I am trying to write a script to compare two csv files with similar columns. I need it to locate the matching column from the other csv and compare any differences. The kicker is I would like the script to be dynamic to allow any number of columns to be entered and it still be able to function. I thought I had a good plan to solve this but turns out I'm running into syntax errors. Here is a sample of a csv I need to compare.

IP address, Notes,  Nmap-SSH,   Nmap-SMTP, Nmap-HTTP, Nmap-HTTPS,
10.0.0.1,   ,       open,       closed,     open,     open,
10.0.0.2,   ,       closed,     open,       closed,   closed,

When I read the csv file I was planning to look for "IF column == open; then; populate this column's array with the IP address" This would have given me 4 lists in this scenario with the IPs that were listening on said port. I could then compare that to my security device configuration to make sure it was configured properly. Finally to the meat, here is what I thought would accomplish creating the arrays for me to search later. However I ran into a snag when I tried to use a variable inside an array name. Can my syntax be corrected or is there just a better way to do this sort of thing?

#!/bin/bash
#
#
# This script compares config_cleaned_<ip>.txt output against ext_web_env.csv and outputs the differences
#
#
# Read from ext_web_env.csv file and create Array
#
        FILENAME=./tmp/ext_web_env.csv
#
        index=0
#
        while read line
          do
# How many columns are in the .csv?
        varEnvCol=$(echo $line | awk -F, '{print NF}')
            echo "columns = $varEnvCol"

# While loop to create array for each column

                while [ $varEnvCol != 2 ]
                  do
# Checks to see if port is open; if so then add IP address to array
                   varPortCon=$(echo $line | awk -F, -v i=$varEnvCol '{print $i}')
                        if [ $varPortCon = "open" ]
                          then
                                arr$varEnvCol[$index]="$(echo $line | awk -F, '{print $1}')"
# I get this error message "line29 : arr8[194]=10.0.0.194: command not found"
                        fi
           echo "arrEnv$varEnvCol is: ${arr$varEnvCol[@]}"
# Another error but not as important since I am using this to debug "line31: arr$varEnvCol is: ${arr$varEnvCol[@]}: bad substitution"
                   varEnvCol=$(($varEnvCol - 1))
                done
                index=$(($index + 1 ))
          done < $FILENAME

UPDATE

I also tried using the eval command since all the data will be populated by other scripts.

but am getting this error message:

./compare.sh: line 41: arr8[83]=10.0.0.83: command not found

Here is my new code for this example:

 if [[ $varPortCon = *'open'* ]]
   then
    eval arr\$varEnvCol[$index]=$(echo $line | awk -F, '{print $1}')
 fi

mklement0 · Accepted Answer

arr$varEnvCol[$index]="$(...)"

doesn't work the way you expect it to - you cannot assign to shell variables indirectly - via an expression that expands to the variable name - this way.

Your attempted workaround with eval is also flawed - see below.

tl;dr

If you use bash 4.3 or above:

declare -n targetArray="arr$varEnvCol"
targetArray[index]=$(echo $line | awk -F, '{print $1}')

bash 4.2 or earlier:

declare "arr$varEnvCol"[index]="$(echo $line | awk -F, '{print $1}')"

Caveat: This will work in your particular situation, but may fail subtly in others; read on for details, including a more robust, but cumbersome alternative based on read.

The eval-based solution mentioned by @shellter in a since-deleted comment is problematic not only for security reasons (as they mentioned), but also because it can get quite tricky with respect to quoting; for completeness, here's the eval-based solution:

eval "arr$varEnvCol[index]"='$(echo $line | awk -F, '\''{print $1}'\'')'

See below for an explanation.