Multidimensional associative arrays in Bash - bash

Question

I'm trying to create a multidimensional associative array but need some help. I have reviewed the page suggested in this SO answer but it confused me even more. So far here is what I have:
The script:
#!/bin/bash
declare -A PERSONS
declare -A PERSON
PERSON["FNAME"]='John'
PERSON["LNAME"]='Andrew'
PERSONS["1"]=${PERSON[#]}
PERSON["FNAME"]='Elen'
PERSON["LNAME"]='Murray'
PERSONS["2"]=${PERSON[#]}
for KEY in "${!PERSONS[#]}"; do
TMP="${PERSONS["$KEY"]}"
echo "$KEY - $TMP"
echo "${TMP["FNAME"]}"
echo "${TMP["LNAME"]}"
done
The output:
1 - John Andrew
John Andrew
John Andrew
2 - Elen Murray
Elen Murray
Elen Murray
As you can see trying to access a specific index of the $TMP array in the for loop returns the whole array.
[Q] What do I need to do in order to separately access the "FNAME" and "LNAME" indexes of the $TMP array inside the for loop?
Thanks.

Answer 1

You can't do what you're trying to do: bash arrays are one-dimensional
$ declare -A PERSONS
$ declare -A PERSON
$ PERSON["FNAME"]='John'
$ PERSON["LNAME"]='Andrew'
$ declare -p PERSON
declare -A PERSON='([FNAME]="John" [LNAME]="Andrew" )'
$ PERSONS[1]=([FNAME]="John" [LNAME]="Andrew" )
bash: PERSONS[1]: cannot assign list to array member
You can fake multidimensionality by composing a suitable array index string:
declare -A PERSONS
declare -A PERSON
PERSON["FNAME"]='John'
PERSON["LNAME"]='Andrew'
i=1
for key in "${!PERSON[#]}"; do
PERSONS[$i,$key]=${PERSON[$key]}
done
PERSON["FNAME"]='Elen'
PERSON["LNAME"]='Murray'
((i++))
for key in "${!PERSON[#]}"; do
PERSONS[$i,$key]=${PERSON[$key]}
done
declare -p PERSONS
# ==> declare -A PERSONS='([1,LNAME]="Andrew" [2,FNAME]="Elen" [1,FNAME]="John" [2,LNAME]="Murray" )'

Answer 2

I understand what you need. I also wanted the same for weeks.
I was confused whether to use Python or Bash.
Finally, exploring something else I found this
Bash: How to assign an associative array to another variable name (e.g. rename the variable)?
Here, I got to know how to assign some string and use it later as command.
Then with my creativity I found solution to your problem as below:-
#!/bin/bash
declare -A PERSONS
declare -A PERSON
PERSON["FNAME"]='John'
PERSON["LNAME"]='Andrew'
string=$(declare -p PERSON)
#printf "${string}\n"
PERSONS["1"]=${string}
#echo ${PERSONS["1"]}
PERSON["FNAME"]='Elen'
PERSON["LNAME"]='Murray'
string=$(declare -p PERSON)
#printf "${string}\n"
PERSONS["2"]=${string}
#echo ${PERSONS["2"]}
for KEY in "${!PERSONS[#]}"; do
printf "$KEY - ${PERSONS["$KEY"]}\n"
eval "${PERSONS["$KEY"]}"
printf "${PERSONS["$KEY"]}\n"
for KEY in "${!PERSON[#]}"; do
printf "INSIDE $KEY - ${PERSON["$KEY"]}\n"
done
done
OUTPUT:-
1 - declare -A PERSON='([FNAME]="John" [LNAME]="Andrew" )'
declare -A PERSON='([FNAME]="John" [LNAME]="Andrew" )'
INSIDE FNAME - John
INSIDE LNAME - Andrew
2 - declare -A PERSON='([FNAME]="Elen" [LNAME]="Murray" )'
declare -A PERSON='([FNAME]="Elen" [LNAME]="Murray" )'
INSIDE FNAME - Elen
INSIDE LNAME - Murray
The problem actually with multi dimensional arrays in bash and specifically in your approach is that you are assigning PERSON array values to the array element PERSONS[1] which is converted to a list and not an assoc array when you assigned it.
And so it no longer will take it as 2 elements of an array as you are not keeping any info about the array data structure in your value.
So, I found this hack to be sufficient with only 1 limitation that you will have to do this each time you want to do store/retrieve values. But it shall solve your purpose.