bash/shell script: evaluating a hashmap key to a variable

Question

Im no bash/shell script expert...what im trying to do the is following: im using array variables as keys to a hashmap. when i iterate over the keys of the hash map i want to evaluate them to the actual array, iterate over the array and print the values in that array out.

arr1=(A B)
arr2=(C D)

declare -A arrmap
arrmap=(
    [arr1]="array 1"
    [arr2]="array 2"
)

for k in "${!arrmap[@]}";
do  
    for f in "${k[@]}";
        do echo "$k: $f";
    done
done

This prints out:

arr2: arr2
arr1: arr1

but what i would like to see is:

A: array 1
B: array 1
C: array 2
D: array 2

EDIT: think i was able to figure it out:

for k in "${!arrmap[@]}"; do  
    eval a=\( \${${k}[@]} \)
    for f in "${a[@]}"; do 
        echo "$k: $f";
    done
done

score 2 · Accepted Answer · answered Apr 02 '23 at 05:42

A good rule of thumb is that if you find yourself wanting to use eval, you're doing something wrong. In this case, a nameref is what you need instead.

Documentation for declare -n:

Give each name the nameref attribute, making it a name reference to another variable. That other variable is defined by the value of name. All references, assignments, and attribute modifications to name, except for those using or changing the -n attribute itself, are performed on the variable referenced by name’s value.

#!/usr/bin/env bash

arr1=(A B)
arr2=(C D)

declare -A arrmap=(
    [arr1]="array 1"
    [arr2]="array 2"
)

for k in "${!arrmap[@]}";
do
    declare -n arr="$k"
    for f in "${arr[@]}"; do
        echo "$f: ${arrmap[$k]}"
    done
done

will output

C: array 2
D: array 2
A: array 1
B: array 1

(Order may vary; hash tables don't iterate over elements in a set predictable order)

score 1 · Answer 2 · answered Apr 02 '23 at 19:41

You can use indirect expansion (${!var}) to do what you want without eval:

#! /bin/bash -p

arr1=(A B)
arr2=(C D)

declare -A arrmap
arrmap=(
    [arr1]='array 1'
    [arr2]='array 2'
)

for k in "${!arrmap[@]}"; do  
    ref="$k[@]"
    for f in "${!ref}"; do
        printf "%s: %s\n" "$f" "${arrmap[$k]}"
    done
done

This code works with Bash version 4.0 (tested), and should work with all later versions. It won't work with older versions because support for associative arrays was introduced in Bash 4.0. Support for namerefs was added in Bash 4.3.
See What does "${!var}" mean in shell script? and BashFAQ/006 (How can I use variable variables (indirect variables, pointers, references) or associative arrays?) for information about indirect expansion in Bash.
The code causes a few harmless Shellcheck warnings. Shellcheck doesn't handle indirect expansion well.
Note that, in common with (the much more maligned) eval, both indirect variable expansion and namerefs are vulnerable to Code injection. This code should be safe, assuming that the contents of arrmap are controlled.
See the accepted, and excellent, answer to Why is printf better than echo? for an explanation of why I replaced echo with printf.

bash/shell script: evaluating a hashmap key to a variable

2 Answers2