retrieve bash array by referencing its name as a variable

Question

I need help with bash expansion.
I want to retrieve array values, GNU bash 5.1.0. The array name shall be a variable. "Just" referencing a variable in bash.

I have an array named "armin" and its name is in variable $gral (works fine):

gral="armin"

Values are assigned:

declare -a ${gral}[1]="milk"
declare -a ${gral}[2]="cow"
declare  ${gral}[7]="budgie"
declare  ${gral}[9]="pla9ne"

fine.

Array exists:

$ echo ${armin[@]}
milk cow budgie pla9ne

Array index exists:

$echo ${!armin[@]}
1 2 7 9

Array and index are fine.

I want to retrieve the array by referencing its name as a variable, not manually. Have plenty of them ...
The variable was set and used before:

$ echo $gral
armin  ## name of our bash array

fine - so far.

Just to show the difference NOT using a variable:

echo ${armin[@]}
milk cow budgie pla9ne

Now attempts to reference a variable (gral) to call the name (armin):

$ echo ${$gral[@]}
-bash: ${$gral[@]}: wrong substitution.

$echo ${"$gral"[@]}
-bash: ${"$gral"[@]}: wrong substitution.
echo ${"gral"[@]}
-bash: ${"gral"[@]}: wrong substitution.
echo ${${gral}[@]}
-bash: ${${gral}[@]}: wrong substitution.

all fail. Some attempts with "eval" as well. Using associative (declare -A) makes no difference.

Rem.: Index works fine this way, no issue. Name is the issue.

I think I am missing something. Maybe the answer was described before, I found a lot of interesting stuff about variables in arrays but did not recognize an answer to my challenge.

Can you please help me find the term to retrieve the array by referencing its name as a variable?

Answer 1

Use namerefs (in Bash >= 4.3):

$ armin=(foo bar doo)
$ declare -n gral=armin      # 'gral' references array 'armin'  
$ gral[123]=quux             # same as 'armin[123]=quux'
$ echo "${gral[@]}"
foo bar doo quux
$ echo "${gral[1]}"
bar
$ echo "${!gral[@]}"         # listing the indexes works too
0 1 2 123

See also: Does bash provide support for using pointers?

Answer 2

As @ilkkachu answers, namerefs are the tool to use here. They make it super easy to pass arrays to functions. For example:

dumpArray() {
  local -n ary=$1
  for i in "${!ary[@]}"; do
    printf "%s\t%s\n" "$i" "${ary[$i]}"
  done
}

That function can handle arrays and associative arrays:

$ declare -a armin=([1]=milk [2]=cow [7]=bugle [9]=pla9ne)

$ dumpArray armin
1   milk
2   cow
7   bugle
9   pla9ne

$ declare -A map=([foo]=bar [baz]=qux)

$ dumpArray map
foo bar
baz qux

The only real problem is you that it's messy to make a nameref reference an array with the same name:

$ ary=(a b c)

$ dumpArray ary
bash: local: warning: ary: circular name reference
bash: warning: ary: circular name reference
bash: warning: ary: circular name reference
bash: warning: ary: circular name reference
bash: warning: ary: circular name reference
0   a
bash: warning: ary: circular name reference
bash: warning: ary: circular name reference
1   b
bash: warning: ary: circular name reference
bash: warning: ary: circular name reference
2   c

So in the function, you make the array have an odd name, like

dumpArray() {
  local -n __dumpArray_ary=$1
  do_stuff_with "${__dumpArray_ary[@]}"
}

Answer 3

Include the "index", i.e. the [@], in the variable that contains the name:

ref=$gral'[@]'
printf '%s\n' "${!ref}"

Output:

milk
cow
budgie
pla9ne

It works with values containing spaces, too.

Answer 4

well,
the answer to dereferecing an array by a name passed as variable (partly resembles pointer)
is (example above re-used):

eval echo "\$$(eval echo "{$gral[@]}")"
with $gral holding the array name

Result:

milk cow budgie pla9ne

You may extend this solution now to iterate bash array (indexed AND associated) content using the index entries.

Glad I found the answer. Thank you all for your help!

Accessing the contents of the contents of a variable.

There are hints leading to the result in abs-guide:
https://tldp.org/LDP/abs/html/abs-guide.html#IVR

Committed thanks to Mendel Cooper; so thankful we have this great gift named Advanced Bash-Scripting Guide.

OK,
eval is ugly due to security concerns.
Try to find a better one ...