How to iterate over an array using indirect reference?

Question

How can I make this code work?

#!/bin/bash
ARRAYNAME='FRUITS'
FRUITS=( APPLE BANANA ORANGE )
for FRUIT in ${!ARRAYNAME[@]}
do
    echo ${FRUIT}
done

This code:

echo ${!ARRAYNAME[0]}

Prints APPLE. I'm tryng to do something similar but with "[@]" to iterate over the array.

Thanks in advance,

Answer 1

${!ARRAYNAME[@]} means "the indices of ARRAYNAME". As stated in the bash man page since ARRAYNAME is set, but as a string, not an array, it returns 0.

Here's a solution using eval.

#!/usr/bin/env bash

ARRAYNAME='FRUITS'
FRUITS=( APPLE BANANA ORANGE )

eval array=\( \${${ARRAYNAME}[@]} \)

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

---

What you were originally trying to do was create an Indirect Reference. These were introduced in bash version 2 and were meant to largely replace the need for eval when trying to achieve reflection-like behavior in the shell.

What you have to do when using indirect references with arrays is include the [@] in your guess at the variable name:

#!/usr/bin/env bash

ARRAYNAME='FRUITS'
FRUITS=( APPLE BANANA ORANGE )

array="${ARRAYNAME}[@]"
for fruit in "${!array}"; do
  echo $fruit
done

---

All that said, it's one thing to use Indirect References in this trivial example, but, as indicated in the link provided by Dennis Williamson, you should be hesitant to use them in real-world scripts. They are all but guaranteed to make your code more confusing than necessary. Usually you can get the functionality you need with an Associative Array.

Answer 2

Here's a way to do it without eval.

See Bash trick #2 described here: http://mywiki.wooledge.org/BashFAQ/006

Seems to work in bash 3 and up.

#!/bin/bash

ARRAYNAME='FRUITS'
tmp=$ARRAYNAME[@]
FRUITS=( APPLE BANANA ORANGE "STAR FRUIT" )
for FRUIT in "${!tmp}"
do
    echo "${FRUIT}"
done

Here's a more realistic example showing how to pass an array by reference to a function:

pretty_print_array () {
  local arrayname=$1
  local tmp=$arrayname[@]
  local array=( "${!tmp}" )
  local FS=', ' # Field seperator
  local var
  # Print each element enclosed in quotes and separated by $FS
  printf -v var "\"%s\"$FS" "${array[@]}"
  # Chop trailing $FS
  var=${var%$FS}
  echo "$arrayname=($var)"
}
FRUITS=( APPLE BANANA ORANGE "STAR FRUIT" )
pretty_print_array FRUITS
# prints FRUITS=("APPLE", "BANANA", "ORANGE", "STAR FRUIT")