Why can't an associative Bash array be assigned with an array?

Question

According to man bash it is possible to assign an associative array with a flat list of key-value pairs:

[…] or a list of words that is interpreted as a sequence of alternating keys and values: name=( key1 value1 key2 value2 ...)

This does work, but it does not work to use an array to get these "alternating keys and values": name=( "${someArray[@]}" )

Consider this example:

#!/usr/bin/env bash

set -u

#declare -A map=(one ONE two TWO)
array=(one ONE two TWO)
declare -A map=("${array[@]}")
# Workaround with `eval`:
#eval "declare -A map=(${array[@]@Q})"

echo "KEYS:"
printf '    %s\n' "${!map[@]}"
echo "VALUES:"
printf '    %s\n' "${map[@]}"

echo "${#map[@]}"

Actual output:

KEYS:
    one ONE two TWO
VALUES:

Expected output:

KEYS:
    two
    one
VALUES:
    TWO
    ONE

Why is that so?

Although probably clear, declare -A map=(${array[@]}) does not work either. — doak
– doak, Commented Jan 11, 2024 at 13:14
Any reference for this behaviour, @mandy8055? It also happens if assigned later, i.e. after declared. — doak
– doak, Commented Jan 11, 2024 at 13:17
No, it doesn't, @Amessihel. I know how to decalre an associative array. This is about using another array for doing this. — doak
– doak, Commented Jan 11, 2024 at 13:18
Furthermore, your claim seems to be wrong, @mandy8055: There is only a single key one ONE two TWO. — doak
– doak, Commented Jan 11, 2024 at 13:20

pmf · Accepted Answer · 2024-01-11 13:30:30Z

6

Include the array parens in the quoted string to preserve the whitespace between items for the declaration. Also, if keys or values themselves contain whitespace, use @Q to quote each item:

array=(one 'O NE' 'tw o' TWO)
declare -A map="(${array[@]@Q})"

echo "KEYS:"
printf '    %s\n' "${!map[@]}"
echo "VALUES:"
printf '    %s\n' "${map[@]}"

echo "${#map[@]}"

KEYS:
    one
    tw o
VALUES:
    O NE
    TWO
2

Tested with GNU bash 5.2.21

answered Jan 11, 2024 at 13:30

pmf

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3 Comments

doak Over a year ago

Indeed, using declare -A map="(${array[@]@Q})" works. Any explanation for this? It looks like Bash is using eval internally for this, doesn't it?

pmf Over a year ago

I'd guess that ${array[@]} is evaluated later than declare -A map=… is parsed. Therefore, declare -A map=(one ONE two TWO) is recognozed as intended whereas declare -A map=(${array[@]}) is prematurely broken down into multiple arguments. This can be mitigated by using quotes as in declare -A map="(${array[@]})" or even declare -A "map=(${array[@]})". The important thing is to provide two arguments to declare: -A and the "content".

F. Hauri - Give Up GitHub Over a year ago

U could use declare -p map array for showing content of map and array variable. Or even this: paste -d '' <(printf ' - %-10s\n' "${!map[@]}") <(printf ' \47%s\47\n' "${map[@]}")

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