SC2149 – ShellCheck Wiki

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Remove $/${} for numeric index, or escape it for string.

Problematic code:

# Regular array
index=42
echo $((array[$index]))

or

# Associative array
index=banana
echo $((array[$index]))

Correct code:

# Regular array
index=42
echo $((array[index]))

or

# Associative array
index=banana
echo $((array[\$index]))

Rationale:

For a numerically indexed array, the $ is mostly pointless and can be removed like in SC2004.

For associative arrays, the $ should be escaped to avoid accidental dereferencing:

declare -A array
index='$1'
array[$index]=42
echo "$(( array[$index] ))"    # bash: array: bad array subscript
echo "$(( array[\$index] ))"   # 42

Exceptions

None.

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