SC2302 – ShellCheck Wiki

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This loops over values. To loop over keys, use "${!array[@]}".

Plus companion warning SC2303: i is an array value, not a key. Use directly or loop over keys instead.

Problematic code:

array=(foo bar)
for i in "${array[@]}"
do
  echo "Value is ${array[$i]}"
done

Correct code:

Either loop over values

for i in "${array[@]}"
do
  echo "Value is $i"
done

or loop over keys:

for i in "${!array[@]}"
do
  echo "Key is $i"
  echo "Value is ${array[$i]}"
done

Rationale:

ShellCheck found a for loop over array values, where the variable is used as an array key.

In the problematic example, the loop will print Value is foo twice. On the second iteration, i=bar, and bar is unset and considered zero, so ${array[$i]} becomes ${array[bar]} becomes ${array[0]} becomes foo.

If you don't care about the key, simply loop over array values and use $i to refer to the array value, like in the first correct example.

If you do want the key, loop over array keys with "${!array[@]}", use $i to refer to the array key, and ${array[$i]} to refer to the array value.

Exceptions:

If you do want to use values from the arrays as keys in the same array, you can ignore these messages with a directive:

declare -A fatherOf=(
  ["Eric Bloodaxe"]="Harald Fairhair"
  ["Harald Fairhair"]="Halfdan the Black"
  ["Halfdan the Black"]="Gudrød the Hunter"
  ["Gudrød the Hunter"]="Halfdan the Mild"
)

# shellcheck disable=SC2302,SC23203
for i in "${fatherOf[@]}" 
do
  echo "${fatherOf[$i]:-(missing)} begat $i"
done

ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.