How can I check if an associative array element exists in my Bash script?

Question

I have an array of animals:

declare -A animals=()
animals+=([horse])

and I want to check if an animal exists or not:

if [ -z "$animals[horse]"]; then
    echo "horse exists";
fi

but this does not work.

Answer 1

In bash 4.3, the -v operator can be applied to arrays.

declare -A animals
animals[horse]=neigh
# Fish are silent
animals[fish]=
[[ -v animals[horse] ]] && echo "horse exists"
[[ -v animals[fish] ]] && echo "fish exists"
[[ -v animals[unicorn] ]] || echo "unicorn does not exist" 

In prior versions, you would need to be more careful distinguishing between the key not existing and the key referring to any empty string.

animal_exists () {
    # If the given key maps to a non-empty string (-n), the
    # key obviously exists. Otherwise, we need to check if
    # the special expansion produces an empty string or an
    # arbitrary non-empty string.
    [[ -n ${animals[$1]} || -z ${animals[$1]-foo} ]]
}

animal_exists horse && echo "horse exists"
animal_exists fish && echo "fish exists"
animal_exists unicorn || echo "unicorn does not exist"