check if tool is installed before running command

code.sh

check_file_exists () {
    local what=${1:-}
    if [[ ! -f "$what" ]]; then
        >&2 echo "Error: Could not find '$what'"    # stderr
        exit 1
     fi
}

check_tool_exists () {
    local what=${1:-}
    local how=${2:-brew install $what}
    if ! command -v $what > /dev/null; then
        >&2 echo "Error: '$what' command not found"     # stderr
        >&2 echo "Try: $how"                            # stderr
        exit 1
    fi
}

check_var_exists () {
    local what=${1:-}
    if [[ -z "${!what}" ]]; then
        >&2 echo "Error: Did not find value for '$what'"    # stderr
        >&2 echo "Try: export $what=something"              # stderr
        exit 1
     fi
}

# Example for command "jq"
if ! command -v jq > /dev/null; then
    >&2 echo "Error: 'jq' command not found"
    >&2 echo "See how to install for your system in https://stedolan.github.io/jq/download/"
    exit 1
fi
# run your command HERE