Once Foundry is installed, the script uses cast to generate a burner wallet stored in an unencrypted keystore file under ./test-keystore/, then it signs and verifies a "hello world!" message.

signature-fun.sh

#!/usr/bin/env bash

# cast is required: https://book.getfoundry.sh/getting-started/installation
[ ! -x "$(command -v cast)" ] && echo "cast is not installed, please run 'curl -L https://foundry.paradigm.xyz | bash'." && exit 1

SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
ETH_KEYSTORE_DIR=$SCRIPT_DIR/test-keystore

if [ ! -d "$ETH_KEYSTORE_DIR" ]; then
    echo "Creating test keystore"
    mkdir $ETH_KEYSTORE_DIR
    export ETH_KEYSTORE=$(cast wallet new "$ETH_KEYSTORE_DIR" --unsafe-password "" | head -1 | sed 's/^.*: //')
else
    echo "Using existing keystore"
    export ETH_KEYSTORE="$ETH_KEYSTORE_DIR/$(ls -1 $ETH_KEYSTORE_DIR | head -1)"
fi

ethAddress=$(cast wallet address --password "")
echo "Using address $ethAddress"

message="hello world!"

# Sign
signature=$(cast wallet sign --password "" "$message")

# Verify
cast wallet verify --address $ethAddress "$message" $signature