shell write formatted text file with environment variables

readme.md

ref: https://stackoverflow.com/questions/12524437/output-json-from-bash-script/12524510

cat_style.sh

#!/bin/sh

cat <<EOF >/path/to/config.json
{ "ip": "$ip"}
EOF

printf_style.sh

#!/bin/sh

printf '{"ip": "$ip" }' >/path/to/config.json