Simple UNIX shell script for currency conversion

currconv.sh

#!/bin/sh
[ -z $2 ] || [ ! -z $4 ] && echo "Usage: currconv FROM TO [AMOUNT]" && exit;

# Parameters must be uppercase
FROM=$(echo $1 | tr 'a-z' 'A-Z' );
TO=$(echo $2 | tr 'a-z' 'A-Z' );

# SAMPLE: {"base":"CAD","rates":{"USD":0.8010888328},"date":"2021-04-23"}
RESPONSE="$(curl -s --http2 "https://api.ratesapi.io/api/latest?base=$FROM&symbols=$TO")";

# Check for API errors
echo $RESPONSE | grep -s "error" && echo "API error: $RESPONSE" && exit;

# Extract rate from response
RATE=$(echo $RESPONSE | cut -f4 -d\: | cut -f1 -d\} );

# If no amount given, output the rate
[ -z $3 ] && echo "Rate between $FROM and $TO: $RATE" && exit;

# Output amount * rate
echo "$3 $FROM is worth $(echo "$RATE*$3" | bc) $TO";