echo -e "use your_keyspace;\r\n show schema;\n" | bin/cassandra-cli -h localhost > mySchema.cdl
I always forget the "-e" on echo, which is why I thought I would blog this. We've started using ".cdl" as the extension, short for "Cassandra DDL".
Coincidentally, you can then load it with:
bin/cassandra-cli -h localhost -f mySchema.cdl
Hope people find this useful.