The documentation for SQLite isn't super clear on how to work with external files directly from the command line. Here's how to read and process SQL from an input file:
sqlite3 database.db < input.sql
And this captures the output to a file as well:
sqlite3 database.db < query.sql > results.txt
Super useful for automating work without having to jump into SQLite itself.
(Alternative approaches are
cat input.sql | sqlite3 database.db and
cat query.sql | sqlite3 database.db > results.txt, respectively, but they aren't as aesthetically pleasing.)