In most hackers' idiolect, the term "string" refers to an implementation-dependent representation of characters. An octet stream or byte stream is the abstraction that UNIX files and the standard I/O streams happen to be instances of. So it wouldn't be wrong per se to use "octet stream" here, but I'm treating each phase of the pipeline as a hypothetical function that operates on text. With less indirection, you could certainly view them as functions operating on streams, but I abstracted that away in my mental picture.