cat file | … vs … <file

Question

Is there a case of ... or context where cat file | ... behaves differently than ... ?

Answer 1

One further difference is behavior on a blocking open() of the input file.

For example, assuming input is a FIFO with no writers, one invocation will not spawn any child programs until the input file is opened, while the other will spawn two processes:

prog ... < a_fifo      # 'prog' not launched until shell can open file
cat a_fifo | prog ...  # 'prog' and 'cat' are running (latter may block on open)

In practice this rarely matters except in contrived circumstances. prog might periodically log or do some cleanup work while waiting for input, for example, which you might want to happen even if no input is available. (Why wouldn't prog be sophisticated enough to open its own input fifo nonblocking?)