[ralph]

The difference between

    cat file | foo
    foo <file

assuming foo only reads stdin so `foo file' isn't possible, is that with the latter the shell will open file for reading on file descriptor 0 (stdin) before execing foo and the only cost is the read(2)s that foo does directly from file.

With the needless cat we have cat having to read the bytes and then write(2) them whereupon foo reads them as before. So the number of system calls goes from R to R+W+R assuming all reads and writes use the same block size and more byte copying may be required.