What is the purpose of using a FIFO vs a temporary file or a pipe?

Question

From APUE

FIFOs can be used to duplicate an output stream in a series of shell commands. This prevents writing the data to an intermediate disk file (similar to using pipes to avoid intermediate disk files).

But whereas pipes can be used only for linear connections between processes, a FIFO has a name, so it can be used for nonlinear connections.

Consider a procedure that needs to process a filtered input stream twice.

mkfifo fifo1
prog3 < fifo1 &
prog1 < infile | tee fifo1 | prog2

We create the FIFO and then start prog3 in the background, reading from the FIFO. We then start prog1 and use tee to send its input to both the FIFO and prog2.

1. How does a FIFO "duplicate an output stream in a series of shell commands"? Isn't this done by tee instead of a FIFO?

2. In the example, mkfifo fifo1 creates a file in the current directory, and fifo1 seems replaceable with a regular file . So what is the point of a FIFO "prevent writing the data to an intermediate disk file"?

3. What do "linear connections" and "nonlinear connections" between processes mean? What does it mean that a FIFO can be used for nonlinear connections, while a pipe can be only used for linear connections between processes?

Thanks.

Answer 1

1. APUE says “FIFOs can be used to duplicate an output stream”, it doesn’t say that FIFOs actually duplicate the output stream. As you point out, the duplication is done by tee in the example.

2. mkfifo creates a FIFO, which is visible as a “file” in the containing directory; but writing to the FIFO isn’t like writing to a file because the data never hits the disk. Pipes, named or otherwise, don’t provide storage for data, they provide communications channels; the writing end of a pipe can’t write data if there’s no receiver, the pipe just passes data along, without storing it. (On most systems pipes are backed by small kernel buffers, to improve performance, but that’s an implementation detail.)

3. Linear connections between processes are pipes which can be represented as a linear graph. In the example, you can represent the last line as

   infile → prog1 → tee fifo1 → prog3
   

   which is linear, but if you try to represent the whole chain, reducing to processing elements, you need

   infile → prog1 → prog2
                  → prog3
   

   which is non-linear (there’s one node in the graph, prog1, which has two exit nodes).