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 how does 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.

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.