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.
How does a FIFO "duplicate an output stream in a series of shell commands"? Isn't this done by
teeinstead of a FIFO?
In the example,
mkfifo fifo1creates a file in the current directory, and
fifo1seems replaceable with a regular file . So what is the point of a FIFO "prevent writing the data to an intermediate disk file"?
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?