brain of mat kelcey

cool bash stuff; mkfifo

April 15, 2010

mkfifo is one of those shell commands provided as part of coreutils that not many people seem to know about.

here's an (semi contrived) example close to something i did the other day to show how awesome it is

say you have a number of largish presorted files; run-00 to run-03; and you want to find the most frequent lines. you could do something like the following...

sort -m run-* | uniq -c | sort -nr | head

however you'll know that from previous posts i just loooove keeping all my data compressed on disk so instead i've got run-00.gz to run-03.gz

without having to uncompress the files to disk i'd have to do something like this...

zcat run*gz | sort | uniq -c | sort -nr | head

but this pains me since it results in completely resorting the stream. i know the input files are sorted so i'd much prefer doing a sort -m than sort

so how can i mix the combo of zcat and a pipe to sort with sort -m wanting the multiple inputs as file descriptors instead of STDIN?

well, mkfifo of course!  it's a way of making a file that acts like a pipe ( a named pipe )

ls | sort

is sort-of, roughly, equivalent to

mkfifo bob
ls > bob &
sort < bob
rm bob

( have to background the ls since the write to the named pipe blocks until the read starts )

apart from being a cool way to get pipes working between totally seperate processes on a box this provides a solution for our original problem

mkfifo p0 p1 p2 p3
zcat run-00.gz > p0 &
zcat run-01.gz > p1 &
zcat run-02.gz > p2 &
zcat run-03.gz > p3 &
sort -m p* | uniq -c | sort -nr | head
rm p[0123]

and all four zcat can burn cpu while avoiding the need to resort.