Bash: create anonymous fifo

bashpipescript

We all know mkfifo and pipelines. The first one creates a named pipe, thus one has to select a name, most likely with mktemp and later remember to unlink. The other creates an anonymous pipe, no hassle with names and removal, but the ends of the pipe get tied to the commands in the pipeline, it isn't really convenient to somehow get a grip of the file descriptors and use them in the rest of the script. In a compiled program, I would just do ret=pipe(filedes); in Bash there is exec 5<>file so one would expect something like "exec 5<> -" or "pipe <5 >6" -is there something like that in Bash?

Best Answer

You can unlink a named pipe immediately after attaching it to the current process, which practically results in an anonymous pipe:

# create a temporary named pipe
PIPE=$(mktemp -u)
mkfifo $PIPE
# attach it to file descriptor 3
exec 3<>$PIPE
# unlink the named pipe
rm $PIPE
...
# anything we write to fd 3 can be read back from it
echo 'Hello world!' >&3
head -n1 <&3
...
# close the file descriptor when we are finished (optional)
exec 3>&-

If you really want to avoid named pipes (e.g. the filesystem is read-only), your "get a grip of the file descriptors" idea also works. Note that this is Linux-specific due to the use of procfs.

# start a background pipeline with two processes running forever
tail -f /dev/null | tail -f /dev/null &
# save the process ids
PID2=$!
PID1=$(jobs -p %+)
# hijack the pipe's file descriptors using procfs
exec 3>/proc/$PID1/fd/1 4</proc/$PID2/fd/0
# kill the background processes we no longer need
# (using disown suppresses the 'Terminated' message)
disown $PID2
kill $PID1 $PID2
...
# anything we write to fd 3 can be read back from fd 4
echo 'Hello world!' >&3
head -n1 <&4
...
# close the file descriptors when we are finished (optional)
exec 3>&- 4<&-