Diff for "BashFAQ/085"

How to write several times to a fifo without having to reopen it?

In the general case, you'll open a new FileDescriptor (FD) pointing to the fifo, and write through that. For simple cases, it may be possible to skip that step.

The problem

The most basic use of NamedPipes is:

mkfifo myfifo
cat < myfifo &
echo 'a' > myfifo

This works, but cat dies after reading one line. (In fact, what happens is when the named pipe is closed by all the writers, this signals an end of file condition for the reader. So cat, the reader, terminates because it saw the end of its input.)

What if we want to write several times to the pipe without having to restart the reader?

Grouping the commands

We have to arrange for all our data to be sent without opening and closing the pipe multiple times.

If the commands are consecutive, they can be grouped:

cat < myfifo &
{ echo 'a'; echo 'b'; echo 'c'; } > myfifo

Opening a file descriptor

It is basically the same idea as above, but using exec to have greater flexibility:

cat < myfifo &

# assigning fd 3 to the pipe
exec 3>myfifo

# writing to fd 3 instead of reopening the pipe
echo 'a' >&3
echo 'b' >&3
echo 'c' >&3

# closing the fd
exec 3>&-

Closing the FD causes the pipe's reader to receive the end of file indication.

This works well as long as all the writers are children of the same shell.

Using tail

The use of tail -f instead of cat can be an option, as tail will keep reading even if the pipe is closed:

tail -f myfifo &

echo 'a' > myfifo
# Doesn't die
echo 'b' > myfifo
echo 'c' > myfifo

The problem here is that the process tail doesn't die, even if the named pipe is deleted. In a script this is not a problem as you can kill tail on exit.

If your reader is a program that only reads from a file, you can still use tail with the help of process substitution:

myprogram <(tail -f myfifo) &
# Doesn't die
echo 'b' > myfifo
echo 'c' > myfifo

Here, tail will be closed when myprogram exits.

Using a guarding process

The pipe will be really closed when the last opened file descriptor will be closed, you can exploit this by keeping a file descriptor opened on a process doing nothing.

An elegant solution is to use a second pipe to control the guarding process:

mkfifo myfifo
mkfifo guard

# keep the fifo opened using a fake writer
: >myfifo < guard &  #note the order is important! 

# if you do <guard first, it will be blocked
# and >myfifo will not be opened until guard is opened

# start the reader
cat myfifo

Now you can use writers in other unrelated process and the pipe will not be closed because of your process keeping it opened.

echo something > myfifo
# reader does not die

When you are finished and want to close the pipe, you just need to open and close the helper pipe to unblock the guarding processs:

: >guard 

An alternative is to use a process doing nothing, killing it in the end:

mkfifo myfifo

while :;do sleep 10000 & wait;done >myfifo &
pid=$!

cat myfifo

kill $pid