continuous reading from named pipe (cat or tail -f)
cat
keeps reading until it gets EOF. A pipe produces EOF on the output only when it gets EOF on the input. The logging daemon is opening the file, writing to it, and keeping it open — just like it does for a regular file — so EOF is never generated on the output. cat
just keeps reading, blocking whenever it exhausts what's currently in the pipe.
You can try this out yourself manually:
$ mkfifo test
$ cat test
And in another terminal:
$ cat > test
hello
There will be output in the other terminal. Then:
world
There will be more output in the other terminal. If you now Ctrl-D the input then the other cat
will terminate too.
In this case, the only observable difference between cat
and tail -f
will be if the logging daemon is terminated or restarted: cat
will stop permanently when the write end of the pipe is closed, but tail -f
will keep going (reopening the file) when the daemon is restarted.
There is also a difference in buffering between cat
and tail -f
. You can check this out:
Create pipe: mkfifo pipe
Start reading pipe using cat
in background: cat pipe &
Open pipe and write to it every second: perl -MFcntl -we 'sysopen(my $fh, "pipe", O_WRONLY | O_NONBLOCK); while() {warn "written: " . syswrite($fh, "hello\n"); sleep 1}'
Now try this with tail -f pipe &
instead of cat
. So you can see that cat
prints lines as soon as they are written to pipe by perl script, while tail -f
buffers them up to 4kb before printing to stdout.