Redirect to a named pipe

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.

The data passing through the pipe is not written or read to and from the filesystem. When creating the named pipe an inode is allocated so that it can be linked to from the directory you created it in; so that's a form of "modifying the filesystem".

Accessing the named pipe will update the inode's access time, writing to the named pipe will update the inode's modification time, so in that indirect way the filesystem is also being modified. That doesn't mean that the data is written to the filesystem; it's just how Linux handles writes to a named pipe.