Difference between > and >> when used with a named pipe

Short answer: > must be followed by a filename or &n (n is a number), and | must be followed by another command invocation.

Details: In shell syntax, a call to some command contains several components. Example:

A=foo 2>/dev/null B=bar cmd arg1 arg2 >file 3>&4 arg3

Here, parameters 2>/dev/null, >file and 3>&4 are special parameters (containing an unescaped >¹), they are used to establish io-redirections, and can appear anywhere in the command line. Filedesciptor 2 is redirected to /dev/null, filedescriptor 1 (implicit) is redirected to file and filedescriptor 3 is redirected to what filedescriptor 4 was linked to.

Then, among remaining parameters, A=foo and B=bar contain =, so they are not considered as the command name: they give specific values to environment variables of the process to be launched.

Then comes the command cmd and the real arguments: arg1, arg2, arg3.

The pipe | is not part of a command invocation, it links two such invocations together. Example:

CC=gcc make 2>&1 | LESS=--quit-at-eof less

Output on filedescriptor 1 by the first process will be received as input on filedescriptor 0 by the second process, through a “pipe” which acts like a buffer.

—
^{1. In fact, the special characters like > are sometimes seen followed by a space. Even though this is allowed, the two (space-separated) strings must be understood as a single ‘entity’.}

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.