You describe the GNU
tail utility. The difference between these two flags is that if I open a file, a log file for example, like this:
$ tail -f /var/log/messages
... and if the log rotation facility on my machine decides to rotate that log file while I'm watching messages being written to it ("rotate" means delete or move to another location etc.), the output that I see will just stop.
If I open the file with
tail like this:
$ tail -F /var/log/messages
... and again, the file is rotated, the output would continue to flow in my console because
tail would reopen the file as soon as it became available again, i.e. when the program(s) writing to the log started writing to the new
On the free BSD systems, there is no
-F option, but
tail -f will behave like
tail -F does on GNU systems, with the difference that you get the message
tail: file has been replaced, reopening.
in the output when the file you're monitoring disappears and reappears.
YOU CAN TEST THIS
In one shell session, do
$ cat >myfile
That will now wait for you to type stuff. Just go ahead and type some gibberish, a few lines. It will all be saved into the file
In another shell session (maybe in another terminal, without interrupting the
$ tail -f myfile
This will show the (end of the) contents of
myfile in the console. If you go back to the first shell session and type something more, that output will immediately be shown by
tail in the second shell session.
cat by pressing Ctrl+D, and remove the
$ rm myfile
Then run the cat again:
$ cat >myfile
... and type something, a few lines.
tail, these lines will not show up in the second shell session (where
tail -f is still running).
Repeat the exercise with
tail -F and observe the difference.