What I'm trying to do is really simple - I want to monitor a file and print its last line to the screen as the file gets updated. From what I know,
tail -F -n 1 myfile.txt
should do exactly that. However, I get strange behaviour: With the "original" myfile.txt, the command works fine and only the last line is printed to the screen. However, as soon as I alter myfile.txt by appending new lines of text, the entire contents of myfile.txt are printed - rather than just the very last line.
I have never used tail before and I might just be getting something terribly wrong here, but surely that's not the expected behaviour? I purposefully use the -F flag so I can manually alter myfile.txt - could that be the reason for it not working?
Help is very much appreciated... Thanks so, so much!
No, that's the way it's meant to work, -n 1 is the initial behaviour, printing only the last line, but -F follows the file beyond that point, and states quite clearly:
output appended data as the file grows;
In other words, it outputs all the appended data that's gone to the file.
If you examine the source code, you'll notice the main() function first processes the -n option and, only at the end, does it call tail_forever(), in which there is no mention of the argument supplied with -n.
If you execute:
( echo 1; echo 2; echo 3 ) >qq
in one window then start up a tail in another:
tail -F -n 1
you should get only the line with 3.
If you then return to the original window and execute:
( echo 4; echo 5; echo 6 ) >>qq
your second window should just output the new lines (and all three of them).
If your second window gives you all six of the lines, it's broken.