“tail -f” vs “tail -F”: Do you know the difference?

Hi all!
Do you know the difference between “tail -f” and “tail -F”?

duvida

Ok, don’t feel bad. It’s very difficult to find someone who knows… And with a reason, I can’t find any link explaining this by Googling.
It’s possible that I don’t know how to search it too. But I searched as I’d search if I didn’t know that… And couldn’t find anything about…

Let’s take a look on –help, so:

[root@mbdbasrvr]# tail --help
Mandatory arguments to long options are mandatory for short options too.
--retry              keep trying to open a file even if it is
inaccessible when tail starts or if it becomes
inaccessible later; useful when following by name,
i.e., with --follow=name
-f, --follow[={name|descriptor}]
output appended data as the file grows;
-f, --follow, and --follow=descriptor are
equivalent
-F                       same as --follow=name --retry
-n, --lines=N            output the last N lines, instead of the last 10
--max-unchanged-stats=N
with --follow=name, reopen a FILE which has not
changed size after N (default 5) iterations
to see if it has been unlinked or renamed
(this is the usual case of rotated log files)
If the first character of N (the number of bytes or lines) is a `+',
print beginning with the Nth item from the start of each file, otherwise,
print the last N items in the file.  N may have a multiplier suffix:
b 512, k 1024, m 1024*1024.
With --follow (-f), tail defaults to following the file descriptor, which
means that even if a tail'ed file is renamed, tail will continue to track
its end.  This default behavior is not desirable when you really want to
track the actual name of the file, not the file descriptor (e.g., log
rotation).  Use --follow=name in that case.  That causes tail to track the
named file by reopening it periodically to see if it has been removed and
recreated by some other program.
Report bugs to .

(Yes, I cutted off  non useful options. But you can check in your SO, if want to verify all options.)

So, ok! The information is there, but isn’t very clear. You have to match some points to understand it. I can’t find examples using -F nor –retry… So, let’s innovate posting about it… 🙂
The effect of F (capital) is the same of “-f file_name –retry”. It basically “still working” if the inode of the file change. Is very useful to systems with log rotation or stuffs like that.

Let me show you:

Session1:

[root@mbdbasrvr]# echo "test1">test.log

Session2:

[root@mbdbasrvr]# tail -f test.log
test1

Session1:

[root@mbdbasrvr]# echo "test2">test.log

Session2:

[root@mbdbasrvr]# tail -f test.log
test1

oook, we truncated the file but the tail didn’t change. But if a use an appen by now?

Session1:

[root@mbdbasrvr]# echo "test2">>test.log
[root@mbdbasrvr]# echo "test3">>test.log
[root@mbdbasrvr]# cat test.log
test2
test2
test3

Session2:

[root@mbdbasrvr]# tail -f test.log
test1
<>
[root@mbdbasrvr]# tail -f test.log
test2
test2
test3

Still not working, unless you restart the command… It’s because inode changed.

Let’s do it with -F (capital), to see the difference:

Session1:

[root@mbdbasrvr]# mv test.log oldtest.log
[root@mbdbasrvr]# echo "new_test">test.log

Session2:

[root@mbdbasrvr]# tail -F test.log
new_test

Session1:

[root@mbdbasrvr]# echo "new_test2">>test.log

Session2:

[root@mbdbasrvr]# tail -F test.log
new_test
new_test2

Session1:

[root@mbdbasrvr]# echo "new_test3">test.log
[root@mbdbasrvr]# echo "new_test4">>test.log

Session2:

[root@mbdbasrvr]# tail -F test.log
new_test
new_test2
tail: test.log: file truncated
new_test3
new_test4

Session1:

[root@mbdbasrvr]# mv test.log oldtest.log
mv: overwrite `oldtest.log'? y
[root@mbdbasrvr]# echo "new_test_with_capital">>test.log
[root@mbdbasrvr]# rm test.log
[root@mbdbasrvr]# echo "xxx new file">test.log

Session2:

[root@mbdbasrvr]# tail -F test.log
new_test
new_test2
tail: test.log: file truncated
new_test3
new_test4
tail: `test.log' has become inaccessible: No such file or directory
tail: `test.log' has appeared;  following end of new file
new_test_with_capital
tail: `test.log' has become inaccessible: No such file or directory
tail: `test.log' has appeared;  following end of new file
xxx new file

Owoooooow! Cool, hãn?
Even we truncate, or move, or remove and recreate it, the command still working! 🙂

Very cool and very useful to some situations… Unfortunately just a few know it… :/
Let’s spread this information by sharing this post?

Have a nice day, see ya!
Matheus.

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